MCAT Organic Chemistry Question 135: Answer and Explanation
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Question: 135
15. Why should esterification reactions NOT be carried out in water?
- A. Carboxylic acids, from which esters are made, are generally insoluble in water.
- B. The polar nature of water overshadows the polar nature of the leaving group.
- C. The extensive hydrogen bonding of water interferes with the nucleophilic addition mechanism.
- D. Water molecules would hydrolyze the desired products back into the parent carboxylic acid.
Correct Answer: D
Explanation:
The presence of water in an esterification would likely revert some of the desired esters back into carboxylic acids. Small carboxylic acids, like formic or acetic acid, are easily dissolved in water, eliminating choice (A). The polarity of water plays little role in affecting the leaving group; if anything, water can be used to increase the electrophilicity of the carbonyl carbon by protonating the carbonyl oxygen—eliminating choice (B). Finally, this is a nucleophilic substitution mechanism, not a nucleophilic addition mechanism, as mentioned in choice (C). Further, hydrogen bonding would likely augment the reaction.