Ester infrared spectra

Vinyl acetate

Methyl benzoate

Methyl methacrylate

Methyl salicylate

Ethly butyrate

The C=O of an ester appears near 1750-1735 which can overlap with some ketone C=O stretches. One can usually eliminate ketones by considering the strong and broad C-O peak at 1300-1000. A ketones absorptions will have a weaker and narrower bands. Compare the ethyl butyrate ester above to the ketone below:

Esters can also conjugate on the side of the single-bonded oxygen:

Apparently this form of conjugation interferes with resonance of the carbonyl group, hence increasing the arbsorption frequency of the C=O bond.

α-keto esters: One might expect two C=O peaks due to the two different carbonyls. In practice, it usually manifests as a shoulder on the main C=O peak or a single broadened band.

β-keto esters: These give a strong intensity doublet for the C=O stretches. Since they do not tautomerize to the same extent as β-diketones, one cannot generally observe the OH stretch from the enol form.

The –C–CO 2R stretch characteristic of an ester is visible in the isopentyl acetate IR spectrum in the 1735–1745 cm -1 range. The –C–H stretches are visible just below 3000 cm -1, and the –C–O and –CO 2 stretches appear as several peaks in the 1050–1300 cm -1 range.

The spectra of ethyl acetate and ethyl benzoate are shown above. Note that the C=O stretch of ethyl acetate (1752) is at a higher wavelength than that of the α, β-unsaturated ester ethyl benzoate (1726). Also note the C–O stretches in the region 1300-1000 cm^-1.