MCAT Organic Chemistry Question 54: Answer and Explanation

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Question: 54

9. S_N1 reactions show first-order kinetics because:

A. the rate-limiting step is the first step to occur in the reaction.
B. the rate-limiting step involves only one molecule.
C. there is only one rate-limiting step.
D. the reaction involves only one molecule.

Correct Answer: B

Explanation:

An S_N1 reaction is a first-order nucleophilic substitution reaction. It is called first-order because the rate-limiting step involves only one molecule. Choice (A) is true, but does not explain why S_N1 reactions have first-order kinetics; the rate-limiting step of an S_N2 reaction is also the first (and only) step of that reaction, but S_N2 reactions have second-order kinetics, not first-order. Choice (C) is a true statement as well, but again does not explain why the reaction is first-order. Finally, choice (D) is incorrect because it is the rate-limiting step, not the reaction overall, that involves only one molecule.

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