MCAT Organic Chemistry Question 165: Answer and Explanation

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Question: 165

15. The IR spectrum of a fully protonated amino acid would likely contain which of the following peaks?

I. A sharp peak at 1750 cm–1

II. A sharp peak at 3300 cm–1

III. A broad peak at 3300 cm–1

  • A. I only
  • B. I and II only
  • C. II and III only
  • D. I, II, and III

Correct Answer: B

Explanation:

Amino acids in their fully protonated form contain all three of the peaks that should be memorized for Test Day: C–O, N–H, and O–H. While statements I and II correctly give the peaks for the C=O bond (sharp peak at 1750 cm–1) and the N–H bond (sharp peak at 3300 cm–1), the peak for the O–H bond is in the wrong place. In a carboxylic acid, the C=O bond withdraws electron density from the O–H bond, shifting the absorption frequency down to about 3000 cm–1. Statement III is therefore incorrect.

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