IR and UV–Vis Spectroscopy of Carboxylic Acids

Carboxylic acids display two distinctive IR absorptions from the –COOH group. The C=O stretch appears between 1710–1760 cm⁻¹, while the O–H stretch produces a broad band at 2500–3500 cm⁻¹. This broad O–H band masks the C–H stretching peaks in the spectrum, making these two features diagnostic for identifying carboxylic acids.

Monomeric carboxylic acids show C=O absorption at 1760 cm⁻¹, but dimerization through hydrogen bonding lowers this frequency to approximately 1710 cm⁻¹. This shift occurs because hydrogen bonding weakens the C=O double bond, reducing its stretching frequency and creating a characteristic lower-frequency band in IR spectra.

In conjugated carboxylic acids, resonance delocalization weakens the C=O double bond character, lowering the C=O stretching frequency to approximately 1690 cm⁻¹. This further reduction compared to non-conjugated acids reflects the decreased bond strength caused by electron delocalization into the conjugated system.

Carboxylic acids absorb UV light at 200–215 nm due to an n→π∗ transition, producing a weak band with molar absorptivity below 100. However, extensive conjugation shifts the absorption maximum to higher wavelengths with dramatically increased molar absorptivity reaching 25,000, enabling detection of conjugated carboxylic acids.

Unconjugated carboxylic acids exhibit weak n→π∗ transitions at 200–215 nm with low molar absorptivity. In highly conjugated carboxylic acids, this transition shifts to longer wavelengths with substantially higher molar absorptivity, allowing conjugated systems to absorb more strongly and be detected more readily in UV–Vis spectroscopy.

IR spectroscopy identifies the –COOH functional group through characteristic C=O and O–H stretches, while UV–Vis spectroscopy detects the n→π∗ transition at 200–215 nm. Combined, these techniques confirm carboxylic acid identity and reveal structural features like conjugation, which shifts UV absorption to higher wavelengths and increases molar absorptivity.

Dimeric carboxylic acids stabilized by hydrogen bonding show a C=O stretch at 1710 cm⁻¹ rather than the 1760 cm⁻¹ observed in monomeric forms. Additionally, small spikes or shoulder bands between 2500–2700 cm⁻¹ in the IR spectrum often indicate hydrogen-bonded carboxylic acids, reflecting the structural changes from dimerization.