3 Challenging Sample MCAT Questions on Organic Chemistry

Bryan Schnedeker

Organic chemistry tends to be one of the most challenging areas of the MCAT for prospective medical school students. Work through each of the following MCAT-style questions and choose an answer, then look at the explanation to help prepare to take the medical school admissions exam.

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Question 1: What is the product of water and pentyl ethanoate under acidic conditions?

A. Ethanoic acid and pentanol

B. Butanol and water

C. Ethanoic acid, two equivalents

D. Pentanol, two equivalents

Explanation: When mixed with water under acidic or basic conditions, esters undergo hydrolysis. The products are a carboxylic acid and an alcohol.

A. Ethanoic acid and pentanol, correct.

B. Butanol and water, incorrect. Pentyl ethanoate does not lose its ester group to become a four-carbon alcohol.

C. Ethanoic acid, two equivalents, incorrect. Hydrolysis of an ester forms carboxylic acid and alcohol.

D. Pentanol, two equivalents, incorrect. Hydrolysis of an ester forms carboxylic acid and alcohol.

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Question 2: In order to extract isobutyric acid from a solution of diethyl ether, one should wash the solution with:

A. One 30 mL wash of water

B. Three 10 mL washes of water

C. One 30 mL wash of hexane

D. Three 10 mL washes of hexane

Explanation: Isobutyric acid has a carboxylic acid group, which is polar, and can readily hydrogen bond. It can be extracted from the organic solvent diethyl ether by washing with a polar solution like water. Multiple small volume washes extract better than one large volume wash. Use of a second organic solvent, hexane, will not extract the isobutyric acid from the diethyl ether.

A. One 30 mL wash of water, incorrect. Multiple smaller volume washes will yield higher extraction results.

B. Three 10 mL washes of water, correct. Isobutyric acid will enter the aqueous layer and wash off of the organic layer of diethyl ether.

C. One 30 mL wash of hexane, incorrect. Hexane is another organic solvent like diethyl ether.

D. Three 10 mL washes of hexane, incorrect. See C.

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Question 3: Which alcohol will most easily react with HCl to form an alkyl halide?

A. Primary alcohol

B. Secondary alcohol

C. Tertiary alcohol

D. All alcohols react equally to form alkyl halides

Explanation: Tertiary alcohols have greater reactivity with hydrogen halides than secondary alcohols -- which in turn have greater reactivity than primary alcohols -- in reactions forming alkyl halides. For tertiary alcohols, the reaction proceeds by an S N1 mechanism which prefers a more substituted alcohol. For small primary and secondary alcohols, there is some reactivity by an S N2 mechanism, but the S N1 mechanism with tertiary alcohols is the most reactive.

A. Primary alcohol, incorrect.

B. Secondary alcohol, incorrect.

C. Tertiary alcohol, correct.

D. All alcohols react equally to form alkyl halides, incorrect.

Bryan Schnedeker is the national MCAT director at Next Step Test Preparation, which provides one-on-one MCAT tutoring programs. He has scored a 41 on the exam and taught the test for over a decade.