Question 21.58: Explain why the carbonyl stretching absorption of thioesters......
Explain why the carbonyl stretching absorption of thioesters occurs at 1690 cm^{−1}, whereas that of acyl chlorides occurs at 1800 cm^{−1}.
Step-by-Step
Neither the sulfur nor the chlorine atom can effectively contribute electrons by resonance to the carbonyl group. Sulfur is less electronegative than chlorine and does not destabilize the dipolar resonance form as much as chlorine does. Thus, the carbonyl group of the thioester has more single bond character than the acyl chloride and requires less energy to stretch. This effect results in an absorption at a lower wavenumber for the thioester.
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