MCAT Organic Chemistry Question 104: Answer and Explanation

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Question: 104

14. 3-hydroxybutanal can be formed by the reaction of:

  • A. methanol in diethyl ether.
  • B. ethanal in base, then in acid.
  • C. butanal in strong acid.
  • D. methanal and ethanal in catalytic base.

Correct Answer: B

Explanation:

This is an example of an aldol condensation, but stopped after aldol formation (before dehydration). After the aldol is formed using strong base, the reaction may be halted by the addition of acid. Butanal in strong acid, described in choice (C), would be likely to deprotonate without gaining the hydroxyl group. Methanal in diethyl ether would not be reactive because diethyl ether is not a strong enough base to abstract the α-hydrogen, eliminating choice (A). Reaction of the two aldehydes methanal and ethanal in catalytic base would form a 3-hydroxypropanal (which would dehydrate to form propenal), not 3-hydroxybutanal.

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