Question

Ketones and esters have very similar spectra because both have C=O bands in their spectra. What band should you look for on the spectrum of an ester that a spectrum of ketone won't have?

Answer 1

The general structure of ketone is R-(C=O)-R’ and that of ester is R-(C=O)-O-R^’. Both ketones are esters have C=O bond. But still you can differentiate between them in IR. C=O stretching band of ketones appears at 1710-1720 cm^-1. C=O stretching band of esters appears at 1735-1750 cm^-1. Since the band appears at different frequencies you can distinguish between them.

In addition to C=O band esters have C-O band as well. This band appears at 1000 – 1300 cm^-1. This band is not there in ketones.