Spectroscopic Features of Carboxylic Acids

IR Spectroscopy

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Identifying carboxylic acids by IR is straightforward. There is always a C-O stretch, often at lower frequency (<1700 cm^-1). In addition, the O-H stretch appears as a distinctly broad band covering the 3500-2500 cm^-1 region. Note that the C-H bands will still be evident. Hydrogen bonding makes this peak very different from that for an alcohol; acid O-H peaks often have some degree of structure to them.

¹H NMR Spectroscopy

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The COOH protons are usually very broad, often to the point of being hard to see above the baseline. They usually appear downfield 10-12 ppm region; they are rarely any further upfield because the chemical environment removes electron density from around the proton.
The acid carbonyl will have an effect similar to that of a ketone on any alpha position protons; this example (benzoic acid) lacks such a feature. Load the spectrum of nonanoic acid to see some of these effects more clearly.

¹³C NMR Spectroscopy

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The carboxyl carbon appears at a characteristic shift between 160-185 ppm. It is often weak because of nOe effects on other carbons in a proton-decoupled spectrum.

Mass spectrometry