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MCAT Organic Question — Reactions

Consider the following reaction:

Alanine Peptide

Reactions 1 and 2 can be respectively characterized as:

A. peptide bond formation, nucleophilic acyl addition

B. polymerization, hydrolysis

C. hydrolysis, peptide bond formation

D. nucleophilic addition, hydrolysis

Click for Explanation

The reaction depicted shows two alanine molecules forming a peptide bond. The process of combining monomers, such as amino acids, into polymers, such as polypeptides is called polymerization. Reaction 2 shows the reverse reaction with water being used to break a peptide bond. Using water to break a bond can be described as hydrolysis. Thus (B) is the correct answer.

A: While reaction 1 is peptide bond formation, the reverse reaction is not an addition reaction. In addition reactions two molecules combine. Here, reaction 2 shows one large molecule being broken down.

C: These are the right labels, but applied backwards.

D: Reaction 2 is hydrolysis, but reaction 1 is not a nucleophilic addition; it’s a substitution reaction. The OH group is being substituted for the amino group of the amino acid on the right. Thus, this is a nucleophilic acyl substitution.

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