Resonance effects infrared, ester

Acyl chlorides have C=0 stretching absorptions at higher wavenumber (1800 cm ) than esters, and amides have C=0 stretching absorptions at lower wavenumber (1650—1655 cm ). These different values reflect the contribution of inductive and resonance effects in the stabilization of the Lewis structure of a carbonyl group compared to the dipolar resonance form. The chlorine atom of acyl chlorides inductively withdraws electrons from the carbonyl carbon and destabilizes the dipolar resonance form, thus leading to an increase in the double bond character of the carbonyl group. Figure 21.5 shows the infrared spectrum of acetyl chloride. 

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