Spectroscopy Tutorial: Esters

IR Spectroscopy

Esters will show prominent C=O stretches at the frequencies listed below. You can also look for the C–O stretch, although this band appears in the same region(s) as a lot of other functional groups.

NMR Spectroscopy

If you are following this tutorial in sequence, you are already familiar with carbonyl groups (ketones, aldehydes), which shift the protons on adjacent carbons downfield. In esters, these protons are shifted to 2-2.2 ppm. The protons on the “other” side of the ester, meaning those attached to the carbon adjacent to a carbonyl group, is shifted less far, to 3.7-4.1 ppm. This is important in determining which “way” the ester function is situated in the molecule, –O-C=O– or -O=C-O–.

Notes on Ester Nomenclature

Esters are named for the parent carboxylic acid. For instance, ethyl butanoate (CH3CH2CH2CO2CH2CH3) is derived from butanoic acid, CH3CH2CH2CO2H, with the ethyl group replacing the –H. Similarly, an acetate is an ester of acetic acid (CH3CO2H), so propyl acetate is CH3CO2CH2CH2CH3. Note some “slang” in ether nomenclature, isoamyl is used more commonly than “3-methyl-1-butyl” and propanoate is the same as propionate, butanoate is the same as butyrate.


Example 4

Example 5