Infra-red spectra of the benzene carboxylic acids

Abstract

Abstract The infra-red absorption spectra of the 12 benzene carboxylic acids and their 4 hydrates have been measured between 2–15 μ as mulls in Nujol. The spectra have been analysed and the principal bands related to particular molecular groupings. The completeness of this series of spectra makes it interesting to study some correlations between molecular structure and infra-red spectra, in particular (a) COOH bands and (b) vibrations associated with the benzene system as a function of the type of substitution. The spectra of the 12 anhydrous acids do not exhibit any free O-H absorption, indicating that there exists in the acids a high inter- and intra-molecular association. The spectra of the hydrates show these compounds to have hydrogen bonds between the CO groups and the water of hydration. Examination of the spectra shows that dimeric COOH gives rise to 5 bands between 2 and 15 μ (5000–665 cm −1 ). The COOH band near 900 cm −1 disappears in the hydrates. This seems to confirm that association in the latter takes place through the water molecules and, also, that it appears advisable to assign this absorption to the out-of-plane O-H deformation vibration in the dimeric ring. This series of acids constitutes an example where existing correlations for the determination of the type of benzene substitution do not apply in full. The electrophylic COOH group is regarded as responsible for the deviations. The spectra are highly specific and make possible a ready characterization of any acid. This should prove useful in any analytical work involving these acids, particularly the higher members.