Question

IR question. Aside from the C=O bond. What are some good ways to differentiate between an estern and a ketone on IR?

Answer 1

Ester is represented as R- C(O) -OR` and a Ketone is as R- C(O) -R`. Both will have carbonyl (C=O) stretching vibrations. But the carbonyl IR stretch in an ester higher than in a ketone.

In ester, you will have a O=C-O-C stretching vibration which is absent in case of ketone.

Now the R` in ester group can be aliphatic or aromatic. The peaks due to O=C-O-C stretching vibration is as follows:

Absorption, cm^-1 Intensity Assignment
aliphatic 1160-1210 s-vs O=C-O-C
aromatic 1250-1310 s-vs O=C-O-C

s-strong and vs-very strong

In ester you will get a strong to very strong intense peak at the above mentioned values, which will be absent in ketone.