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Carboxylic acid infrared spectra

The obvious way to recognize an acid is by noting both a OH and a C=O stretch. The C=O stretch for the monomer is at 1760-1730, but carboxylic acids form dimers even in dilute solutions, the dimer C=O stretch is at 1730-1700. As usual for carbonyls, conjugation lowers it and a halogen on the α-carbon increases it.

The C-O stretch for an acid is near 1260.

O-H out-of-plane bending produces a broad, medium-weak band around 930.

Isobutyric acid

Benzoic acid

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