TLC – Flashcard
Unlock all answers in this set
Unlock answersquestion
Sketch of apparatus
answer
Fig 8.9 and Fig 1 from Handout
question
Uses for this TLC experiment (know more from book)
answer
a way to tell if compound is pure a way to see what compounds are in a sample by comparison monitor progress of a reaction determine identity of two substances determine the appropriate conditions for a column chromatography separation to monitor column chromatography
question
analytic technique vs. purifying technique
answer
Melting Point and TLC are analytic techniques Recrystallization and Distillation are purifying techniques
question
TLC is a commonly used tool to
answer
observe separation of biomolecules in biochemistry identifying illicit drugs or dyes in fibers in forensic chemistry analyzing components of medicinal plants
question
General Chromatography Technique
answer
based on components of a mixture being differentiated by exposure by two competing phases
question
Tools for the Experiment
answer
1. TLC plates 2. Capillary tubes 3. Developing chamber 4. Standards 5. Reference solution 6. Unknown 7. Solvents
question
TLC plates
answer
plastic plates covered with an adsorbent material (stationary phase) The material we will be using is silica but alumina is another common adsorbent
question
For TLC, a binder like calcium sulfate added to
answer
hold adsorbents on the plate
question
You must know what happens if you silica for very nonpolar compounds and if you use alumina for very polar compounds
answer
Silica: Samples travel all the way too the top of the plate or will travel with the solvent front
question
Capillary tubes (open at both ends)
answer
called spotters
question
Developing chamber
answer
a bottle with a top with filter paper inside
question
Standards
answer
Acetaminophen, Aspirin, Caffeine (solids)
question
Reference solution
answer
a mixture of acetaminophen, aspirin, and caffeine
question
Unknowns
answer
anacin, excedrin, tylenol
question
Solvents
answer
methylene chloride for dissolving unknowns - eluent (mobile phase) ethyl acetate
question
What are typical characteristics for solvents used as eluents? Why?
answer
having low boiling points that allow them to be easily evaporated and low viscosities that allow them to migrate rapidly
question
Solvent rises up the TLC plate by capillary action (rising up; opposite gravity) which is the ability of a
answer
liquid filling the spaces b/w the solid particles in opposition to the gravitational force
question
Extra Considerations for TLC eluent
answer
Mixture to be separated must be soluble in eluent - if not the spotted sample will stay on the spotting line Intermolecular attractions between eluent and compounds determine - solubility of compounds in mobile phase - like dissolves like
question
Using eluent with too high a polarity with respect to samples
answer
all spots will move to top of TLC plate ; more polar an eluent the faster a compound moves
question
Using eluent with too low a polarity with respect to samples
answer
all spots will stay towards the bottom of the TLC plate;
question
What do polar compounds require
answer
Require polar eluents to overcome attractions to the adsorbent
question
If you use a mixture of solvent, the pair of solvents MUST be
answer
miscible
question
Stages of Experiment
answer
Spotting Developing Visualizing
question
Spotting Stage (Preparation)
answer
Draw a line using a pencil 1 cm from the bottom of the TLC plate (spotting line) Draw a line 5 mm from the top of the plate (solvent front). Make sure that the lines are drawn lightly. Do not flake off adsorbent.
question
Why shouldn't you flake off adsorbent?
answer
If flaked a gap might form that may stop the flow of the eluent or cause eluent to run crookedly
question
Why is pencil always used to mark a plate and why is ink never used to mark the plate?
answer
It will chromatograph like any other organic compound and give flawed results; Because the graphite (carbon) is inert
question
Spotting
answer
Use Capillary tubes to take sample Then lightly touch it to the TLC plate to apply a spot of sample onto the spotting line (1 cm line). Be careful again not to flake off adsorbent (spotter has sharp edges) Label spots with a pencil
question
To avoid streaking spots
answer
Try to make spots small and compact - by touching lightly on the TLC plate Also allow the solvent to dry completely between application to the same spot. Otherwise the spot will become too large Sample should not be too concentrated
question
What problem could streaking spots lead to?
answer
Streaking may lead to the overlap of two or more compounds, thus skewing results
question
If a sample contains many components then spots may just run together and
answer
appear as a streak
question
Additional points: Don't put spots too
answer
close together on spotting line (1 cm between each spot) - they may bleed into each other close to the edge - leads to inaccurate RFs ( not enough solvent or adsorbent surrounding spot - unequal forces for each spot)
question
When it is not balanced and it goes up crooked how do you know results are accurate?
answer
you don't
question
Note: the sample needs to be concentrated enough-otherwise
answer
no spots will be visible during visualization stage
question
Developing Stage
answer
Place TLC plate in a developing chamber that has 4mm high level of ethyl acetate. Make sure solvent level is below spotting line so that solvent does not dissolve samples on the spotting line
question
Why is it suggested that one uses tweezers to place TLC plate in developing chamber? !!!
answer
Don't want surface of hand touching the top of TLC plate (we will handle by holding by sides) gloves still have chemicals/oils you don't want to get on TLC plates
question
The RF for a compound is a constant from one experiment to the next providing the following are constant:
answer
- solvent system - adsorbent (silica or aluminum) - thickness of adsorbent (difficult to control) - amount of material spotted (difficult to control) - temperature (difficult to control) Keeping the last three constant is difficult so relative RFs are used
question
What can you do if two different compounds have the same RF in one eluent?!!!!
answer
changing the stationary or mobile phase will usually effect their separation
question
Two samples of the same compounds will
answer
always have the same relative RF in every eluent
question
You should know the relative polarity of functional groups
answer
alkane, alkyl halides, alkenes, dienes, aromatic hydrocarbons, ethers, esters, ketones, aldehydes, amines, alcohols, phenols, carboxylic acids, sulfonic acids.
question
Column chromatography (purifying technique)
answer
used for separation and purification of both solids and liquids when carrying out microscale experiments
question
What is the difference of how column chromatography works vs. thin-layer chromatography? Where do you put solvent?
answer
column: mixture loaded on top (solvent poured on top) TLC: mixture loaded on bottom : solvent is reservoir on the bottom
question
Separation begins with a relatively low polarity solvent. Why?
answer
to allow compounds to adsorb to stationary phases then polarity of solvent is slowly increased to desorb compounds and allow compounds to move with the mobile phase
question
What may occur if polarity is changed too suddenly?
answer
cause heat evolution as the alumina or silica gel adsorbs the new solvent
question
Stages
answer
Packing column Adding sample Eluting column
question
What is the problem of collecting very large fractions?
answer
column chromatography is a length process; may get more than one compound in a particular fraction
question
Isolating Separated Compounds; What is TLC's role in this stage
answer
Spotting some of the starting material or the product on the TLC plate as a standard will help in the identification
question
Limitation for TLC and column chromatography-
answer
cannot be used on volatile compounds having boiling points below 150 degrees C
question
Experiment- 1st TLC plate
answer
spot 3 standards -develop with ethyl acetate -visualize using UV - trace spots with pencil and take measurements
question
Experiment- 2nd TLC plate
answer
spot unknown (note #) and reference solution - develop using ethyl acetate - visualize using UV - trace spots with pencil and take measurements
question
From 1st TLC plate, you should be able to identify the spots of the reference solution on the 2nd plate
answer
From reference solution you should be able to identify your unknown using the information on your handout
question
Why should you make sure the bottom of the TLC plate is level with bottom of developing chamber
answer
if not then solvent will rise up unevenly on TLC plate
question
What effect might moving or jostling the development chamber during the developing stage have on how it rises?evenly? unevenly?!!!
answer
the eluent will rise up unevenly
question
Make sure top is placed back on so
answer
solvent does not evaporate
question
Evaporation of solvent during this stage causes the !!!
answer
development of the TLC plates to be messy and ineffective !!!
question
Before developing a new TLC plate
answer
add solvent to correct level
question
Make sure solvent does not rise up past solvent front (5 mm line) Why?
answer
if it does, spots in visualization stage will be difficult to evaluate; RF would be too high
question
Make sure filter paper is in the dev. chamber
answer
to saturate dev. chamber w/ solvent vapors
question
Avoid leaning plate against the filter paper because!!
answer
Eluent on the filter paper can absorbed by adsorbent and interfere with ascending eluent. Put filter paper on one side plate on another
question
Where should the solvent come from?
answer
bottom
question
This extra eluent may prevent the compounds travelling from
answer
moving up the plate in a straight line
question
Leaning
answer
have wetness come from sides since filter paper is wet
question
Visualizing stage
answer
Dry the TLC plate Since the solvents and solutions you are using are colorless, you need to use visualization technique
question
Two common ways to visualize plates
answer
1. UV light-used for conjugated compounds (don't shine on eyes; shine on plate) - Shine on TLC spots and you will see purple spots, take pencil and trace the spots. Any place where there isn't spots it would appear florescent green. 2. I2 vapor - for most compounds (usually with double bonds)
question
Other methods
answer
stains, charring are associated with the presence of a particular functional group.
question
During development, samples travel along with solvent up the TLC plat. Each pure compound shows up as
answer
one dev. spot directly above where it was spotted on the spotting line.
question
Does a single spot on a TLC spot guarantee a single substance. Why or why not?!!!
answer
No ; pure' must see 1 spot but if its 1 spot it is not necessary pure.
question
The more polar a compound, the ____ farther it travels up a TLC plate
answer
less
question
Retardation Factor (RF)
answer
distance sample traveled/distance solvent traveled
question
The more polar the _________the RF
answer
Smaller
question
TLC is typically used to determine the correct adsorbent and solvent for column chromatography. Why is that possible?
answer
TLC and column chromatography use some absorbents or TLC and column chromatography both use silica and alumina
question
TLC can be used to monitor column chromatography. What specifically does the TLC do to achieve this purpose?
answer
Analyze fractions of column chromatography
question
What advantage is obtained when a TLC is used to determine the number of components in a mixture?
answer
It is sensitive, fast simple, and inexpensive analytical technique
question
Which use of TLC allows for one to determine the optimum time to halt a reaction?
answer
Monitoring progress of a reaction
question
You spot a sample that contains a mixture of 3 compounds of varying polarities on a TLC plate and develop it. Assuming that all other factors (adsorbent, technique) were appropriate, what kind of solvent would cause all the spots to congregate in the following manner when visualized under UV light?
answer
spots congregated at top of plate A solvent with too high a polarity in respect to the mixtures spots congregated at bottom of plate a solvent with too low a polarity in respect to the mixtures
question
You spot two very polar compounds on a TLC plate and develop it in order to distinguish the two compounds. Which adsorbent would most likely produce the result where all spots stay at the bottom of the plate when visualized under UV light if all other factors (solvent, technique) are accounted for?
answer
A solvent that is too low in polarity
question
During the development stage of the TLC experiment, a student accidentally left the top off the developing chamber and the eluent evaporated. Why is this a problem?
answer
Because the developing chamber wouldn't be saturated with vapors and the TLC plate would become ineffective and messy
question
A student develops a TLC plate and realizes during the visualization stage that the eluent had dissolved the samples during the development stage. How could the student have prevented this situation?
answer
By making sure the eluent is below the spotting line
question
During a TLC experiment, a student used only 0.5 mL of solvent to dissolve a solid unknown instead of the necessary 1.5 mL before spotting the unknown on the TLC plate. What is the likely observation on the developed plate when it is visualized?
answer
There would be too much sample during spotting which would cause poor separation of the compound and it would be smeared
question
You take a TLC of a compound and you find that the Rf of the compound is much higher than expected. What action would have produced the above result?
answer
solvent rose up past solvent front
question
Why is it important that solvents for TLC (eluents) have low boiling points?
answer
So they can saturate the developing chamber with their vapors effectively
question
Why is it important that solvents for TLC (eluents) have low viscosities?
answer
Allows them to migrate rapidly
question
What advantage is obtained when a TLC is used to determine the number of components in a mixture?
answer
Aids in planning further analytical and separation steps
question
What is the reason a TLC would be taken after one carries out a distillation?
answer
To examine effectiveness of purification
question
One of the uses of TLC is to monitor the progress of a reaction. What kind of info does TLC provide in this instance?
answer
When to halt rxn and the effect of changing temp, concentrations and solvent w/o having to isolate product
question
Why can TLC be use to determine the appropriate conditions for a column chromatographic separation?
answer
Adsorbents used for TLC same as adsorbents for column chromatography
question
One of the uses of TLC is to determine the number of components in a mixtures. What does this help in achieving?
answer
Aids in planning further analytical and separation step
question
How do you ensure that you make your spots on the spotting line of the TLC plate small and compact?
answer
By touching the capillary tube lightly on the TLC
question
A student accidentally lets the eluent run all the way to the top of the TLC plate...
answer
The spot in the visualization phase will be difficult to evaluate or the Rf will be too high
question
A student develops a TLC plate w/o placing filter paper in the developing chamber, why would this omission hinder the rising of the solvent up the TLC plate?
answer
Developing chamber would not be saturated with solvent vapors
question
Why is it important to remove the tlc plate from the developing chamber as soon as the solvent reaches the solvent front?
answer
Rf will be too high or spots at visualization stage will be difficult to evaluate
question
List 2 precautions you take while spotting
answer
Spots must be small and compact and sample must not be too concentrated
question
Why should one not stick the capillary tube directly into the reagent bottle
answer
To avoid contamination
question
What precaution does one have to take when marking a tlc plate?
answer
Not to flake off the absorbent
question
What might happen if you use silica as your absorbent to distinguish between a set of very polar solutes
answer
Samples will not migrate very far from spotting line
question
You can use 2 different visualization methods to visualize colorless substances, list which method is used for what kind of colorless compound
answer
Uv- conjugated and iodine for most compounds
question
What is important that the top of the chromatography jar be kept on while developing a tlc plate?
answer
So solvent does not evaporate
question
Why is it important not to dissolve the unknowns in more than 1 ml of methylene chloride?
answer
Samples will be too dilute to be spotted
question
Spotting precautions:
answer
Make sure lines are drawn lightly on plate so as to not flake off adsorbent If flaked, the solvent will not rise up the plate effectively Lightly touch capillary tubes to the plate when spotting Try to avoid spots from being smeary Try to make spots small and compact Sample should not be too concentrated
question
Developing precautions:
answer
Make sure solvent level is below spotting line So that the solvent does not dissolve the samples on the spotting line Make sure top is placed back on developing chamber So that solvent does not evaporate - evaporation of solvent during this stage causes the developing of the plates to be messy and ineffective (before developing a new plate, add solvent to correct level) Make sure solvent does not rise up past solvent front If it does, spots in visualization stage will be difficult to evaluate (RF would be to high) Make sure filter paper is in developing chamber The reason is to saturate developing chamber with solvent vapors
question
Visualizing precautions:
answer
Since the solvents and solutions you are using are colorless, you need to use visualization techniques: UV light - used for conjugated compounds I2 vapor - for most compounds (brown spot) During development, samples travel along with solvent up a TLC plate Each pure compound shows up as one developed spot directly above where it was spotted on the spotting line. More polar a compound, less is travels up a TLC plate
question
Why should bores of capillaries be small
answer
To avoid smearing of spots
question
What are typical characteristics for solvents used as eluents
answer
Low boiling points that allow them to be easily evaporated and low viscosities that allow them to migrate rapidly
question
Characteristics of solvents that allow them to migrate rapidly
answer
Low boiling point, low viscosity, high fluidity
question
Adsorbent materials and specific uses (what kinds of adsorbents would be better for what kind of substances)
answer
Alumina- when anhydrous, is more active - it will adsorb substances more strong; adsorbent of choice when the separation involves relatively nonpolar substances such as hydrocarbons, alkyl halides, ethers, aldehydes, and ketones Silica- less active- used to separate more polar substrates such as alcohols, carboxylic acids, and amines
question
Table 8.2 (order of solute migration)
answer
Fastest?slowest: Alkanes > alkyl halides > alkenes > dienes > aromatic hydrocarbons > aromatic halides > ethers > esters > ketones > aldehydes > amines > alcohols > phenols> carboxylic acids > sulfonic acids
question
Uses of TLC and how it is used - all 6
answer
To determine the number of components in a mixture To determine the identity of two substances If two substances spotted on the TLC plate give spots in identical locations, they may be identical. If the spots are not the same, the substances cannot be the same To monitor the progress of a reaction By sampling a reaction from time to time, you can watch reactants disappear and reappear using TLC. The optimum time to talk the reaction can be determined, and the effect of changing such variables as temperature, concentrations, and solvents can be followed without having to isolate the product To determine the effectiveness of a purification To determine the appropriate conditions for column chromatography Column chromatography is used to separate and purify larger quantities of material. The correct adsorbent and solvent used to carryout the chromatography can be determined rapidly by TLC (which would be unsatisfactory for the large quantities of material) To monitor column chromatography As the column chromatography is carried out and the solvent is collected in small flasks, the fractions can be analyzed by TLC to determine which ones have the desired components of mixture
question
Effects of using too polar or too nonpolar a solvent in respect to sample
answer
Too polar- all spots will ove to top of TLC plate Too nonpolar- all spots will stay towards the bottom of the TLC plate
