lab questions orgo
if a particular compound in a TLC experiment has an equilibrium constant of 1.0, what Rf value would you expect?
0.5
What approximate mass of benzoic acid is expected if the benzoic acid/4-chloroaniline/benzil mixture is 1:1:1, 2:1:1, or 1:1:2?
0.5 g, 0.75 g, or 0.38 g
Benzoic acid and 2-naphthol both have melting points of 121-122 ˚C. Which melting point would be most reasonable for a 1:1 mixture of benzoic acid and 2-naphthol?
115-122 ˚C
Use this phase diagram to estimate the composition of the initial distillate in the simple distillation of a mixture of 75% A and 25% B. (Using a ruler and your cursor will probably be helpful.) What is the % of A in the distillate (enter just the number, not the % symbol).
23
What temperature would be recorded by the thermometer in Figure 4?
51 ˚C
If an impure sample has a melting point of 55-68 ˚C, what is a possible melting point for the compound when it is pure?
68-70 ˚C
In a simple distillation of a mixture of two liquids with boiling points of 83 and 97 ˚C, the distillation temperature for the first drops of distillate will most likely be
86 ˚C
Ethanol, an alcohol, is much more polar than our alkyne product. Why is ethanol used as our recrystallization solvent?
A large polarity difference between solvent and solute is desirable in recrystallization.
If extracting organic compound X from water into diethyl ether, how could you increase the concentration of X dissolved in the ether?
Add NaCl
If the desired compound crystallizes and gets filtered along with the insoluble impurities and activated charcoal, what would be a good next step?
Add some hot solvent to the filter.
Which of the following will increase the amount of X dissolved in solvent A?
Addition of more solvent A
A few drops of bromine solution are added to a solution of a mixture of two unknowns, producing a colorless solution. What is the most appropriate conclusion?
At least one of the unknowns is an alkene.
Peak height cannot be taken as proportional to amount of compound. Why not?
Because peak widths can vary.
While the alkene is thought to react with Br2 that is liberated from tribromide anion, another possibility is the direct reaction with tribromide anion. Why is this less favorable?
Because the nucleophile must approach a negative anion.
The instructions say that you should calculate the theoretical yield of each products as if it is the only product. Why is it that only one calculation is needed?
Because the two products have the same molecular weight.
rank in order of increasing polarity: CO, CO2, CH4
CH4, CO2, CO
Given that solvents are typically present in higher amounts and have lower boiling points than solutes, which of the following chromatograms looks most typical?
Chromatogram A has the largest peak with the smallest retention time, consistent with the expectations for the solvent.
While it may be necessary to move the sand to adjust the rate of distillation, this should not be done more than necessary. Why?
Continually increasing and decreasing the heat can lead to oscillations in the data.
What is another name for trans-stilbene?
E-1,2-diphenylethene
A flowchart is an alternative way to describe a procedure. What advantage does it have?
Gives a quick overview of strategy
Why is the O-H stretch of a carboxylic acid broader than that of an alcohol?
Hydrogen bonding leads to broadness, and H-bonding is stronger in the acid.
While protic solvents speed up SN1 reactions, they tend to slow down SN2 reactions. What is a possible reason?
Hydrogen bonding to the nucleophile
Treatment with NaI/acetone can be said to be a very general SN2 test. What does "general" mean in this context?
It works for any alkyl chloride or bromide.
If a compound with a very high boiling point that could not be volatilized was injected into a GC, what would be the result?
It would remain in the injection port.
Presuming that everything was done correctly in your experiment, would you expect the second crop of crystals to have more or less mass than the first crop?
Less mass, since the solution had less compound.
Which of the following changes would lead to a higher retention time?
Longer column
Infrared spectroscopy gives us information about the vibrations of a compound based on the absorption of infrared light of different frequencies (energies). What does "spectroscopy" mean?
Measurement of the interaction of light with matter.
Wrapping with foil is more often needed for a fractional distillation. Why is this?
More heat is needed, so preventing heat dissipation is needed.
What is needed to make a phenol water soluble?
NaOH
If the solution contained both insoluble impurities and activated charcoal, after the filtration, how are these separated from each other?
Neither is desired, so they need not be separated.
If no precipitate is observed in 5 minutes at 50 ˚C, is it safe to say that this compound cannot react at all by SN2?
No - it could be a case that takes longer.
Will an alkene always show an sp2 C-H peak, even if the C=C peak is not observable?
No, because the alkene might not have an sp2 C-H bond.
Is it expected that the percent recovery should be near 100%?
No, because we started with three compounds.
Is AgNO3 a catalyst in these SN1 reactions?
No, it gets used up.
Since ketones and carboxylic acids (dimers) absorb in the same range, does this mean that IR spectroscopy cannot distinguish between them?
No, the O-H region should distinguish them.
Is separation of cyclohexane and toluene expected to work well by simple distillation?
No, the boiling points differ by much less than 70 ˚C.
Would this compound also show a C=C IR peak in the 1685-1665 range?
No, the second C=O would further weaken the C=C, shifting the peak to even lower frequency.
Would it be good to wrap the horizontal arm of the distillation head in foil?
No, we need this arm to stay cool.
Once a compound is vaporized in the injection port, does it remain in the gas phase throughout the GC analysis?
No, when the compound is adsorbed on the stationary phase, it is not gaseous.
The instructions say we need the theoretical yield in mmol, g, and mL. Does this mean we need to calculate theoretical yield three times?
No. We just need to calculate it once and then can convert the units.
The all-caps for "DO NOT DISTILL TO DRYNESS" suggests that this is a safety issue. What is the issue?
Nonvolatile solutes left in the distillation flask might become explosive, especially when heated.
what is the main reason that silica gel is so polar?
O-H bonds
IR absorption requires a change in dipole moment. Why does it matter whether a particular bond has a bond dipole or not?
Stretching a bond that has no bond dipole will not change the dipole moment very much.
The double-dagger symbol by the bracket in this equation signifies what?
That the marked species is a transition state.
What effect does H-bonding in the carboxylic acid dimer have on the C=O bond?
The C=O bond is weakened.
This fractional distillation might actually take longer than if we just distilled a mixture of the two alkenes. Why?
The alkenes must first be formed before they can distill.
Tertiary alkyl halides are too sterically hindered for reaction by SN2, but they are fast in SN1, in spite of the steric hindrance and the fact that the nucleophile still has to approach in the second step. Why is this?
The carbocation intermediate is very reactive.
After recrystallizing 100 mg of compound, 105 mg of crystals were obtained. What is the most likely explanation?
The crystals are not yet dry.
What would be the likely problem with cooling the hot solution in ice-water?
The crystals formed would not be pure.
In a phase diagram like the one in Figure 5, what determines whether the distillation proceeds toward the left or toward the right in the diagram?
The direction we have to go to get horizontally from the liquid to the vapor line
Why is the fluted shape preferred for a hot filtration over the more common cone formed by folding twice and opening?
The fluted shape uses the entire filter paper surface, leading to faster filtration.
The reported melting point for biphenyl is 70-71 ˚C, but your recrystallized sample of biphenyl gave a melting point of 73-74 ˚C. What might be the problem?
The heating rate for your melting point determination was too high.
What does it mean that the 50-mL beaker is tared?
The mass of the empty beaker was recorded.
If you wanted to save most of the acetominophen from this experiment for later analysis, what should you save?
The mother liquor
Why are second-crop crystals often less pure than first-crop crystals?
The mother liquor has more impurities relative to desired compound.
By changing from AgNO3/ethanol to AgNO3/ethanol/water, what else could change besides the rate of reaction?
The product
Given that the SN1 reaction involves both alkyl halide and nucleophile, why is it called unimolecular?
The rate-determining step involves only one molecule.
The point of recrystallization is to purify the compound, but a seed crystal that is added is just as impure as what you started with. Why is this not a problem?
The seed crystal is small and its impurities are insignificant.
Which would you expect to start melting first?
The starting mixture (less pure)
Why are O-H, C-H, and C=O IR stretching absorptions reliably observed when these bonds are present?
These bond types have non-zero bond dipoles.
Acetic acid is a highly polar solvent. Why would it be used for this reaction?
To dissolve the pyridinium tribromide.
Why is concentrated HCl (which is 12 N) used to acidify rather than 3 N HCl?
To minimize the amount of water used.
What is the purpose of using NaHCO3 in this procedure?
To neutralize any phosphoric acid that might have distilled.
What is the purpose of the NaCl here?
To remove water from the ether.
Bromine was added to a known alkene as a positive control, but a red solution was obtained. What is the most likely explanation?
Too much bromine was added.
If you have a neutral organic compound that is contaminated with basic impurities, what single extraction (washing) step would best purify the compound?
Treat with aqueous acid
How can an amine be made water soluble?
Treating with HCl
Why do the compounds precipitate from the aqueous solution when they were soluble before?
Treatment with base or acid makes the compounds neutral and no longer soluble in water.
Why do C-C triple bonds absorb at higher frequency than C-C double bonds?
Triple bonds are stronger than double bonds.
Why do we use two 5-mL portions of 3 N HCl instead of a single 10-mL portion?
Two portions more efficiently extract the basic compound.
If you are doing an extraction in a centrifuge tube and wish to remove the upper layer, what is the correct approach?
Use a pipet to remove the lower layer, then remove the upper layer and put the lower layer back.
Why should the distillate not run down the side of the receiver vial?
We need to be able to count the drops.
Why is the product dibromide washed with methanol instead of acetic acid?
We want to be sure the acetic acid is gone before the next step.
IR spectra usually have peaks that are aimed down. Why is this?
When light is absorbed, less light is transmitted.
What does "meso" designate about the dibromide?
Which diastereomer
Which reaction of alkenes is faster?
With Br2
If we had a fractionating column with 200 theoretical plates, would it be possible to distill any significant amount of the higher-boiling liquid B?
Yes, but only after the A was essentially all distilled.
Why did we need to distill? Would not GC have worked to show us the alkene ratio if we injected the reaction mixture?
Yes, but this would not have been good for the GC.
Just because analysis of the fingerprint region is difficult, is that a reasonable excuse for organic chemists to not try to analyze this region?
Yes, if the easier regions provide the desired answer, there is no need to risk making a mistake in the difficult region.
If you felt the outside of the fractionating column in Figure 4, would it feel any different at the top vs. at the bottom.
Yes, it would not be as hot at the top.
A phase diagram such as in Figure 5 might appear to be just a theoretical graph. Is there any way to actually know what the liquid and vapor curves really look like for a given pair of liquids?
Yes, we can collect the actual data by doing a number of simple distillations.
Aqueous sodium bicarbonate (NaHCO3) will serve to draw which compound into the aqueous layer?
a carboxylic acid
Channels and cracks are best avoided in column chromatography. What usually causes these to form?
admission of air into a packed column
which compound is more polar than 1-butanol?
ammonium acetate
where is the filter paper in the figure
behind the TLC plate
if a compound has Rf=0.57 with diethyl ether as eluant, switching to which solvent is expected to give Rf < 0.57
benzene (less polar)
The best solvent for recrystallization will provide
both high and low solubility
what causes the solvent to rise up the plate
capillary action
Why is ice-cold solvent used for washing the crystals?
cold solvent minimizes the redissolution of the crystals.
What do you do if the TLC analysis showed the first component in fractions 3-5 but did not show any of the second compound in any fractions?
collect more fractions
By applying enough vacuum, it might be possible to reduce the boiling point of a liquid to room temperature or below. What would then be more difficult for such a vacuum distillation?
condensation
Select all choices that correctly describe possible variations in the column chromatography method.
different stationary phases, dry or wet packing of column, one or more than one mobile phases, addition of sample mixture in solution or dry on absorbent, use of air pressure or just gravity
which of the following compounds would be hardest to visualize with chemical visualization agents?
dodecane (alkane)
The purpose of the TLC plate in this section can be said to be
following the reaction
gas chromatography (GC) sometimes goes by a different name. which of the following makes sense?
gas-liquid chromatography
A mixture of hexanoic acid and 2-methylphenol was dissolved in diethyl ether and extracted with 3 N NaOH. Which compounds will be removed from the diethyl ether layer?
hexanoic acid and 2-methylphenol
the most polar solvent should give the ______ Rf value
highest
if a spot that is visible in an iodine chamber is not visible under UV light, what can be concluded about this compound?
it does not absorb UV light
Diethyl ether is not often used as a recrystallization solvent. Which do you think is the correct reason for this?
its boiling point is 35°C
if a non polar compound has Rf 0.44 in 2:1 hexane/ethyl acetate, what will the Rf be in 4:1 hexane/ethyl acetate
lower than 0.44
In a distillation, what method is typically used to condense the vapor?
lowering the temperature
According to Zaitsev's rule, which product is expected to be the major one?
most substituents
What is a possible consequence of using more than the minimum solvent needed?
no crystals will form at all
Reviewing the structures in Figure 7, which compound would you expect to have a higher Rf?
phenacetin
Which compound, phenyl acetate or phenethyl alcohol, would your expect to elute first from the column?
phenyl acetate
Raoult's Law says that the partial pressure of each component in the vapor over a liquid mixture
reflects both the amount of this component in the liquid and its volatility.
if the TLC plate is a layer of silica gel coated onto glass, the class is called the
support
The liquid line in a phase diagram represents
the boiling points of liquids of different compositions.
If ferrocene and acetylferrocene were not stable, what observations might have been made when you tried to separate them by column chromatography?
the colors changed as they moved down the column
What conclusion would you make from the following TLC analysis? (no separation)
the column did not work at all
A particular point on the vapor line in a phase diagram represents
the composition of vapor over a liquid that boils at this temperature.
what is the most polar compound on a TLC plate?
the one traveling the least- lowest Rf
Here is a slightly simpler plan for loading the sample: transfer the 1 mL of sample solution to the top of the column, and immediately fill the column with eluent and apply pressure to collect fractions. What, if anything, would go wrong with this plan?
the separation would be poor
why is no visualization method described in this section?
the spots will already be visible
Would taking smaller fractions possibly have given a better result on this column?
yes, but not very much. it is possible that a small amount of clean compounds would be obtained