NMR and Mass Spectroscopy of Carboxylic Acids

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.