A kinetic study of the solvolysis of some diphenylmethyl chlorides

Abstract

A study has been made of the electronic effect of substituents upon the first-order rates of solvolysis of a number of formal derivatives of benzyl chloride. These solvolyses, which proceed through a unimolecular mechanism, were studied in a mixture of 90% ethanol and 10% acetone at various temperatures, allowing the energies of activation to be calculated. A good linear free-energy correlation was found using the Brown-Okamoto substituent constant, sigma+.

A comparison of substituent effects in the alpha-(2-fluorenyl)- and alpha-(4-biphenylyl)- systems shows that the transmission of electronic substituent effects across the aryl systems is enhanced by increased co-planarity of the aryl fragment. The relative effects of substituents upon the observed rate are proportional, but not identical, in the two systems. This proportionality factor may be linked with an interplanar angle of 45° in biphenyl.

The effect of inserting p-C₆H₄- the group between the substituent and the reaction centre is the same in the benzhydryl system (rho = -4.4), cf. -1.2 for the (alpha-Ci-biphenylyl)benzyl chlorides) and in the t-cumyl system, where rho = -1.21 for substituent effects in the solvolysis of alpha-(4-biphenylyl)-isopropyl chloride.

Transmission of substituent effects in the 2,6-dimethylbenzhydryl chlorides is less (rho = -3.6) than in the benzhydryl chlorides, allowing the estimation of a mean transmission coefficient. Using this, together with allowances for both the steric and the electronic effects of the two ortho-methyl groups, an attempt was made to calculate the various contributions of steric and electronic factors to the observed reaction rate. The average discrepancy between calculated and experimental values of the free energy of activation is +/- 0.13 kcal mole-1. No saturation of electronic effect occurs in those derivatives of 2,6-dimethyl-benzhydryl chloride studied here.

A general mechanism for the transmission of electronic effects of substituents across aryl systems is discussed.

Twenty-four new chlorides and their precursor have been synthesised; new routes have been investigated in the preparation of alpha-(6-substituted-2-fluorenyl)benzyl chlorides.