CHEM 237 REVIEW
When comparing melting temperature ranges of pure and impure samples, which of the following statements describes the melting temperature range of an impure sample? Select all answers that apply.
The melting temperature range of a pure sample is narrower than that of an impure sample. The melting temperature range of an impure sample is lower than that of a pure sample.
What error is introduced if the bulb of the thermometer, or the thermocouple, is too high in the distillation apparatus?
The recorded boiling point will be lower than actual because the bulb is not exposed to the hot vapor.
Cyclohexanol is converted to cyclohexanone by hypochlorous acid. Excess hypochlorous acid is removed by adding sodium bisulfite. The aqueous reaction mixture is then saturated with sodium chloride and placed in a separatory funnel with methylene chloride (dichloromethane). What's in the organic (bottom) layer?
methylene chloride cyclohexanone
To avoid spilling chemicals on others or yourself.
never point the seperatory funnel at anyone, including yourself
To avoid a build up of pressure inside the glassware.
vent the separatory funnel through the stopcock
The retention factor, or Rf, is a number that describes the position of a spot on a chromatography plate relative to the position of the solvent on the chromatography plate. What are the minimum and maximum values of Rf?
0-1
What region of an infrared spectrum is most useful for distinguishing between isomers that contain the same functional group?
1400-400 cm-1
A two-component mixture of volatile liquids was separated by gas chromatography using a 5 meter capillary column. The separation was insufficient with this column to see resolution of both fractions. Without changing any of the other parameters, what length of column must be used to double the efficiency of this chromatographic separation?
20 meters
Rank the SN1 reaction rates following from fastest to slowest for the following compounds: 3-chloro-3-methylpentane chloromethane 2-chloro-3-methylpentane 1-chloro-3-methylpentane
3-chloro-3-methylpentane 2-chloro-3-methylpentane 1-chloro-3-methylpentane chloromethane
A student was given a sample of crude acetanilide to recrystallize. The initial mass of the the crude acetanilide was 146 mg. The mass after recrystallization was 123 mg. Calculate the percent recovery from recrystallization.
84.2%
The solubility of acetanilide is 3.53 g in 100 mL of chloroform at 0 °C, and 29.1 g in 100 mL of chloroform at 60 °C. What is the maximum percent recovery that can be achieved for the recrystallization of acetanilide from chloroform?
87.9%
To test for free chloride ions; an indication of a tertiary/benzylic carbocation.
A few drops of silver nitrate solution were added to the organic product
In which of the following ways is the use of an infrared spectrophotometer fitted with an ATR sample advantageous over a beam-through-sample spectrophotometer? Select all answers that apply.
ATR allows for the use very small samples. The same ATR sample holder may be used for liquid samples and solid samples.
During a solvent extraction, a student lost track of which layer was the aqueous layer and which was the organic layer. What simple, non-destructive test could be used to determine which layer is which?
Add a little distilled water to see which layer the water adds to.
Sodium hypochlorite and glacial acetic acid combine to make hypochlorous acid. Hypochlorous acid oxidizes cyclohexanol to cyclohexanone. At the end of the reaction, any excess hypochorous acid must be destroyed. How is the destruction of HOCl accomplished?
Add sodium bisulfite, NaHSO3.
Infrared Spectra are recorded using an ATR detector. What does ATR stand for?
Attenuated Total Reflectance
Why is it good practice to perform a mixed melting point determination at two different ratios of unknown to known materials in the melting point capillary?
Because it is possible that one particular combination of materials generates a eutectic.
The Williamson ether synthesis between 2-naphthol and allyl bromide in the presence of strong base and benzyltriphenylammonium chloride as a phase transfer catalyst in dichloromethane solution is known to proceed in reasonable yield. Why will the reaction fail to work if the solvent (NAOH) is switched to ethanol while retaining the same reactants, phase transfer catalyst, and base?
Because the base would deprotonate ethanol and create a second nucleophile.
The Williamson ether synthesis between 2-naphthol and allyl bromide in the presence of strong base and benzyltriphenylammonium chloride as a phase transfer catalyst in dichloromethane solution is known to proceed in reasonable yield. Why will the reaction fail to work if the reactants (2-naphthol and allyl bromide) are switched to 2-bromonaphthalene and allyl alcohol while retaining the same phase transfer catalyst, solvent, and base?
Because the bromo group is a poor leaving group when attached to an aromatic ring.
Detergents are soluble in both organic and aqueous phases.
Benzyltributylammonium chloride was added to the reaction mix
Which of the following statements about infrared spectroscopy is not true? A. Infrared spectroscopy can detect impurities within a sample. B. Infrared spectroscopy is non-destructive. C. Infrared spectroscopy can be used to analyze almost any kind of sample. D. Infrared spectroscopy is good for detecting and identifying functional groups of a sample.
C. Infrared spectroscopy can be used to analyze almost any kind of sample.
Finely powdered drying agents can clog the G.C. syringe.
Calcium chloride was used as a drying agent instead of magnesium sulfate
Which of the following are commonly used oxidizing agents? Select all that apply.
Chromic acid Sodium dichromate
What effect would each of the following changes to the conditions of a GC separation have on the retention time for a sample. Use the pop-up menus to identify the change The column temperature is increased. The carrier gas flow is decreased. The amount of sample injected is doubled.
Decreases Increases Increases
In the reaction to produce allyl 2-naphthyl ether, 2-naphthol is first reacted with aqueous sodium hydroxide to make sodium 2-naphtholate, which then goes on to react with allyl bromide and benzyltributylammonium chloride phase transfer catalyst, with dichloromethane as a solvent. What is in the organic (bottom) layer
Dichloromethane allyl bromide allyl 2-naphthyl ether benzyltributylammonium chloride
Eugenol, the essential oil found in cloves, is insoluble in water. The alcohol group in eugenol is dramatically more acidic than regular straight-chain alcohols. How could eugenol be separated from a mixture of other non-acidic molecules?
Dissolve the mixture in dichloromethane, shake with sodium hydroxide, collect the aqueous layer, and reacidify to force precipitation of the eugenol.
To avoid the introduction of a spurious peak into the GC trace.
Do not rinse the syringe with acetone, rinse it with the solution instead
Acetyl coenzyme A precursor Acetogenin
Eugenol
Tests positive for bromine, potassium permanganate, and ferric chloride tests?
Eugenol
There are four major methods of purification in the chemistry laboratory. Which of the following techniques are included in the four most common methods for separation and purification? Select all that apply.
Extraction Recrystallization Distillation
T/F Gloves and goggles are not required when handling sodium hydroxide.
False
T/F It is acceptable to handle TLC plates with bare hands.
False
T/F Lachrymators like allyl bromide are specifically dangerous because of their explosion hazard.
False
T/F Methylene chloride is flammable.
False
T/F Pipets that contain only water can be disposed of in the trash.
False
T/F Silica is toxic.
False
T/F The primary risk associated with dichloromethane is its flammability.
False
T/F Unused allyl bromide can be disposed of in the aqueous waste.
False
Several liters of the sample need to be separated
Fractional Distillation
Isopentyl diphosphate precursor Terpene
Limonene
What physical properties prevent a sample mixture from being analyzed by gas chromatography (GC)? Select all that apply.
One or more of the components is not volatile. One or more of the components is thermally unstable.
A student is comparing two samples of crystalline organic solids. A TLC analysis shows a single spot for each sample and the co-spot with the same Rf value. Which of the following would provide the most conclusive evidence that the two samples are the same compound?
Perform a mixture melting point.
Less than 1 mL of the sample is available The components of the liquid mixture are not thermally stable The components of the liquid mixture have identical boiling points Only a tiny amount of a pure sample is needed
Preparative GC
The carrier gas in a G.C. separation is usually an inert gas such as nitrogen, helium, or argon, but can also be hydrogen. What are the pros and cons of using hydrogen as a carrier gas in a G.C. separation?
Pros: Cheaper than helium Can be generated at the GC instead of from gas tanks Greater difference in mass between the carrier gas and sample than nitrogen Cons: Explosion hazard
What is the major underlying principle of chromatography? (including TLC)
Separation will be achieved if one component adheres to the stationary phase more than the other component does.
A TLC plate was run in 1:1 hexane and acetone, but when developed the plate only showed baseline spots. What change could be made to get the spot(s) to move further up the plate?
Spot a new plate and run in 1:4 hexane/acetone
Base can cause ground glass pieces to fuse together.
The air condenser and claisen head were greased before use
Which of the following statements about the collection of a G.C. trace is true. Select all that apply.
The amount of sample detected is proportional to the peak area. The stationary phase is a very high-boiling liquid adsorbed onto fire brick.
Which of the following statements best describes a eutectic?
The composition of a mixture of two components that generates the lowest possible melting point.
The non-polar ether product can be separated from the polar by-products.
The contents of the reaction flask were passed through a column of silica.
To avoid loss of sample.
The detector fitted to the Prep GC is a TCD not a FID
Which of the following statements about melting point determination are true? Select all that apply.
The lower limit of the melting temperature range occurs when the first crystal starts to melt. Pure substances typically have a melting range of less than 2 degrees Celsius. The upper limit of the melting temperature range occurs when the last remaining crystal melts.
How would the melting range of a pure organic material be changed if there was a tiny shard of broken glass inside the melting point capillary?
The melting range would not be affected.
To make the polar impurities less soluble in the organic layer.
The organic layer was shaken with saturated sodium chloride solution
To gently neutralize the acidic reaction.
The organic layer was shaken with sodium bicarbonate solution
Which of the following experimental errors would lead to the observation of peaks trailing in on the G.C. trace?
The oven temperature is too low. The column is overloaded.
Which of the following statements best describes density?
The ratio of a substance\'s mass to its volume, measured in g/mL
To exclude water from the product and push the equilibrium to the right.
The reaction is performed in conc. hydrochloric acid, in which the organic halide is not soluble
Adding a strong electrolyte can help the layers separate.
The reaction mix is shaken with concentrated sodium chloride
Potassium chloride precipitates as potassium hydroxide reacts with the chloroalkane.
The reaction mixture was decanted from the precipitate before extraction
Dichloromethane solvent is highly volatile.
The reaction vial was stoppered before it was stirred
To avoid making the reaction mix become an emulsion.
The reaction was stirred at a moderate, not rapid, pace.
Both Br2 and HCl undergo electrophilic addition to alkenes. Why is Br2 used as a test for unsaturation instead of HCl?
The reaction with bromine can be followed because of a visible loss of color.
In order to perform a bromine test on your compound, it must be treated with a dilute solution of bromine. Which of the following results will indicate the presence of an alkene?
The reddish color of the bromine will disappear.
Which of the following experimental errors would lead to the observation of peaks trailing off on the G.C. trace?
The sample is too concentrated. The column is overloaded.
How is it possible to separate two liquids with exactly same boiling point by gas chromatography (G.C.) when it cannot be done by fractional distillation?
The use of a polar GC column will separate the liquids based on differences in polar interactions and volatility.
A 0.7 mL mixture of pentanol and heptanol are separated and collected using preparative gas chromatography on a GC fitted with a polar column and a TCD detector. During the separation, why are the injection port and detector heated to a higher temperature than the column? Select all that apply.
The wire in the thermal conductivity detector needs to be cooled by the eluent in order to register a signal. A heated injector creates flash vaporization which allows separation at temperatures lower than the boiling point of the sample.
Why is there usually a piece of filter paper in the TLC developing chamber?
To create an atmosphere saturated with developing solvent.
Test for the presence of oxidizing agent.
Touch a drop of the reaction mix onto the piece of KI-starch paper
T/F Acetone is flammable.
True
T/F Concentrated acetic acid is corrosive.
True
T/F Ethyl acetate is flammable.
True
T/F Hexane is flammable.
True
T/F Hypochlorous acid is an oxidizing agent.
True
T/F Methylene chloride is a probable human carcinogen.
True
T/F Sodium hydroxide can cause severe damage to skin and eyes.
True
The ether product is colorless.
Ultraviolet light was shone on the TLC plates
For a successful TLC experiment, you need to pay attention to the details. From the list of instructions and tips below, select the correct itmes.
Use only a plain pencil to draw a starting line and mark the compounds' spots. Never use a pen or marker! Cover the developing chamber with a watch glass. Do not permit the solvent to rise to the very top of the TLC plate. Do not disturb the developing chamber, do not move it or carry around. Once the solvent front is getting close to the top of the plate, remove the plate and mark the solvent front. The solvent level in the chamber should be lower than the starting line where you spot the samples. Use a micropipet or glass capillar to put a small amount of the sample solution on the starting line.
To avoid dangerous vibration due to imbalance.
Vials are placed into the centrifuge in pairs, 180 degrees apart
To preserve the purity of the sample because there is a lag between being detected and being eluted.
Wait for the second peak to be on the rise before beginning its collection
In the reaction to produce allyl 2-naphthyl ether, 2-naphthol is first reacted with aqueous sodium hydroxide to make sodium 2-naphtholate, which then goes on to react with allyl bromide and benzyltributylammonium chloride phase transfer catalyst, with dichloromethane as a solvent. What is in the aqueous (top) layer?
Water sodium 2-naphtholate sodium hydroxide benzytributylammonium chloride
Which of the following statements describe why moisture must be excluded from any sample injected into the G.C. Select all answers that apply.
Water contains dissolved salts that can degrade the column. Water signals are very broad and make interpretation of other peaks difficult.
When is recrystallization not a suitable method to purify an organic material?
When the organic material is not a solid.
Convert the benzocaine into a water-soluble salt.
add acid to the three component mixture
To precipitate the benzocaine.
add base to the aqueous acidic layer mixture
Generate the oxidizing agent.
add bleach drop wise into a flask containing acetic acid
Destroy the oxidizing agent.
add sodium bisulfite solution
Dry the reaction product.
combine the organic layers over anhydrous potassium carbonate
Tests positive for bromine and potassium permanganate tests?
cyclohexene limonene
Compared to the dichromate oxidizing agents, hypochlorous acid (prepared from household bleach) is ... (Select all answers that apply)
environmentally sound. inexpensive. safe
For a packed column, doubling the length essentially doubles the number of theoretical plates. This will increase the effectiveness of the separation by the square root of 2. In order to double the separation, one would need to increase the column length _______.
four times
Rank the following molecules in terms of their carbonyl stretching frequency, ν(C=O), in the infrared spectrum. benzaldehyde pentanal cinnamaldehyde
pentanal benzaldehyde cinnamaldehyde
Control the rate of reaction.
place the reaction flask in an ice-water bath
In the presence of hydrochloric acid, 3,7-dimethyl-3-octanol is converted to 3-chloro-3,7-dimethyloctane via an SN1 mechanism. SN1 and E1 mechanisms are usually in competition with each other. Why, in this case, is the elimination product not observed?
the alkene formed by any E1 reaction would react with HCl to form the same product as the substitution reaction
For SN1 solvolysis of t-butyl chloride, rank the solvents from fastest reaction to slowest reaction: acetone methanol ethanol water acetic acide formic acid
water formic acid methanol ethanol acetone acetic acid
Cyclohexanol is converted to cyclohexanone by hypochlorous acid. Excess hypochlorous acid is removed by adding sodium bisulfite. The aqueous reaction mixture is then saturated with sodium chloride and placed in a separatory funnel with methylene chloride (dichloromethane). What's in the aqueous (top) layer?
water sodium chloride
