MCAT Chemistry Question #1: Answer and Rationale

1. The progress of Reaction 2 can be monitored by observing what change to the IR spectrum of the product mixture?

A) Appearance of a broad peak at 3400 cm–1

B) Disappearance of a broad peak at 3400 cm–1

C) Appearance of a sharp peak at 1700–1750 cm–1

D) Disappearance of a sharp peak at 1700–1750 cm–1

Answer Analysis

The correct answer is C.

Rationale: This question requires the test-taker to combine knowledge of infrared (IR) spectroscopy with reasoning about the structural differences between the products and reactants of Reaction 2. You must be able to visually recognize various functional groups and recall that each functional group has a characteristic IR absorbance. After recognizing the presence of additional carbonyl groups in the products of Reaction 2, you’ll need to know the characteristics absorbance for this type of functional group. You can conclude that appearance of a peak between 1700–1750 cm–1 in the IR spectrum would provide an effective way to monitor formation of a carbonyl-containing product mixture.

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