MCAT Organic Chemistry Review

Separations and Purifications

Answers and Explanations

1.    BFractional distillation is the most effective procedure for separating two liquids that boil within a few degrees of each other. Ethyl acetate and ethanol boil well within 25°C of each other and thus would be good candidates for fractional distillation. Fractional distillation could also be used for the liquids in choice (C), but would require lower pressures because of their high boiling points.

2.    CBy extracting with sodium hydroxide, benzoic acid will be converted to its sodium salt, sodium benzoate. Sodium benzoate, unlike its conjugate acid, will dissolve in an aqueous solution. The aqueous layer simply has to be acidified to retrieve benzoic acid. Choice (A) is incorrect because diethyl ether and tetrahydrofuran are both nonpolar and are miscible. Hydrochloric acid will not transform benzoic acid into a soluble salt, so choice (B) is incorrect. Finally, choice (D) is incorrect because protonated benzoic acid has low solubility in water.

3.    DHexane is less polar than ether and, therefore, is less likely to displace polar compounds adsorbed to the silica gel. This would decrease the distance these polar compounds would travel, decreasing Rf values.

4.    DIn column chromatography, as in TLC, the less polar compound travels most rapidly. This means that 1-naphthalenemethanol,with the highest Rf value, would travel most rapidly and would be the first to elute from the column.

5.    DIt is more effective to perform four successive extractions with small amounts of ether than to perform one extraction with a large amount of ether.

6.    AThis is an example of reverse-phase chromatography. The solvent system is polar, which means that the most polar compound will travel the furthest up the card, resulting in the largest Rf. This gives compound III the largest Rf, which corresponds to spot A.

7.    BBecause methylene chloride is denser than brine (salt water), the organic layer will settle at the bottom of the funnel. Methylene chloride is nonpolar, so it cannot mix with brine, eliminating choice (C).

8.    BSilica gels are polar. Polarity is used to selectively attract specific solutes within a nonpolar solvent phase. Although silica gels have other properties, this is the most important to TLC.

9.    BIn this question, three substances must be separated using a combination of techniques. The first step should be the most obvious: remove the sand by filtration. The remaining compounds—benzoic acid and naphthalene—are still dissolved in ether. If the solution is extracted with aqueous base, the benzoate anion is formed and becomes dissolved in the aqueous layer, while naphthalene, a nonpolar compound, remains in the ether. Finally, evaporation of the ether will yield purified naphthalene.

10.CThis is the only option that would be effectively separated by a simple distillation. Choice (B) would require vacuum distillation because the boiling points are over 150°C. In choices (A) and (D), the boiling points are within 25°C of each other and would therefore require fractional distillation in order to be separated.

11.CAffinity chromatography, using the target for the biological effector or a specific antibody, would work best in this case. It will specifically bind the protein of interest and keep it in the column.

12.DBecause this solution is composed of a much larger molecule and a much smaller molecule, size-exclusion chromatography would effectively remove the smaller insulin molecule into the fraction retained in the column and allow the titin to be eluted. Affinity chromatography,choice (C), could also be used, but comes with a risk of rendering the titin unusable; the eluent run through an affinity chromatography column often binds to the target molecule.

13.BEach of these is the mobile phase of the system, in which the solutes are dissolved and move. The stationary phase in gas chromatography is usually a crushed metal or polymer; the stationary phase in paper chromatography is paper.

14.CWarm or hot solvent is generally used in gravity filtration to keep the desired product soluble. This allows the product to remain in the filtrate, which can then be collected. In this case, the student likely used solvent that was too cold, and the product crystallized out. The product should be present in the residue.

15.BBecause the lactoferrin proteins are likely to be charged, as is the resin described in the question, this is an example of ion-exchange chromatography. The charged protein molecules will stick to the column, while the remainder of the milk washes through and can later be washed off of the column and collected.

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