13.8: NMR and Mass Spectroscopy of Carboxylic Acids
In ¹H NMR spectroscopy, acidic protons (–COOH) of carboxylic acids are highly deshielded and absorb far downfield, at around 9–12 ppm. The chemical shift value depends on the concentration and solvent used.
While α protons of carboxylic acids absorb at 2–2.5 ppm, β protons absorb further upfield.
Carboxylic acids are easily identified by dissolving them in deuterium oxide, which results in a rapid exchange of the acidic protons with deuterium. This leads to the disappearance of the acidic proton signal in the spectrum.
In 13C NMR spectroscopy of carboxylic acids, carbonyl carbons absorb around 160–180 ppm. However, they absorb at a lower chemical shift (or at a higher field) than the aldehydes and ketones’ carbonyl carbons. It is due to the high shielding effect of the carboxylate oxygen’s unshared pair of electrons. α carbons of carboxylic acids absorb at 20–40 ppm.
In mass spectroscopy, carboxylic acids show a small molecular ion peak. McLafferty fragmentation of carboxylic acids yields even-numbered mass fragment ions (base peak) with the loss of an alkene. Another prominent fragmentation happens with the loss of an alkyl radical, yielding a resonance-stabilized cation (has odd mass). Aromatic carboxylic acids give prominent fragment ion peaks with the loss of –OH and –C=O, apart from the molecular ion peaks.