MCAT Organic Chemistry Question 160: Answer and Explanation
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Question: 160
10. In an IR spectrum, how does extended conjugation of double bonds affect the absorbance band of carbonyl (C=O) stretches compared with normal absorption?
- A. The absorbance band will occur at a lower wavenumber.
- B. The absorbance band will occur at a higher wavenumber.
- C. The absorbance band will occur at the same wavenumber.
- D. The absorbance band will disappear.
Correct Answer: A
Explanation:
Carbonyl groups (C=O) in conjugation with double bonds tend to absorb at lower wavenumbers because the delocalization of π electrons causes the C=O bond to lose double-bond character, shifting the stretching frequency closer to C–O stretches. Remember that higher-order bonds tend to have higher absorption frequencies, so loss of double-bond character should decrease the absorption frequency of the group.