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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

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.