Question 21.57: Would you expect the carbonyl stretching absorption of acyl ......

The bromine atom cannot effectively contribute electrons by resonance to the carbonyl group. It is less electronegative than chlorine and does not destabilize the dipolar resonance form as much as chlorine does. Thus, the carbonyl group will have more single bond character in the acyl bromide, and the bond will require less energy to stretch. This effect results in an absorption at a lower wavenumber for the acyl bromide.