IR and UV–Vis Spectroscopy of Carboxylic Acids

Overview

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 cm⁻¹, while H-bonded carboxylic acids (dimers) exhibit stretching absorptions at a lower frequency, 1710 cm⁻¹. The C=O bond of the conjugated aliphatic and aromatic carboxylic acids shows an even lower stretching frequency at 1690 cm⁻¹.

In IR spectra of carboxylic acids, the stretching absorptions for the C–H and O–H bonds overlap and exhibit absorption around 3000 cm⁻¹, 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 cm⁻¹.

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.

Procedure

Carboxylic acids are easily detected via IR spectroscopy by identifying two characteristic absorptions formed from C=O and O–H stretching of the –COOH group.

In general, the –C=O group of a monomeric carboxylic acid absorbs at 1760 cm⁻¹. However, dimerization due to hydrogen bonding lowers the stretching frequency to ∼1710 cm⁻¹.

In conjugated systems, as resonance delocalization weakens the C–O double bond, the stretching frequency is further lowered to 1690 cm⁻¹.

Additionally, the O–H bond stretching of the –COOH group generates a broad band at ∼2500–3500 cm⁻¹, masking the C–H stretching peaks.

In UV–Vis spectroscopy, carboxylic acids absorb UV light in the range of 200–215 nm, which corresponds to an n→π∗ transition. It is a weak band with a molar absorptivity of <100.

However, the presence of an extensive conjugation shifts the absorption maxima to a higher wavelength with increased molar absorptivity of 25,000.