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13.5: IR and UV–Vis Spectroscopy of Carboxylic Acids

JoVE Core
Organic Chemistry

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IR and UV–Vis Spectroscopy of Carboxylic Acids

13.5: IR and UV–Vis Spectroscopy of Carboxylic Acids

In IR spectroscopy of carboxylic acids, the C=O bond shows a characteristic band between 1710 and 1760 cm⁻¹, and the O–H bond exhibits a broad band between 2500 and 3300 cm⁻¹.

However, the stretching absorptions for the C=O bond vary depending on the structure of carboxylic acids. The C=O bond of the free carboxylic acids shows a higher stretching frequency, 1760 cm1, while H-bonded carboxylic acids (dimers) exhibit stretching absorptions at a lower frequency, 1710 cm1. The C=O bond of the conjugated aliphatic and aromatic carboxylic acids shows an even lower stretching frequency at 1690 cm1.

In IR spectra of carboxylic acids, the stretching absorptions for the C–H and O–H bonds overlap and exhibit absorption around 3000 cm1, having sharp C–H peaks and a broad O–H peak. Many carboxylic acids also exhibit small spikes or shoulder bands between 2500 and  2700 cm1.

UV–vis spectroscopy of the carboxylic acids shows a weak band at a lower wavelength of 200–215 nm due to an n→π transition. However, in highly conjugated carboxylic acids, the n→π transition shifts to a higher wavelength with higher molar absorptivity.


Keywords: IR Spectroscopy UV-Vis Spectroscopy Carboxylic Acids C=O Bond O-H Bond H-bonded Carboxylic Acids Conjugated Carboxylic Acids N→π* Transition

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