Overview

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.

Procedure

In the ¹H NMR spectra of carboxylic acids, the highly deshielded –COOH protons appear far downfield—in the range of 9 to 12 ppm.

In contrast, the shielded α protons appear upfield—between 2 to 2.5 ppm, while the β protons appear further upfield.

In deuterated solvents, the exchange of the acidic –COOH proton with deuterium results in the disappearance of the corresponding proton signal, which serves as an identification of carboxylic acids.

The ¹³C NMR spectra of carboxylic acids show a strongly deshielded –C=O carbon peak at ~160 to 180 ppm, while the shielded α carbons absorb at ~20 to 40 ppm.  

The mass spectra of aliphatic carboxylic acids display a significantly small molecular ion peak, along with the base peak corresponding to a fragment formed by McLafferty rearrangement.

The second strongest peak is generated by a resonance-stabilized cation, formed by the loss of an alkyl radical.

Aromatic carboxylic acids show intense molecular ion peaks followed by other fragmentation peaks, signifying loss of –OH and –C=O groups.