An electron spin resonance study of the photolysis of carboxylic acids, neat or in non-polar solvents: regiospecific loss of a hydrogen atom from the α-position

Abstract

When carboxylic acids RCH₂CO₂H (R = H, Me, Et or Pr) are photolysed, either neat or in carbon tetrachloride, the e.s.r. spectra of only the radicals RĊHO₂H are observed. Isobutyric acid, Me₂CHCO₂H, similarly shows the spectrum of the radical Me₂ĊCO₂H at room temperature, but at lower temperatures the spectrum of the radical Me₂CH· can also be observed. Pivalic acid, Me₃CCO₂H, shows only the spectrum of the Me₃C· radical. This is in contrast to the behaviour of the acids in polar solvents, when they normally show Norrish l cleavage of the alkyl–acyl bond, or photoreduction by the solvent. Two models are considered, to account for the regiospecific formation of the radicals RĊHCO₂H. The first assumes that cleavage of a C–OH bond in a hydrogen-bonded dimer yields an HO· radical which is hydrogen-bonded to a carbonyl group of the acid [RCH₂C(OH)=O ⋯ HO˙], and this species then undergoes regiospecific intramolecular transfer of α-H. The second assumes initial formation of the RCH₂˙ radical, which then abstracts hydrogen from a second molecule of acid.