Free-radical decarboxylation of carboxylic acids as a concerted abstraction and fragmentation reaction

Two variants of the reaction of radicals with the carboxyl group of carboxylic acids, namely, RO (2) (aEuro cent) + R (i) COOH -> ROOH + R (i) CO (2) (aEuro cent) and RO (i) (aEuro cent) + R (j) COOH -> ROOH + R (i) (aEuro cent) + CO2 are theoretically analyzed. It is demonstrated by the intersecting-parabolas method that if the reaction proceeded via the formation of an intermediate carboxyl radical, it would be much slower than is actually observed. Quantum-chemical calculations carried out by the density functional method using the nonempirical functional PBE have shown that the reactions of the methyl radical with the carboxyl group of acetic, butyric and vinylacetic acids include concerted H atom abstraction and C-C bond breaking. In the framework of the intersecting-parabolas model, an algorithm has been developed to calculate the activation energy and rate constant for X-aEuro cent + R (i) COOH -> XH + CO2 + R (i) (aEuro cent) reactions, where X = R-aEuro cent, ROaEuro cent, HOaEuro cent, ArOaEuro cent, Ar2NaEuro cent or H-aEuro cent.