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Chem 29a – Exam 1 – Flashcards

Lewis Edwards

7 July 2022

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Chromatography

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- sep. mixture (ions/compounds) between a stationary phase and a mobile phase. - the 2 phases can be solid-liquid, liquid-liquid, gas-liquid - types: TLC, Column, GC

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TLC

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- based on polarity, H-bonds - seperates mixture of dyes into its components between stationary phase and mobile phase - how far each dye will move along the plate will depend on the strength of the dye-mobile vs. dye-stationary IMF - stationary phase = silica absorbent (solid) - mobile phase = developing solvent (liquid) - component dots = mixture to be seperated

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Distillation

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- separation. of immiscible liquids in a mixture based on bp - factors that influence boiling point: IMF and molecular weight - two types: simple and fractional

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Chromatography basics - long version

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1. Apply mixture to stationary phase (silica gel) 2. Immerse in mobile phase 2. Mobile phase (developing solvents) passes over stationary phase in a defined direction 3. The components in the mixture sep as a result of the equilibria between mobile phase and stationary phase (free and absorbed state) 4. The free/absorbed state depends on the strength of its interaction (IMF) to the stationary and mobile phase

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TLC overview

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1. Spot the silica plate with mixture (Plate = Stationary phase) at the solvent origin 2. Immerse spotted plate in developing solvent (Solvent = mobile phase) 3. Developing solvent rises through the silica plate by capillary action 4. The mixture separates as a result of the equilibria between the mobile phase and stationary phase (free state vs. absorbed state 5. The more tightly a compound binds to the statioanry silica absorbent, the slowly it moves aup on the plate - When the silica gel is the stationary phase, the developing solvent moves nonpolar substances up the plate most rapidly 6. The TLC plate is removed fromthe developing chamber when the solvent front is 1-1.5cm from the top 7. Observe the positions of the compounds through KMNO4 stain or UV

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TLC basics

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- stationary phase = silica absorbent - mobile phase = develoing solvent - component spot = mxiture

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free state

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- completely dissolved in the liquid or gaseous mobile phase

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absorbed state

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- stuck on the surface of the stationary phase

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The equilibrium of the mixture between free-absorb state equilibrium between the mobile phase and the stationary phase

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- depends on IMF/polarity of: - remember polarity is also determined by structure 1. Polarity/Size/Structur of the components in the mixture 2. Polarity/Size of the stationary phase 3. Polarity of the solvent

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Intermolecular Attractions (IMF)

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- determine how far a compound moves on TLC 1. H-bonding 2. Dipole-dipole 3. Induced dipole/Van der Waals

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H-bonding

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- strongest IMF - between hydrogen atom and a highly electronegative atom - H bond donor and H-bond acceptor - Best H bond donors: O, N, halogens (have a H) - Best H bond acceptors: O, N, F - EX: The hydroxyl group in methanol is an H bond donor (NOT the CH3, since its not an electronegative atom).

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Dipole-dipole interactions

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- electrostatic attractions between polar molecules - acetone, acetonitrile, dimethyl sulfoxide

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induced dipole interactions/Van der Waals

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- weakest interaction - rapid and random e- movement - TEMPORARY DIPOLE within a molecule - a secondary molecule induced is called an induced dipole - hexane and another dipole can force hexane to become induced. one side is partially +, one side aprtially -

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Silica in TLC

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- easily hydrated, dessicant, absorbs water - The electropositive silicon and the electronegative oxygen create a very polar stationary phase. Therefore, the more polar the molecule to be separated, the stronger the attractive force to the stationary phase. - polar and able to form H-bonds - polar molecules are strongly attracted to silcia and will move slower on TLC - nonpolar molecules are weakly attracted to silica and will move faster on TLCD

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Developing solvents in TLC

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- The mobile phase can be polar, int. polar, or nonpolar **Polar solvents move farther on TLC than nonpolar solvents - the molecule has to be on its free state in order to travel on the plate - In general, non-polar developing solvents move non-polar compounds, and polar solvents move polar compounds

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Distance Travelled by compounds (general)

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Non-polar compounds spend less time interacting with silica, more time in the mobile phase • Polar compounds spend more time interacting with silica, less time in the mobile phase • Nonpolar compounds are always going to win the race whether the compound is polar solvent or not • Although order of elution can generally be predicted, only performing the experiment will give definite answers

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retention factor

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- distance traveled by compound/distance travelled by solvent front The larger an Rf of a compound, the farther the distance it travels on the TLC plate

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polarity

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- sep of charge, dipoles - results in diff of electronegativity

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polar molecule

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- has a permanent dipole e.g: water, acetic acid, ethanol (water is bent e- are being drawn towards the oxygen - In general, O, N halogens increase polarity but it depends on the shape

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nonpolar molecule

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- does not have a permanent dipole - hexane, toulene

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Uses of TLC

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1. Purity: # of components in a mixture 2. Identity: identity of compounds 3. Monitor rxn progress - disappereance of limiting reagent and appearance of product 4. Conditions for a column chromtography 5. To monitor fractions from column chromatography

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Hypothesis for TLC: Is a green spot a mix of two or more spots

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- purity question - # of components in a green spot Conclusion: - We see a little residue of the green spot - The green spot contains at least two spots

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Is the yellow spot in the green marker Sunset yellow, or a different yellow dye?

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- identity question - Spot green, sunset yellow, and a cospot of both. Conclusion: - Green separated in two spots like expected - sunset yellow ran the same distance as the yellow spot in the green and had the same streaking pattern , its the same

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Prelab 1 - Hypothesis and Design TLC experiment

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- Hypothesis: Eryth B is a common dye that can be found in purple markers. - 95% water/ethanol solvent system, which is more similar to the polarities of the markers - Marker, spot, and Eryth B will be spotted at the solvent origin. - conclusion: 3 spots travel to the same height with the same RF values and color streaking patterns after development and UV visualization

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Postlab 1: Can you combine all 3 dyes that will allow separation for at least one of the dyes?

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- Fast Green, Tatrazine, and Eryth B were spotted on the solvent origin. - Observation: 3 spots formed three big circles with the same RF value - Conclusion: - Most likely all three dyes had the same polarity since they contain all the acidic sulfonic groups and NaO ionic bonds - In future experiments, use diff solvent system

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stationary phase

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- usually polar, silica gel most common

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mobile phase

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- pure solvent or a mixture of solvents - depends on the polarities of the compounds in the mixture being separated - mostly nonvolatile, since volatility can lead to loss of sample by evaporation

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Some mixtures of dyes contain amines or carboyxilic acids that hydrogen bond to the plate and produce really low RF values, what solvent system should you use?

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- Acetic acid for carboxylic acids - Ammonium hydroxide for amines - Make sure that addditives are miscible for the developing solvent

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Answers to pitfalls of TLC 1. Solid doesn't stick 2. Don't see any spots 3. Spots are at the solvent front 4. RF values aren't the same to literature value 5. RF values are too similar (+/- 0.05

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1. Make sure to dissolve solid 2. a. Not enough sample was plate - too dilute, too volatile b. Sample was dissolved in the developing jar, since immersed past the developing solvent c. Not visualizing properly - UV or KMNO4 not heated 3. Developing solvent was too par 4. RF values aren't the same because the developing solvent contained a soluble impurity (Waterin acetone vs. pure acetone) 5. Another solvent can be tested

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18.1) When 2-propanol was used as the developing solvent, the two substance moved with the solvent front (RF = 1) during the TLC analysis on the silica gel plate. Can we conclude that they are identical? If not, what additional experiments would you perform?

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- By definition, RF values are always less than 1. - An Rf value of 1 means the solvent front and spot travelled close together - RF value is too high - 2-propanol is too polar of an developing solvent

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Chromotography elution sequence by FG (increasing polarity)

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Higest/Fastest (elute with nonpolar mobile phase) 1. Alkane, Alkene, Alkyl 2. Aromatic HC, hallides 3. Ethers 4. Ketones, Aldehydes 5. Esters 6. Amines 7. Alcohols 8. Phenols 9. RCOOH 10. Sulfonic acid (Slowest - need more polar mobile phase to elute)

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Chromotography mobile phase/solvent polarities

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1. Hexane 2. Toluene 3. Methyl-t-butyl ether (RO-fork) 4. DCM 5. Ethyl acetate 6. Acetone 7. Ethanol, Methanol 8. Acetonitrile 9. Water 10. Acetic acid

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18.2)The RF value of a compound A is 0.34 when a TLC plate is developed in pentane and 0.44 when the plate is developed in diethyl ether. Which solvent would be better separating a mixture of A and B by TLC ?

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- The more optimal solvent is the diethyl ether because it creates a RF value that is closer to 0.5 with because that has intermediate polarity -

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Why don't idenical RF values necessarily mean the same compound?

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- TLC plates must be run under the same exact conditions (stationary phase, mobile phase, and temperature) - Many organic molecules have the same physical properties - mp, color, so it makes sense that would have the identical RF - number is only significant when the same chromatographic conditions are used

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Purpose Lab 1 - TLC

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- test hypothesis - to identify dye components in markers (identification) - to determine number of dyes in marker (purity)

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Purpose Lab 2 - Distillation and Chromotography

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- seperate binary mixture of ethyl acetate and n-butyl acetate and dye solution using simple and fractional distillation, LC, GC. - The purities of our samples will be assessed using GC and TLC

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Purpose Lab 3 - Who Else Has My compound

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- Identify unknown compound and see who else has it using physical properties: 1. Solubility - dissolve in diff solvents, test for polarity, pH groups 2. Melting Points - governed by IMF 3. TLC - polarity 4. Recrystallization - purification of compound

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18.4) Compounds 4-tert butyl-cyclohexonol and 4-tert-butylcyclohexanone. Using silica gel plates, which of the two compounds have the higher RF value when ethyl acetate is the developing solvent? Which will have the higher RF value when dichloromethane is the developing solvent?

answer

- Cyclohexanol has the lower RF value because it is more polar. It interacts more with the stationary

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Liquid/Column Chromography

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- most common method of purification of compounds in organic chemistry - sep of a mixture based on polarity and H-bonding - sep. mixture (ions/compounds) between a stationary phase and a mobile phase.

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TLC vs. Column Chromotography (4)

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1. Application TLC is qualitative - get compounds information, purity + identity. Column is quantiatitve/qualitative - can also isolate compounds 2. Stationary phase TLC - Silica gel is on a plate Column - Silica gel is supported on a column 3. Mobile phase TLC - Developing solvent, travel UP the plate Column - Elution solvent, travel DOWN the plate 4. Amount of sample - TLC - small amount of sample - Column - wide range of sample quantities 5. Polarity - Both Separation is based on polarity, H-bonding

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Analyzing fractions b yTLC-which compound will elute from the column first?

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Observation: - The green spot is further up the plate on TLC, means it is less polar Conclusion - Green dot will elute first on column chromography

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Silica Column Chromatogrpahy protocol

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1. Prep the elution sample (mix of dyes) and test tubes 2. Prep properly packed column of absorbent - glass wool, sand, ten add silica slurry, so absorbent doesn't flow through 3. Add the sample to column (wait until meniscus is just below silica absorbent - do not let solvent level fall) 3. Elute the column with elution solvents 4. Collect the eluted compounds in fractions (test tubes) 5. Analyze purity by TLC 6. Recover compounds by evaporating the elution solvents

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Sources of Confusion and common pitfalls in Column Chromotography 1. Solvent started to boil 2. Poor separation of components -- a. Polarity of Elution solvent or Too much b. Preparing sample improperly c. Overloading the column d. Channeling e. Nonhorizontal bands 3. Column became dry 4. Diffused bands or tailing occured

answer

1. Exotheric reactions between polar solvents and the absorbent can produce heat. ADD SOLVENT SLOWLY (silica + diethyl ether) 2a) If the solvent is too polar, the sample mixture will elute too quickly. If the solvent is not polar enough, the sample will elute too slowly and wide bands. Too much elution solvent - excess will carry mixtures carry the mixtures components the column 2b) How well sample preped and applied to column 2c) Overloading the column - too much or too little absorbent 2d) Channeling - occurs because of an air bubble or applying sample to vigourously, the bands start to seep down to the next band 2e) Result if the column is not perfectly vertica. Poor separation can result because the lower part of one band may coeleute with the upperband 3) If the solvent level falls below the top of the absorbent, it can become dry and fall from the column. Be sure that the absorbent is covered with solvent throughout 4) If the elution solvent flows through too slowly or is not polar enough to displace the desired compounds, poor reparation

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Dry silica poses an ___________________________. Preprare silica slurry in the fume hood

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inhalation hazard

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What is silica slurry

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mixture of silica and elution solvent

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How do you clean up TLC chips/residual silica from the benchtop?

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Use damp paper towel to wipe benchtop clean from residual silica gel

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Dispose of used TLC plates and silica in___________

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solid waste

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Why is it imp. ohat the level of the elution solvent not drop below the absorbent?

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- If the solvent level falls below the top of the absorbent, it can become dry and fall from the column. - Compromise the effectiveness of the column

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What effect will the following factors have on liquid chromatographic separation? a) Not clamped in vertical position b) collection of large fractions c) slow flow rate of mobile phase

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a) elution dye mixture will appear to have nonhorizontal bands. bands will co-elute resulting in poor seperation b) the fractions will be impure. Each fraction wil lcontain a mixxture of dyes rather than isolated single dyes c) very slow rate of the mobile phase - column was packed too much, bands will widen and coelute with each other.

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elution solvent

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- mobile phase in liquid chromography - disloade compounds absorbed on an LC column - Nonpolar compounds bind less tightly on a polar absorbent and disloade more easily with the nonpolar solvent - nonpolar solvents exit gfirst - the more polar solvents must be eluted or washed out of the column, with more polar solvents

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Gas Chromatography (GC)/Gas-liquid chromatography

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- based on bp - analyzes VOLATILE mixtures, between stationary phase and mobile phase - stationary phase = polar column - mobile phase = inert gas (air) - mixture = binary acetate mixture - the compound that will elute first will be the one with the lower bp

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Uses of GC

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1. Determine purity 2. Determine fractions of components present in a sample 3. Indentity: identify unknown samples - ONLY if known standards are avaialble

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GC vs. TLC vs. LC

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1. Amount of sample GC/TLC: Small LC: Large 2. Compound Identity GC/TLC: Not really unless known samples are available LC: Able to isolate compounds 3. Mobile phase-compound intearaction TLC/LC: Yes, there are competing IMF GC: No, the mobile phase does not interact with the compounds. The inert gas simply carries them down the column

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GC basics

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1. Sample is injected into the port 2. Sample is vaporized in the heated chamber 3. Sample vapor is carried to the column by the mobile phase (inert gas). Does not interact 4. Separation occurs in the column, based on the attraction to the column. If it has a lower bp, it spends more time in the vapor phase and is carried along faster 5. Chromatogram is recorded

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GC - sources of confusion and common pitfalls 1. No peaks appear - clogged syringe 2. overlapping peaks, poor separation a. overloading column b. pushing syringe slowly 3. More peaks then expected

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1. Injecting the sample into the wrong injection port or using a clogged syringe. Rinse with acetone. 2. a. Overloading the column can cause broad peaks and poor seperation. Sample is too concentrated or the injection volume is too large b. The syringe may have been pushed to slowly - the sample reaches the column before the entire sample has vaporized 3. Making a second injection before the first chromogram is complete. The compounds must be allowed to fully elute before the next injection is made.

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GC: How can the following factors effect retention time? 1. High temperature of column 2. Too fast Mobile phase (carrier gas) flow rate

answer

1. Low Retention Time - Sample will elute too quickly through the column. It will not interact as wiell with the stationary phase and result in a low retention time. 2. Low Retention Time - If mobile phase too fast, the sample vapor will spend more time in its free state/mobile phase rather than interacting with the stationary phase. This may result in overlapping repeating peaks

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Pemanent pipoles and bp

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- Molecules with permanent dipoles can show higher bp - Higher bp than molecules of the same molecular weight - Propane just ha van der waals - di-methyl ether has higher bp

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Alcohols and bp

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- Alcohols have high bp when compared to structurally similar compounds - due to H-bonding. Recall H-donor and H-acceptor

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Molecular Weight and bp

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Increased MW - larger surface area - more IMF - more IMF - higher bp

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Isomers and bp

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- isomers have the same type of IMF and MW but diff structure - Linear HC are elongated - more surace area - more IMF - higher boiling point - Branched HC - blobbed - less surface area - less IMF - lower bp - T-butanol and n-butanol are constitutional isolers. T-butanol in blob form is spherical, n-butanol is sort of oval. N-butanol has a higher poiling point.

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Distillation basics

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1. Sample is heated until boiling (to vaporize the liquid). Heat source = sand bath 2. Vapors leave the flask in the distillation column 3. Some vapors condense and return to the flask as a liquid = Vaporization-condensation cycle - Cycles repeat and the vapor phase becomes enriched with the lower bp component 4. Vapor reaches thermometer, records temp. of vapor, ready distill 5. Vapors reach cool condensing arm, condenses back to liquid 6. Condensed liquid flows into collecting vial

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Simple vs fractional

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Simple = Fewer vaporization-condensation cycles/theoretical plates 1. Less effective, sep high bp from low bp 2. Shorter condensing arm 3. Shows a consistent linear slope of temperature increase as we reach the bp of the mixture, No indication of distilattion into receiving vials of ethyl acetate and n-butyl acetate at respective boiling point Fractional: More vaporization condensation cycles/theoretical plates 1. more effective, better seperation of mix that differ by less than 60°C 2. longer copper packed column - more surface area 3. Once ethyl acetate boils out shows a temperature bump of n-butyl acetate, indicates disllation at the respective bp

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Distillation Setup 1. Thermometer probe 2. Boiling chips... 3. Tight rubber seals 4. Ice bath for receiving vial 5. Don't distill to dryness(3 reasons)

answer

1. Placed below distilation head So the thermometer will accurately read the temp of the vapures about to distill 2. Ensures smooth and even heating, prevents bumping and superheating 3. So that the vapors don't escape during the condensing down of the distillate, if it happens you get less sample 4. To ensure efficient condensation, maximum recovery of distillate 5. (i) Flask may break (ii) Mixture get charred (iii) peroxides (oxidizes)

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12.3) A student carried out a simple distillation on a compound known to boil at 124°C and reported an observed bp of 116°-117°C. Gas chromatographic analysis of the product showed that the compound was pure, and a calibration of the thermometer indicated that it was accurate. What procedural error might the student have made in setting up the distillation apparatus.

answer

- The thermometer is reading much lower than the expected bp - This can be due to the fact that the vapor has not yet reached the thermometer bulb. - Or the thermometer was positioned improperly.

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Solubility

answer

- compound solubility in solvent gives information about the polarity, pH-sensitive group, IMF - like dissolves like - polar: H-bonding, dipole-dipole, Van-der-waals - Acid FG dissolves in basic solutions (high pH) - Basic FG dissolves in acidic solutions (low pH) - Gibbs free energy . Negative = Favorable

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Melting points

answer

- compare melting point RANGE of our known to literature value - mp is determined by IMF. high mp = strong IMF = polar low mp = low IMF = nonpolar - pure compounds have a narrow melting point range while impure compounds have a broad melting point range - if the mp are similar, mixed melting point can be used with a standard

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Recrystillization

answer

- based on solubility - common purification technique of solids, separates solids from the impurities that remain soluble in the solvent - the optimal recrystilization solvent will be determined via TLC. low solubility at RT but high solubility at high T - success is measured by percent recovery

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percent recovery

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mass of crystallized sample/mass of crude sample x 100%

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Solid separated from solid

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1. TLC 2. Column Chromography 3. Extraction

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Solid separated from liquid

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1. Filtration 2. Evaporation

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Liquid separated from liquid

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- immiscible liquids allow for physical seperation 1. Simple Distillation 2. Fractional Distillation

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Solubility-Structure Observations 1. Insoluble in MeOH, Ace, EtOH, H2O 2. Insoluble in HCl, NaOH, NaHCO3 3. Soluble in hexane, toulene, and DCM What are the conclusions? (Polarity --> IMF --> Structure)

answer

1. Does not dissolve in polar solvents a. Polarity: nonpolar end of spectrum b. IMF: lacks H-bonding or dipole-dipole forces c. Structure: eliminate compounds containing H-bonds or dipole-dipole forces 2. Does not dissolve at diff PH a. Polarity: does not contain pH sensitive groups b. : lacks alcohol, phenol, amine, carboxylic acids c. SttrEliminate compounds containing these FG. Use pH to verify 3. Dissolves in nonpolar solvents - proofs 1+2 that is nonpolar - contains only van-der-waals forces

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Structure-solubility (Structure--> Polarity) Given the strcture of your identified compound, do the solubility

answer

- Napthalene - Structure: Two fused benezene rings - Polarity: Nonpolar - IMF: No H-bonding or dipole-dipole, only induced dipole and van der waals - since solubility is governed by IMF we should expect it should dissolve in nonpolar solvents - polarity: No FG

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Choose two compounds from the list of unknowns that you expect to separate by column chromatography and two different compounds that would likely be difficult to separate. Explain your choices, including the structures of compounds

answer

1. Easily to separate: Napthalene and Benzoic acid a. Similar enough in their FG so we can use the same elution solvent but also diff enough in their polarity so that we can achieve a good separation of fractions. b. Both share benzene rings - BA - 1, Napthalene -2. But BA has RCOOH group, more polar, will elute last , more time interacting with the absorbent 2. Hard to separate: Napthalene and Trans-stilbene - Too similar in functional groups and polairities

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Solvation is spontaneous when

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- solvation = solute is completely separated and surrounded by solvent molecules. ?G < 0 § Recall ?G = ?H - T?S § We want a large negative ?H § We want a large, positive ?S

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Enthalpy of Solvation: When dissolving solute X into solvent Y, three processes occur:

answer

1. Breaking up intermolecular forces of X (?H1 > 0, endothermic, using energy) 2. Breaking up intermolecular forces of Y (?H2 > 0, endothermic, using energy) 3. Forming new intermolecular forces between X and Y (?H3 < 0, exothermic) For favorable solvation, process 3 must release enough energy to overcome the energy uptake of processes 1 + 2: ?Hsln = ?H1 + ?H2 + ?H3

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E.g. "Dissolve" hexanes in water.

answer

1. Hexane being nonpolar, lacks permanent dipole-dipole interacts, but it has induced dipole interactions. 2. Water is polar has permanent dipoles and H-bonding, which is hardest to break. It 3. The solution - Hexanes in water are immiscibe. The energy required to break up this int and int, that inergy is too much to overcome the energy that is released froom the dipole-induced dipole interactions of forming new interactions.

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Entropy of solvation

answer

- increase in disorder in the system. ? A little beaker contains solvent Y and solute X. The solute X is nice and organized at the bottom. No mixture ? Mix to dissolve ? You observe the increase in entropy because you are disorganizing the solvent so that the solute can get disorganized in it. It is randomly dispersed, more solute molecules surrounded by solvent

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Filtration

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1. To separate a solid product from a reaction mixture or from recrystallization solution 2. To remove solid impurities from a solution 3. To separate a production solution from a drying agent after aqueous extraction

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Confusion and common pitfalls - filtration 1. Filtrate looks cloudy 2. Filter paper is clogged

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1. Wrong type of filter paper was used. Used filter paper of a wrong diamer, both the liquid and solid particles creep around the edges

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9.1) Why would a Hirsch funnel be more effective than a Buchner funnel for a small-scale vacuum filtration

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Hirsch funnels come in smaller sizes than Buchner funnels and the plate that holds the filter plate has a smaller surface area in Hirsch funnels. There is inherently less loss using a Hirsch funnel for small-scale filtrations.

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9.3) Why should a hot recrystallization solution be filtered by gravity rather than by vaccum filtration?

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The solution would cool too rapidly during vacuum filtration and crystallize prematurely.

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9.4) Explain why the filter flask can become quite cold to the touch during a vaccum filtration

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- because the filtered solution evaporates the solvent used in recrystallization is a volatile organic compound, it will evaporate rapidly during the filtration process. Evaporation is a cooling process.

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9.5) Why must the seal be broken in a vaccum filtration before the flow of water to a water aspirator is turned off?

answer

If the seal is not broken in a vacuum filtration before water flow is turned off, water may be sucked back into the filter flask.

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9.6) To perform each of the following tasks which type of filtration apparatus would you use? (a) Remove 0.3g of solid impurities from 5 mL of a liquid (b) Collect crystals obtained from recrystallizing an organic solid from 20 mL of solvent

answer

a) Pasteur pipet packed with glass wool and a filter aid (e.g. Celite); b) Hirsch funnel, adapter, filter paper, and vacuum filtration flask.

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Recrystallization steps

answer

1. Dissolved in hot water solvent 2. Slowly cooled to RT 3. Cooled in ice-water bath to decrease solubility 4. Filtered and Recovered in Hirsch funnel 5. Wash filtrate with cold solvent 6. The crystal dried and was weighed, % recovery

question

Track the impurities 1. Dissolved in hot water solvent 2. Slowly cooled to RT 3. Cooled in ice-water bath to decrease solubility 4. Filtered and Recovered in Hirsch funnel 5. Wash filtrate with cold solvent 6. The crystal dried and was weighed, % recovery

answer

1. No desired product. Hot water contains impurities. 2+3) The desired purified product was contained in the new crystal and impurities remained in the solution. They did not fit well in the new crystal lattice. The new crystals were much layer. 4+5) The desired product was the precipitate (on top of filter paper) and the impurities that remained in the filtrate.

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melting point technique

answer

1. Introduce dry sample into capillary tube, sealed at one end 2. place tube in mp apparatus 3. adjust the rate of heating so that the temperature rises at a moderate rate

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Sources of confusion and common pitfalls for mp 1. Mp ranges aren't consistent 2. sample disappeared while measuring mp 3. sample turned brown and didn't melt

answer

1. a. Rate of heating - heating really fast (more than 1-2°C per minute) may lead to observed mp range that is higher than the correct one. b. too much sample 2. sample sublimination - solid to gas without passing the liquid phses. 3. decompose instead of melting or decompose and melt. This is indicated as the mp and d 186°C d

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Sources of confusion and common pitfalls for recrystaillization 1. Compound doesn't crystallize 2. Solution turned cloudy + oily rather than crystalline

answer

1. Too much solvent -- evaporate solvent and start over. SOmetimes its superstaturated and requires a kinetic barier 2. Too little solvent -- compound is insoluble at too high a tmeperature. Melting point near the boiling point of the solvent A compound oils ut, impurities dissolve in the oil and impede recrystillization

question

14.1) A student performs two mp determinations on a crystalline product. In one, the capillary tube contains a sample that is about 1-2 mm in heat and the mp range is 141-142. In the other, the sample height is 4-5mm, and the mp range is found to be 141°-145.° Explain the broader mp range observed for the second sample. The reported melting point range for the compound is 143°C.

answer

- too much sample was used In the second melting point determination, using a sample height of 4-5 mm, it took longer for the larger sample to melt. Because the temperature was rising at a steady rate, the second melting range was greater; its high end was 2 above the actual melting point

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14.2) Another student reports an mp range of 136-138°C for the compound and mentions that the rate of heating was about 12° per minute. NMR analysis of this studen's product does not reveal any impurities.

answer

- heating too fast. heating faster than 1-2°C per minute may lead to an observed mp that is higher than the correct one. Heating a melting-point sample so the temperature rises too fast can cause either a high or low melting range. A low melting point range is caused by the inability of the liquid in the thermometer bulb to heat quickly enough to reflect the real temperature of the sample.

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14.3) A compound melts at 120-122 on one appartus and at 128-129 on another. Unfortunately neither apparatus is calibrated. How might you check the identity of the sample without calirbrating either apparatus?

answer

A mixed melting point experiment could be used to determine whether the two compounds have the same identity. Alternatively, the melting point of both compounds could be measured on one apparatus to learn if they are likely to have the same identity without calibrating either melting-point apparatus.

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15.2) The solubility of a compound is 59g/100 mL In boiling MeOH, and 30g/100 mL in cold MeOH whereas its solubility in water is 7.2g/100 mL at 95 C and 0.22 g per 100 mL at 2°C. Which solvent would be better for recrystiliation

answer

- Water is the better recrystallization solvent

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15.3) Explain how the rate of crystal growth can affect the purity of a recrystilized compound

answer

- the slower the cooling, the larger and max selectivity and the purer the crys -If a compound is crystallized too quickly, impurities can be trapped in the crystal lattice

question

15.4) In what circumstances it it necessary to filter a hot recrys. sln

answer

A filtration of the hot mixture is necessary if an insoluble impurity is present or if activated carbon is used to remove colored impurities.

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15.5) Why should a hot recrystilization solution be filtered by gravity rather than by vaccum

answer

A hot recrystallization solution should be filtered by gravity to avoid premature crystallization. Vacuum filtration cools the solution to a much greater extent and produces excessive crystallization of the desired compound during the filtration. Some of the solid may be lost in the bottom portion of the Buchner funnel.

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extraction

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- sep. based on solubility - process of one phase to another - two types: 1. solid-liquid: solute transfer from solid to liquid phase ex) tea leaves--> caffeine, vanilla-->vanillin, poppy flowers --> morphine 2. liquid-liquid: solute transfer from one liquid phase to another - neutral - pH sensitive

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liquid-liquid extraction

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- sep. based on solubility - transferring of solute from one liquid phase to another - the two phases/extraction solvents are immiscible - governed by IMF 1. Neutral - neutral extraction solvent using water 2. pH sensitive - using HCl, NaOH, 5% NaHCO3

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Good extraction solvents are _________

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Immiscible. Between water and: diethyl ether ethyl acetate DCM

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Example of separating two species Desired product = Organic (Square) Unwanted product = Inorganic biproduct (circle)

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1. START: Organic phase + 2 products 2. ADD aqeous phase (water) 3. TRANSFER to sep funnel 4. MIX layers, the products settle in the phase they are most stable in Top layer - Organic Bottom layer - Aqeous 5. Physically separate the layers. Collect layers in a labeled container (Run bottom layer first): Container 1 - Aqeous layer with unwanted byproduct Container 2 - Organic layer with desired product 6. Column Chromotography

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Before extraction - things to think about 1. Solubilities of compounds, 2. Denstities and miscibilities of solvents 3. pH sensitive functional groups

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1. Choose the right extraction solvent. Both compounds should be able to dissolve/soluble in the solvent. 2. a. Density: Keep track of which layer is which using density. Usually organic layer is at the top, less dense than water, and water is the bottom. DCM is at the bottom, it is an exception b. Miscibility tells about the success of the extraction. The solvents must be immiscible with one another. i.e. Ethanol is a solvent that is miscible with water you would not choose it. 3. pH sensitive FG can change the solubilities of compounds

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two types of liquid-liquid extraction: 1. neutral 2. pH-sensitive

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1. Neutral: Organic phase and water = No change in pH necessary to achieve seperation 2. pH = some oranic FG are pH sensitive, their solubility is altered by pH pH Sensitive FG = RCOOH, phenols, alcohols, amines

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Example of Neutral Extraction + recovery 1. Both compounds soluble in MeCN 2. Product insoluble in H2O 3. Byproduct soluble in H2O

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1. Remove MeCN - misscible with water 2. Add Et2O (diethyl ether) - introduces the organic phase dissolvents both compounds 3. Add water - introduces the organic phase 4. Mix + separate layers Top layer - desirered organic product Bottom layer - unwanted byproduct 5. Make sure you label collection flask 6. Dry the organic layer - wash with brine, dry over MgSO4 7. Recover the organic solvent by evaporation

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phenols FG for extraction

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- special type of alcohol, weaker than RCOOH, needs stronger base like NaOH (not 5% NaHCO3) for pH change

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how soluble compounds are with the same FG depends on

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the size of the R group The larger the R the more insignificant/weaker the FG becomes

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Example of pH sensitive - RCOOH Extract + Recover RCOOH + NaHCO3 --> COO- Na+ + H2CO3 1. Both compounds are soluble in CH2Cl 2. Both compounds are insoluble in water 3. Available chem: CH2Cl, water, 5% NaHCO3, HCl

answer

1. Dissolve both compounds in CH2Cl 2. Add 5% NaHCO3 --> introduces aq. basic layer - This deprotenates the acetic acid and converts it into its basic form (carboxylate) that is soluble in aq layer - form CO2, we observe gas evolution 3. Shake and separate 4. Recover by adding HCl - neutralizes the NaHCO3 - observe gas evolution - back to neutral, water insoluble, precipiatates out of solution 5. Recover by filtration

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Example of pH sensitive extraction - phenolRPh-OH + NaOH --> RPhO- Na+ + H2O

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- weaker acid than RCOOH - needs stronger base

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Example of pH sensitive - Amine R-NH2 (lone pair) + HCl --> RNH3+ + Cl- Extract and Recover

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1. Dissolve in DCM - organic phase 2. Add HCl - aq phase - NH2 donates e- and accepts H+ becomes acidic 3. Amine will state in the aqeous layer 4. Recover by adding NaHCO3 - takes the proton off and becomes neutral amine

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Extraction procedure protocol!! Trans-stilbene and Methyl 1-4 aminobenzoate **Remember to use a purification method after extraction and recovery

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1. Prepare 200 mg binary mix in test tube and add to microfuge tube 2. Add HCl - Aqeous layer with Aminobenzoate - (R-NH2) in methyl-4 amino benzoate protenates and converts to (R-NH3+), conjugatic acid, which increases its solubility in the aqueous layer 3. Add T-butyl methyl ether - Organic layer with transtilbene 4. Seperate layers Container 1 - aq. layer with aminobenzoate Container 2 - organic layer trans-stilbene 5. Recover the aqeous layer by saturating it with 5% NaHCO3, removes proton and converts it back to its neutral amine form 6. Recover the organic layer by wash it with brine and running it through an MgSO4 drying column, and evaporating organic solvent under vaccum 7. Determine a percent recovery for each compound 8. Attempt to perfrom recrystilization 9. Determine purity of compounds using mp and TLC

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Common pitfalls - extraction 1. Throwing away desired product 2. Too much drying agent

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1. Make sure to keep both layers just in case 2. Too much drying agent can cause a loss of product by its absorption. Add more organic solvent to minimize the loss of product **You know when the organic layer is dry when the drying agent is not clumpy anymore

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10.1) An extraction procedure specifies that an aq sln containing dissolved organic material be extracted with 10 mL of diethyl ether. A student removes the lower layer after the first extraction and adds the 10 mL portion of ether to the upper layer. After shaking the funnel, the student observes only one liquid phase with no interphase

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Diethyl ether is less dense than water, so ether is the upper layer in the extraction, which means the student removed the aqueous layer. Adding ether to the remaining ether layer will produce only one phase.

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10.2) A crude nonacidic product mix dissolved in diethyl ether contains acetic acid. Describe an extraction protocol that can be used to remove the acetic acid from ether.

answer

See flashcards for RCOOH extraction

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10.3) What precautions should be observed when an aqeous sodium carbonate solution is used to extract an organic solution containing traces of acid

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- gas evolution

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10.4) WHen two layers form during a pteroleum ether/water extraction, what would be to tell which layer is which if the desniteis were not available

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- if water forms a glob - its organic layer - if water dissolves - its aqeous layer

question

11.1) Which would be more of an effective drying agent CaCl2 or CaCl2 * 6H2O. Explain

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CaCl2 is better because it has more of a capacity to suck up water and form a hydrate. CaCL2 ° 6H20 already has a hydrate, its NOT dry, would not able to suck up the water

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11.2) a) Too little dtying agent? b) Too much drying agent

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If too little drying agent us used, the organic layer will still contain water, which may interfere with the subsequent use of the liquid (e.g. evaporation) and act as an impurity. b) Using too much drying agent may cause the product to be adsorbed onto the surface of the drying agent, resulting in significant loss and a reduced yield

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11.4) KOH Is an excellent driyng agent for some organic compounds. Wout it be better for RCOOH or RNH2? Why?

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KOH is a basic drying agent it is useful for sucking up amines

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15.1) A good recrystallization solvent is

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good recrystallization solvent is one in which the compound to be recrystallized is soluble when the solvent is near the boiling point and only sparingly soluble when the solvent is at room temperature or below.

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Simple vs. fractional physical distillation

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boils all the way u the column to the top to hit the distillation head vs. the copper wire gives it more surface area, sort of "filtering" through the copper wire

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How does IMF influece bp?

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Strong IMF lead to higher bp - more energy required to break the bonds, so that the liquid can transition in to the vapor phase. For example, alocohol such as n-butanol has a higher IMF then diethyl ether (similar MW) because H-bonds are stronger than dipole-dipole forces. Generally,, molecules with higher MW have higher surface area, which leads to higher bp. Highly branched structures are an exception

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Safety Information in Lab

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1. Use MgSO4 instead of CaCl2 for drying liquids 2. Use SiC boiling chips instead of boiling sticks 3. Never heat in a closed system. 4. Do not dry solvents in disposeable test tubes, use PYREX tubes 5. Prepare silica slurry in hood, dry silica is an inhalation hazard 6. Be careful when acidifying the bicarb solution, CO2 will form

question

frit vs. filter paper

answer

- filter paper is on top of the frit - allows for

1 of

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question

Chromatography

answer

- sep. mixture (ions/compounds) between a stationary phase and a mobile phase. - the 2 phases can be solid-liquid, liquid-liquid, gas-liquid - types: TLC, Column, GC

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TLC

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Distillation

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- separation. of immiscible liquids in a mixture based on bp - factors that influence boiling point: IMF and molecular weight - two types: simple and fractional

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Chromatography basics - long version

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TLC overview

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TLC basics

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- stationary phase = silica absorbent - mobile phase = develoing solvent - component spot = mxiture

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free state

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- completely dissolved in the liquid or gaseous mobile phase

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absorbed state

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- stuck on the surface of the stationary phase

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The equilibrium of the mixture between free-absorb state equilibrium between the mobile phase and the stationary phase

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Intermolecular Attractions (IMF)

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- determine how far a compound moves on TLC 1. H-bonding 2. Dipole-dipole 3. Induced dipole/Van der Waals

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H-bonding

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Dipole-dipole interactions

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- electrostatic attractions between polar molecules - acetone, acetonitrile, dimethyl sulfoxide

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induced dipole interactions/Van der Waals

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Silica in TLC

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Developing solvents in TLC

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Distance Travelled by compounds (general)

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retention factor

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- distance traveled by compound/distance travelled by solvent front The larger an Rf of a compound, the farther the distance it travels on the TLC plate

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polarity

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- sep of charge, dipoles - results in diff of electronegativity

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polar molecule

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- has a permanent dipole e.g: water, acetic acid, ethanol (water is bent e- are being drawn towards the oxygen - In general, O, N halogens increase polarity but it depends on the shape

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nonpolar molecule

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- does not have a permanent dipole - hexane, toulene

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Uses of TLC

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Hypothesis for TLC: Is a green spot a mix of two or more spots

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- purity question - # of components in a green spot Conclusion: - We see a little residue of the green spot - The green spot contains at least two spots

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Is the yellow spot in the green marker Sunset yellow, or a different yellow dye?

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Prelab 1 - Hypothesis and Design TLC experiment

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Postlab 1: Can you combine all 3 dyes that will allow separation for at least one of the dyes?

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stationary phase

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- usually polar, silica gel most common

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mobile phase

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- pure solvent or a mixture of solvents - depends on the polarities of the compounds in the mixture being separated - mostly nonvolatile, since volatility can lead to loss of sample by evaporation

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Some mixtures of dyes contain amines or carboyxilic acids that hydrogen bond to the plate and produce really low RF values, what solvent system should you use?

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- Acetic acid for carboxylic acids - Ammonium hydroxide for amines - Make sure that addditives are miscible for the developing solvent

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Answers to pitfalls of TLC 1. Solid doesn't stick 2. Don't see any spots 3. Spots are at the solvent front 4. RF values aren't the same to literature value 5. RF values are too similar (+/- 0.05

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question

answer

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Chromotography elution sequence by FG (increasing polarity)

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Chromotography mobile phase/solvent polarities

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1. Hexane 2. Toluene 3. Methyl-t-butyl ether (RO-fork) 4. DCM 5. Ethyl acetate 6. Acetone 7. Ethanol, Methanol 8. Acetonitrile 9. Water 10. Acetic acid

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- The more optimal solvent is the diethyl ether because it creates a RF value that is closer to 0.5 with because that has intermediate polarity -

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Why don't idenical RF values necessarily mean the same compound?

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Purpose Lab 1 - TLC

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- test hypothesis - to identify dye components in markers (identification) - to determine number of dyes in marker (purity)

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Purpose Lab 2 - Distillation and Chromotography

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- seperate binary mixture of ethyl acetate and n-butyl acetate and dye solution using simple and fractional distillation, LC, GC. - The purities of our samples will be assessed using GC and TLC

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Purpose Lab 3 - Who Else Has My compound

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question

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- Cyclohexanol has the lower RF value because it is more polar. It interacts more with the stationary

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Liquid/Column Chromography

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TLC vs. Column Chromotography (4)

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Analyzing fractions b yTLC-which compound will elute from the column first?

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Observation: - The green spot is further up the plate on TLC, means it is less polar Conclusion - Green dot will elute first on column chromography

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Silica Column Chromatogrpahy protocol

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question

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Dry silica poses an ___________________________. Preprare silica slurry in the fume hood

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inhalation hazard

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What is silica slurry

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mixture of silica and elution solvent

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How do you clean up TLC chips/residual silica from the benchtop?

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Use damp paper towel to wipe benchtop clean from residual silica gel

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Dispose of used TLC plates and silica in___________

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solid waste

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Why is it imp. ohat the level of the elution solvent not drop below the absorbent?

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- If the solvent level falls below the top of the absorbent, it can become dry and fall from the column. - Compromise the effectiveness of the column

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What effect will the following factors have on liquid chromatographic separation? a) Not clamped in vertical position b) collection of large fractions c) slow flow rate of mobile phase

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elution solvent

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Gas Chromatography (GC)/Gas-liquid chromatography

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Uses of GC

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1. Determine purity 2. Determine fractions of components present in a sample 3. Indentity: identify unknown samples - ONLY if known standards are avaialble

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GC vs. TLC vs. LC

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GC basics

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GC - sources of confusion and common pitfalls 1. No peaks appear - clogged syringe 2. overlapping peaks, poor separation a. overloading column b. pushing syringe slowly 3. More peaks then expected

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GC: How can the following factors effect retention time? 1. High temperature of column 2. Too fast Mobile phase (carrier gas) flow rate

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Pemanent pipoles and bp

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- Molecules with permanent dipoles can show higher bp - Higher bp than molecules of the same molecular weight - Propane just ha van der waals - di-methyl ether has higher bp

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Alcohols and bp

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- Alcohols have high bp when compared to structurally similar compounds - due to H-bonding. Recall H-donor and H-acceptor

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Molecular Weight and bp

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Increased MW - larger surface area - more IMF - more IMF - higher bp

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Isomers and bp

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Distillation basics

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Simple vs fractional

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Distillation Setup 1. Thermometer probe 2. Boiling chips... 3. Tight rubber seals 4. Ice bath for receiving vial 5. Don't distill to dryness(3 reasons)

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question

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- The thermometer is reading much lower than the expected bp - This can be due to the fact that the vapor has not yet reached the thermometer bulb. - Or the thermometer was positioned improperly.

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Solubility

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Melting points

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Recrystillization

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percent recovery

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mass of crystallized sample/mass of crude sample x 100%

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Solid separated from solid

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1. TLC 2. Column Chromography 3. Extraction

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Solid separated from liquid

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1. Filtration 2. Evaporation

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Liquid separated from liquid

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- immiscible liquids allow for physical seperation 1. Simple Distillation 2. Fractional Distillation

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answer

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Structure-solubility (Structure--> Polarity) Given the strcture of your identified compound, do the solubility

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question

answer

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Solvation is spontaneous when

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- solvation = solute is completely separated and surrounded by solvent molecules. ?G < 0 § Recall ?G = ?H - T?S § We want a large negative ?H § We want a large, positive ?S

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Enthalpy of Solvation: When dissolving solute X into solvent Y, three processes occur:

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E.g. "Dissolve" hexanes in water.

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Entropy of solvation

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Filtration

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Confusion and common pitfalls - filtration 1. Filtrate looks cloudy 2. Filter paper is clogged

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1. Wrong type of filter paper was used. Used filter paper of a wrong diamer, both the liquid and solid particles creep around the edges

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9.1) Why would a Hirsch funnel be more effective than a Buchner funnel for a small-scale vacuum filtration

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9.3) Why should a hot recrystallization solution be filtered by gravity rather than by vaccum filtration?

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The solution would cool too rapidly during vacuum filtration and crystallize prematurely.

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9.4) Explain why the filter flask can become quite cold to the touch during a vaccum filtration

answer

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9.5) Why must the seal be broken in a vaccum filtration before the flow of water to a water aspirator is turned off?

answer

If the seal is not broken in a vacuum filtration before water flow is turned off, water may be sucked back into the filter flask.

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a) Pasteur pipet packed with glass wool and a filter aid (e.g. Celite); b) Hirsch funnel, adapter, filter paper, and vacuum filtration flask.

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Recrystallization steps

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question

answer

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melting point technique

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1. Introduce dry sample into capillary tube, sealed at one end 2. place tube in mp apparatus 3. adjust the rate of heating so that the temperature rises at a moderate rate

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Sources of confusion and common pitfalls for mp 1. Mp ranges aren't consistent 2. sample disappeared while measuring mp 3. sample turned brown and didn't melt

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question

Sources of confusion and common pitfalls for recrystaillization 1. Compound doesn't crystallize 2. Solution turned cloudy + oily rather than crystalline

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question

answer

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answer

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answer

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- Water is the better recrystallization solvent

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15.3) Explain how the rate of crystal growth can affect the purity of a recrystilized compound

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- the slower the cooling, the larger and max selectivity and the purer the crys -If a compound is crystallized too quickly, impurities can be trapped in the crystal lattice

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15.4) In what circumstances it it necessary to filter a hot recrys. sln

answer

A filtration of the hot mixture is necessary if an insoluble impurity is present or if activated carbon is used to remove colored impurities.

question

15.5) Why should a hot recrystilization solution be filtered by gravity rather than by vaccum

answer

question

extraction

answer

question

liquid-liquid extraction

answer

question

Good extraction solvents are _________

answer

Immiscible. Between water and: diethyl ether ethyl acetate DCM

question

Example of separating two species Desired product = Organic (Square) Unwanted product = Inorganic biproduct (circle)

answer

question

Before extraction - things to think about 1. Solubilities of compounds, 2. Denstities and miscibilities of solvents 3. pH sensitive functional groups

answer

question

two types of liquid-liquid extraction: 1. neutral 2. pH-sensitive

answer

question

Example of Neutral Extraction + recovery 1. Both compounds soluble in MeCN 2. Product insoluble in H2O 3. Byproduct soluble in H2O

answer

question

phenols FG for extraction

answer

- special type of alcohol, weaker than RCOOH, needs stronger base like NaOH (not 5% NaHCO3) for pH change

question

how soluble compounds are with the same FG depends on

answer

the size of the R group The larger the R the more insignificant/weaker the FG becomes

question

answer

question

Example of pH sensitive extraction - phenolRPh-OH + NaOH --> RPhO- Na+ + H2O

answer

- weaker acid than RCOOH - needs stronger base

question

Example of pH sensitive - Amine R-NH2 (lone pair) + HCl --> RNH3+ + Cl- Extract and Recover

answer

question

Extraction procedure protocol!! Trans-stilbene and Methyl 1-4 aminobenzoate **Remember to use a purification method after extraction and recovery

answer

question

Common pitfalls - extraction 1. Throwing away desired product 2. Too much drying agent

answer

question

answer

question

10.2) A crude nonacidic product mix dissolved in diethyl ether contains acetic acid. Describe an extraction protocol that can be used to remove the acetic acid from ether.

answer

See flashcards for RCOOH extraction

question

10.3) What precautions should be observed when an aqeous sodium carbonate solution is used to extract an organic solution containing traces of acid

answer

- gas evolution

question

10.4) WHen two layers form during a pteroleum ether/water extraction, what would be to tell which layer is which if the desniteis were not available

answer

- if water forms a glob - its organic layer - if water dissolves - its aqeous layer

question

11.1) Which would be more of an effective drying agent CaCl2 or CaCl2 * 6H2O. Explain

answer

CaCl2 is better because it has more of a capacity to suck up water and form a hydrate. CaCL2 ° 6H20 already has a hydrate, its NOT dry, would not able to suck up the water

question

11.2) a) Too little dtying agent? b) Too much drying agent

answer

question

11.4) KOH Is an excellent driyng agent for some organic compounds. Wout it be better for RCOOH or RNH2? Why?

answer

KOH is a basic drying agent it is useful for sucking up amines

question

15.1) A good recrystallization solvent is

answer

question

Simple vs. fractional physical distillation

answer

boils all the way u the column to the top to hit the distillation head vs. the copper wire gives it more surface area, sort of "filtering" through the copper wire

question

How does IMF influece bp?

answer

question

Safety Information in Lab

answer

question

frit vs. filter paper

answer

- filter paper is on top of the frit - allows for

Chem 29a – Exam 1 – Flashcards

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