NMR and Mass Spectroscopy of Carboxylic Acids

The acidic –COOH protons of carboxylic acids are highly deshielded and appear far downfield at 9 to 12 ppm in 1H NMR spectra. In contrast, the shielded α protons appear between 2 to 2.5 ppm, while β protons absorb further upfield. The exact chemical shift depends on concentration and solvent used.

Carboxylic acids are easily identified by dissolving them in deuterium oxide, which causes rapid exchange of the acidic –COOH protons with deuterium. This exchange results in the disappearance of the corresponding proton signal in the 1H NMR spectrum, confirming the presence of a carboxylic acid functional group.

In 13C NMR spectroscopy, the carbonyl carbons of carboxylic acids absorb at approximately 160 to 180 ppm, appearing strongly deshielded. This range is lower than aldehydes and ketones due to the high shielding effect of the carboxylate oxygen's unshared pair of electrons. The α carbons absorb at 20 to 40 ppm.

The McLafferty rearrangement of aliphatic carboxylic acids produces the base peak, an even-numbered mass fragment ion formed by loss of an alkene. This fragmentation is characteristic of carboxylic acids and appears as the strongest peak in their mass spectra, alongside a smaller molecular ion peak and a second prominent peak from resonance-stabilized cations.

Aromatic carboxylic acids display intense molecular ion peaks followed by significant fragmentation peaks representing loss of –OH and –C=O groups. This contrasts with aliphatic carboxylic acids, which show significantly small molecular ion peaks and prominent base peaks from McLafferty rearrangement, reflecting the greater stability of aromatic molecular ions.

Loss of an alkyl radical from carboxylic acids generates the second strongest peak in their mass spectra, a resonance-stabilized cation with odd mass. This fragmentation is particularly prominent in aliphatic carboxylic acids and represents a major decomposition pathway alongside the McLafferty rearrangement.

Carboxylic acid carbonyl carbons absorb at lower chemical shifts (higher field) than aldehydes and ketones due to the high shielding effect of the carboxylate oxygen's unshared pair of electrons. This electron density shields the carbonyl carbon, reducing its deshielding and causing it to appear upfield relative to other carbonyl-containing compounds.