# Ochem 12A lab exam – Flashcards

## Unlock all answers in this set

question
The melting point is defined as
the temperature at which the solid and liquid phases exist in equilibrium with each other.
question
In the lab, what we measure and record as a melting point is actually a melting range.
True
question
The presence of impurities in a solid sample will tend to cause the melting point range to be
question
If a sample is not finely pulverized, there will be air spaces between the particles when the sample is loaded into the capillary tube. This will cause the melting point to be
question
Pulverized solid sample should be packed into the capillary tube to a depth of approximately
2 mm or 1/8th of an inch.
question
According to the textbook, heating a sample too fast will most likely cause the measured melting point to be
too low.
question
When determining the melting point of an unknown compound for the first time, it is good practice to prepare two capillary tubes, then
run the first tube at a high heating rate (to get a "ballpark" figure for the melting point), allow the instrument to cool 15 degrees, and then run the second tube at a heating rate of about 2 degrees per minute.
question
Part of the melting point experiment involves the technique of "mixed melting points." To show you understand how this works, answer the following question: You have two samples which you know to be pure benzoic acid (mp = 122-124) and pure 2-naphthol (mp = 123), but you don't know which is which. You will determine their identities by the technique of mixed melting points. To do this, you grind together a 50/50 (1:1) mix of one of the unknowns along with pure benzoic acid. You load this mix into a capillary melting point tube. You load the unknown you used in the mix into another capillary tube. Now you melt the two samples in the same machine side by side. The two tubes have identical melting point ranges. Therefore, you can conclude that
the unknown compound you used in the mixture was benzoic acid.
question
A compound has been recrystallized in the lab. Its melting point was found to be 117 - 127 degrees C. From this, we can conclude that this sample is
impure.
question
If the addition of a sample of compound A lowers the melting point of compound X, X and A cannot be identical.
True
question
A procedural error that can cause the melting point to be too high is?
all of these
question
The presence of an impurity will cause the melting point to be ___________.
question
According to our lab text, heating a sample too fast is likely to result in a melting point that is artifically ______.
low
question
The melting point and the freezing point of a pure substance are different temperatures. True or False?
False
question
In theory, does a melting "point" exist?
Yes, because the melting point is the temperature where solid and liquid exist in equilibrium. This temperature should be attainable under very carefully controlled conditions.
question
Two of the unknowns in expt 3.3 were trans-cinnamic acid (lit mp = 133 degrees) and urea (lit mp = 133-5 degrees). Say your unknown had a measured mp range of 132-4 degrees.
You cannot say for certain which is your unknown compound.
question
A substance melts sharply at 135 degrees. Is it a pure compound?
Not necessarily. It could possibly be a eutectic mixture.
question
Compound A and compound B have approximately the same melting point. When a 50:50 mixed sample of A and B is prepared, its melting point differs from that of both A and B by being _________.
question
It is okay to re-use a melting point sample to make a repeat measurement.
No, because the sample may have oxidized or undergone some other form of thermal decomposition
question
You measure a melting point for your unknown sample of 132-4 degrees. From our list of possible unknowns, you think it is either trans-cinnamic acid (lit mp = 133 degrees) or urea (lit mp = 133 - 5 degrees). You prepare a capillary tube containing a 50:50 mix of your unknown plus urea. You then melt both samples side by side in the melting point apparatus. If your unknown compound is urea, you will see
that the contents of both capillary tubes melt simultaneously.
question
Selection of an appropriate recrystallization solvent is extremely important. In a good recrystallization solvent, the solute you wish to recrystallize will be "soluble _____, insoluble _____."
hot, cold
question
Which of the following is NOT a characteristic or quality of a good recrystallization solvent?
cannot be miscible with water
question
Why is it necessary to cool a hot, supersaturated solution SLOWLY?
rapid cooling will cause precipitation thereby trapping impurities in the crystals
question
One of the most commonly asked questions in O-chem lab is "How much solvent should I use for my recrystallization?" The answer is always the same:
use the minimum amount
question
When preparing to vacuum filter your crystals, it is important to remember this when you set up:
all of these
question
It is common for some crystals to remain in the crystallization flask after you have poured out the crystal slurry onto the filter paper. You should...
wash the crystals from the flask using a small amount of ice-cold recrystallization solvent
question
We learned in General Chemistry that water is an excellent solvent for many ionic inorganic compounds. Consequently, water, because of its highly polar nature, should never be used as a recrystallization solvent for nonionic organic compounds.
False ;Water is highly polar, and it often makes a good recrystallization solvent for organic compounds with highly polar, hydrogen-bond-capable organic molecules. Examples of such molecules would be sugars (mono- or di-saccharides like glucose and sucrose) or some carboxylic acids, including amino acids. In this experiment, we will recrystallize benzoic acid and acetanilide from water.
question
Which of the following is NOT a technique that would induce crystallization?
question
After filtration, crystals should be left on the filter paper with the vacuum on for at least 10 minutes. Otherwise,
two of these
question
You look up the melting point of the amino acid serine and find that it is listed as "228d." What does that reported melting point mean?
"d" stands for decomposition
question
In the first part of experiment 3.2, you recrystallized benzoic acid from hot water. Based on what you know about water as a solvent and the solubility of organic compounds in water, do you think water would be a commonly used recrystallization solvent?
No ;Water is a very polar solvent which also has the capacity to form hydrogen bonds with oxygen and nitrogen atoms in organic molecules. Water is a very good solvent for ionic compounds, be they inorganic or organic salts. Because of its hydrogen bonding ability, water is also a good solvent for sugars (carbohydrates) because these molecules contain multiple -OH groups. However, since most other organic molecules lack multiple -OH groups, they have little or no solubility in water; think oil and water. Most organic molecules are simply too nonpolar to dissolve to any extent, even in hot water. Therefore water is not often used as a recrystallization solvent in organic chemistry.
question
When performing a vacuum filtration, some of your crystalline product remains behind in the recrystallization flask. In order to collect this product, you should
Wash the crystals out of the flask with a small volume of ice-cold recrystallization solvent
question
For which of the following situations is it appropriate to perform a recrystallization?
The melting point of your product is depressed and melts over a wide range.
question
In General Chemistry, we learned that 'like dissolves like," i.e. ionic or polar solutes dissolve in polar solvents, and nonpolar solutes dissolve in nonpolar solvents. When we are seeking an appropriate recrystallization solvent for our reaction product, should we try to match the polarity of the solvent to the polarity of our product? (Think about it!)
No, we don't want to match the polarities because then the solute will be soluble at room temperature in the solvent. This prevents crystallization!
question
At the end or a recrystallization, where should the impurities be located?
in the mother liquor
question
The goal of the recrystallization procedure is to obtain purified material with a maximized recovery. What will be the effect on the recovery if, in the dissolution step, an unnecessarily large volume of solvent is used?
the recovery will be lower
question
The goal of the recrystallization procedure is to obtain purified material with a maximized recovery. What will be the effect of washing the crystals obtained after filtration with fresh hot recrystallization solvent?
the recovery will be decreased
question
What will be the effect on purity and/or recovery if crystallization is accelerated by immediately placing the flask of hot solution in an ice-water bath?
purity will be decreased, but recovery will be about the same
question
During this experiment, you may have observed that a second crop of crystals forms in the mother liquor in the filter flask. It is tempting to collect this second batch of crystals and add it to your first batch. Is this a good idea?
Not usually. The second batch of crystals will typically be much less pure than the first batch because all, or most, of the impurities concentrated in the mother liquor.
question
Why are high boiling solvents seldom used as recrystallization solvents?
two of these
question
Never heat a closed system, otherwise it may ____________.
explode
question
The _______________ boiling point of a liquid is measured at 760 torr (1 atm).
normal
question
A liquid will boil when its equilibrium vapor pressure equals the total (ambient) pressure. We can tell the boiling process has begun
when the liquid begins to bubble
question
Imagine a pure liquid containing a soluble but nonvolatile "impurity." Examples might be salt water, or some polyethylene molecules dissolved in hexane. The presence of nonvolatile "impurities" will have no effect on the boiling point of the liquid.
False
question
The presence of a volatile impurity in a liquid (like a little water in some acetone) will have no effect on the boiling point of the liquid.
False
question
What is the relationship between the volatility of a liquid and its vapor pressure?
the higher/greater the volatility of a liquid, the lower its boiling point
question
What is the relationship between a liquid's vapor pressure and its temperature?
vapor pressure is directly proportional to temperature
question
In expt 4.2, why did you add a boiling stone to the test tube of liquid?
to prevent bumping
question
Say you forgot to add the boiling stone to the test tube before you began heating the liquid. Now the liquid is hot. Can you, or should you, add the boiling stone at this point?
No, because the addition of the stone may cause rapid boiling and the hot liquid may splash out of the test tube.
question
You should wait to take the boiling point of the liquid (in expt 4.2) until the reflux ring is
above the top of the thermometer bulb
question
Why don't we put the test tube of unknown liquid in a beaker of boiling water? In other words, why not use boiling water as a heat source instead of a hot sand bath?
because the temperature of boiling water cannot exceed 100 degrees and some organic liquids boil above that temperature
question
At the boiling point, the temperature of a pure liquid in the test tube (or distillation flask) will be ____________ the temperature of the pure vapor above the liquid.
equal to
question
Which of these does NOT apply to a simple distillation?
a chemical reaction between a liquid and at least one other substance, which requires heat in order to occur
question
Consider a SIMPLE distillation of a 1:1 mixture of two liquids, X and Y. X has a LOWER boiling point. Y has a HIGHER boiling point. What would you predict for the composition of the initial distillate collected?
more X than Y ;According to the Law of Partial Pressures, the vapors will contain both X and Y. However, since they have different BP's and different vapor pressures, they will NOT contribute equally. Since X has the lower BP, it is more volatile (higher vapor pressure) and the vapors will be enriched in X. In a simple distillation, this mixture will be collected as the distillate.
question
During a distillation, how can you tell if you are distilling a pure liquid?
the head temperature will remain constant
question
During the simple distillation of sugar water, the pot temperature will be ___________ the head temperature.
greater than
question
During the simple distillation of sugar water, the head temperature will _____________.
remain constant
question
During the simple distillation of sugar water, the pot temperature will ___________?
increase
question
When connecting the cooling water hoses to the condenser, the incoming water should go in the ________ connection, and the outgoing water should go out the _______ connection.
bottom, top
question
When positioning the distillation flask in the sand bath, you should
position the flask way down in the sand in order to maximize heat transfer and speed heating
question
When setting up for a simple distillation, it is very important that you
do all of these
question
According to our lab text, it is acceptable to use simple distillation to separate two volatile liquids as long as their boiling points differ by at least ________ degrees Celsius.
40 to 50
question
In a miniscale distillation, the top of the thermometer bulb (of the thermometer placed at the head of the distillation apparatus) should be adjacent to the entrance opening to the condenser. If the thermometer is placed BELOW the opening of the condenser,
the measured temperature will be too high
question
In a miniscale distillation, what will be the effect on the measured temperature if the thermometer bulb is placed ABOVE the opening to the condenser?
the measured temperature will be too low
question
Assume you are separating two or more volatile liquids by fractional distillation, and that you have a thermometer measuring the pot temperature and another thermometer measuring the head temperature. When the first drops begin coming over in the condenser, the head temperature will be
lower than the pot temperature
question
In experiment 4.3, you separated a 1:2 mixture of cyclohexane and toluene. The fractionating column you used was about 4 inches long, and you packed it with some steel wool. All other things being equal, what do you think would happen if you were to repeat this experiment with an identical column that was twice as long?
the separation efficiency should be improved because the longer column will make for a greater top-to-bottom heat differential
question
The ratio of the amount of condensate returning to the stillpot and the amount of vapor removed as distillate per unit time is known as the ___________________.
reflux ratio
question
True or False? A simple distillation, rather than a fractional distillation, should be used to obtain pure drinking water from sea water.
True
question
True or False? A simple distillation, rather than a fractional distillation, should be used for separating benzene (bp @ 1 atm = 80 degrees) from toluene (bp @ 1 atm = 111 degrees).
False
question
True or False? A simple distillation, rather than a fractional distillation, should be used for obtaining gasoline (bp range @ 1 atm ~ 38 - 204 degrees) from crude oil.
False
question
True or False? A simple distillation, rather than a fractional distillation, should be used for removing diethyl ether (bp @ 1 atm = 35 degrees) from p-dichlorobenzene (solid, mp = 174-5 degrees).
True
question
True or False? A simple distillation, rather than a fractional distillation, should be used for separating oxygen (bp @ 1 atm = -183 degrees) from nitrogen (bp @ 1 atm = -196 degrees) in liquid air.
False
question
Extraction is a commonly used technique in the organic chemistry lab. Extraction is used to separate various compounds based on differences in their
solubilities
question
Extraction (and chromatography) involves the partitioning of substances between two immiscible phases. In experiment 5.4, the two immiscible phases are
ether and nutmeg
question
The purpose of experiment 5.4 is to isolate the natural product called trimyristin. Trimyristin is a(n)
fat
question
Trimyristin is only one component of nutmeg. How are the other components of nutmeg removed in this experiment?
They are removed by gravity filtration.
question
Why isn't ether used as the recrystallization solvent?
because the trimyristin is very soluble in ether, even at room temperature
question
Trimyristin is only one component of nutmeg. How are the other components of nutmeg removed in this experiment?
gravity filtration
question
Considering the solubility properties of trimyristin, would water be a good choice as an extraction solvent for obtaining trimyristin from nutmeg?
No
question
What is the purpose of adding acetone to the crude product?
acetone is the recrystallization solvent used to purify the crude trimyristin ;Trimyristin is highly soluble in ether at room temperature thereby making ether a poor choice for a recrystallization solvent. In acetone, trimyristin is "soluble hot, insoluble cold."
question
Indicate which of the following are reasons for performing a simple distillation in this experiment.
To isolate the nonvolatile solute; To remove the solvent from the solute.
question
Nutmeg is a hard seed that comes from a tree grow in Malaysia and the Caribbean. In this experiment, we use powdered nutmeg instead of the hard seed because
the hard seeds have a very small surface area compared to the same mass of powdered nutmeg -- and the low surface area means the extraction of trimyristin will take a long time
question
The % recovery of trimyristin is very low in this experiment. That is because
trimyristin is only a very small component in nutmeg
question
Why did the directions say to use a gentle reflux rate rather than a vigorous one?
because we want to avoid boiling off the ether
question
What would happen in this experiment if you used 10 mL of acetone instead of 10 mL of ether as the extraction solvent?
some of the trimyristin might come out of solution during the gravity filtration step when the solution cools as it filters -- this would reduce the recovery of trimyristin
question
Consider the structure of trimyristin (as on page 172 of the text). Do you agree that overall the trimyristin molecule is more polar than nonpolar?
Disagree
question
A student in another O-chem class (let's say at Saddleback), performed experiment 5.4 just as you did and reported a 40% recovery of trimyristin. Based on your own experience with this experiment, do you believe this student's result is right or wrong?
Wrong
question
In the trimyristin extraction experiment, we spoke of the trimyristin partitioning between two immiscible phases. In TLC, we also speak of partitioning between two immiscible phases called the mobile phase and the stationary phase. In this experiment where we separate pigments in a leaf, the stationary phase is
silica gel
question
In this experiment the mobile phase is
whatever organic solvent(s) I choose to use
question
Which statement below is correct?
Silica gel is a polar adsorbent.
question
The term eluant refers to
all of the above
question
If experiment 6.2 is performed properly, you should observe at least four different leaf pigments on your TLC plate. The highest spot on the TLC plate will be that of the most _______________ compound.
nonpolar
question
The instructions for experiment 6.2 tell you to put a pencil dot about 1 cm from the bottom of the TLC plate. This will be your "target" where you will eventually spot your plate with the leaf solution. What do you think might happen if you made an ink dot instead of a pencil dot?
There might be one or more pigments in the ink that could chromatograph along with the leaf pigments.
question
In the lab you synthesized a colored compound. You know that there is the possibility that you could have made some undesirable byproducts in the reaction. You recrystallize your product and you think it is pure. You can obtain the melting point of the compound, but since you can't find the melting point of the colored compound in the literature, you decide to use TLC to determine the compound's purity. You run TLC experiments using different solvent combinations and the most spots you ever see on the plate is two. You conclude that
your compound contains (at least) one impurity
question
It is important to mark the distance traveled by the solvent as well as the distance traveled by the spots. This is so
you can calculate acccurate Rf values for the individual pigments
question
How would you calculate the Rf value for spot A?
2 cm divided by 2.5 cm
question
The textbook contains a fairly lengthly discussion of solvent polarity and eluting power. Based on this discussion, what kind of solvent would most likely move a polar compound up the TLC plate the fastest/farthest?
a very polar solvent
question
In experiment 6.3, _____________ was the stationary phase and ______________ and ____________ were the mobile phases.
alumina, petroleum ether, dichloromethane
question
When separating a mixture in normal phase chromatography (like we did), it is better to start with a more polar eluant first and then change to less polar eluants as you go along. True or False?
False
question
Of the two compounds we separated in this experiment, fluorene and fluorenone, _____________ is the more polar.
fluorenone
question
The most appropriate chromatographic method for purifying 5 grams of material would be __________. (either TLC or column chromatography)
column chromatography
question
Of the two solvents we used in this experiment, ______________ has the most eluting power. Is this solvent the most polar of the two, yes or no? ______
dichloromethane, yes
question
The preparation of 1-bromobutane in experiment 14.4 is an example of __________ reaction.
an SN2
question
In the preparation of 1-bromobutane, the nucleophile in this reaction is
Br-
question
In the preparation of 1-bromobutane, the electrophile is
CH3CH2CH2CH2OH
question
By definition, nucleophiles are species that have at least one pair of nonbonding electrons and bear a negative charge.
False
question
The rate determining step in the formation of 1-bromobutane is
attack of the nucleophile with simultaneous elimination of the leaving group
question
The reaction in this experiment is an equilibrium, as shown in equations (14.9) and (14.10) in the textbook. For the reverse reaction, i.e. the formation of 1-butanol from 1-bromobutane, what species would be the electrophile?
CH3CH2CH2CH2Br
question
Since the formation of 1-bromobutane is an equilibrium reaction, we use the following technique to drive the reaction to the right:
both A and B
question
In experiment 14.4, the formation of 1-bromobutane, what is the leaving group in the substitution reaction?
water
question
1-Bromobutane is an example of a __________ alkyl halide.
primary
question
Nucleophilic substitution is a general reaction for aliphatic compounds in which the leaving group is attached to an _______- hybridized carbon.
sp3
question
The dehydration of 4-methyl-2-pentanol, and the dehydration of alcohols in general, is a reversible reaction.
True
question
The dehydration of 4-methyl-2-pentanol, and the dehydration of alcohols in general, is an example of an equilibrium reaction. In these kinds of reactions, we usually take certain steps to maximize the formation of the desired product. In the dehydration of 4-methyl-2-pentanol we encouraged the formation of the alkene product by
removing the alkene product from the reaction vessel by fractional distillation
question
In this experiment, you added several spatula-tips full of anhydrous potassium carbonate to the liquid you had just distilled. The purpose of the potassium carbonate was to
two of these
question
If the potassium carbonate were NOT removed from the liquid product prior to the final distillation, the potassium carbonate might relase its bound water molecules back into the liquid once the pot temperature reached 100 degrees. Then one of the purposes of the potassium carbonate would be defeated.
Agree
question
Please refer to Figure 10.10 on page 354 of the lab text. This Figure shows the gas chromatography (GLC) analysis of the product mix from the dehydration of 4-methyl-2-pentanol as done in this experiment. Note that there are four different product peaks! The Figure caption identifies what compound is responsible for each peak. The peaks and their respective compounds are listed below. Match the compounds with the mechanism that formed them. Peak 3, the major products, cis- and trans-4-methyl-2-pentene Peak 5, the major byproduct, 2-methyl-2-pentene Peak 2, 4-methyl-1-pentene Peak 4, 2-methyl-1-pentene
E1, Zaitsev product E1 with hydride shift E1, Hofmann product E1 with two hydride shifts
question
Why does the alcohol starting material have a higher boiling point than the alkene product(s)?
two of these
question
Why is it proposed that the alcohol functional group is protonated by acid before dehydration can occur via EITHER an E1 or E2 mechanism?
because -OH is a poor leaving group
question
In this experiment, the substrate molecule is 2-bromo-2-methylbutane which is a __________ alkyl halide, and the base is potassium hydroxide which is a ________ base.
tertiary, strong
question
The product(s) in this reaction (experiment 10.2A) is/are formed by the _______ mechanism.
E2/concerted
question
While it is possible that more than one product will be formed in this reaction, the major organic product should be
2-methyl-2-butene
question
The major product in this reaction would also be known as
the Zaitsev product
question
How is the product(s) isolated in this experiment?
fractional distillation
question
In this experiment, the 2-bromo-2-methylbutane serves as the __________, and the potassium hydroxide serves at the __________.
electrophile, nucleophile
question
The substitution product of this reaction is
unlikely because the alkyl halide is too sterically hindered
question
This experiment calls for using 25 mL of a 4 M solution of KOH in 1-propanol. How many moles of -OH will be initially present in this reaction? Your answer must contain two numbers and a decimal point -- one of the numbers can be (but does not have to be!) a zero, e.g. 50., or 5.0, or 0.5.
0.1
question
This experiment calls for using 2.5 mL of 2-bromo-2-methylbutane. The molar mass and density of 2-bromo-2-methylbutane are 151.0 g/mol, and 1.18 g/mL, respectively. How many moles of 2-bromo-2-methylbutane are initially present? You must give your answer in decimal form, not scientific notation! And your answer must contain two significant figures, no more, no less. If your answer is a fraction, you must use a zero before the decimal point, e.g. 0.XX.
0.020
question
In this experiment, the reaction mixture is refluxed for 1.5 hours and then the product is distilled. Because no materials are being removed from the reaction flask during reflux, it is okay to do the reflux part of the experiment without a magnetic stir bar.
False
question
The difference between experiments 10.2A and 10.2B are the bases. The base in 10.2B is potassium tert-butoxide. The major difference between this base and the one used in 10.2 A is
the size; i.e. the more sterically demanding organic base, t-butoxide, will favor abstraction of the more accessible proton
question
The favored product in this reaction is
the Hofmann product
question
The mechanism of this reaction involves
a transition state where the bromine and hydrogen are anti-periplanar
question
The name for this general type of reaction is
dehydrohalogenation
question
Why does the base used in this reaction, potassium t-butoxide, favor the E2 over the E1 mechanism?