Role of solvent in alkyl substituent effects

Abstract

From the energies of electronic transitions that place a high electron demand on alkyl substituents (e.g., the “principal” transition of p-alkylnitrobenzenes in the gas phase) it is concluded that the inherent order of electron release is: Bu t > Pr i > Et > Me. For RCH 2 groups larger spreads are found, in the release order: Pe neo > Bu i > Pr > Et > Me. A Baker-Nathan effect has been found in the principal electron-transition energies of p-alkylnitrobenzenes and p-alkylacetophenones in basic solvents. This effect is attributed to steric hindrance to solvation of the electron-deficient aromatic ring in the vicinity of the alkyl substituent. In electronic transitions of the type in which a substituent is called upon strongly to accept negative charge (e.g., the “principal” transition of p-alkylphenols in the gas phase) it has been found that alkyl groups are apparently better electron acceptors than a hydrogen. Rate constants and heats and entropies of activation obtained in the solvolysis of 3,5-dimethyl- and 3,5-di- tert.-butyl-benzhydryl chlorides are compared with corresponding results on mono m- and p-alkylbenzhydryl chlorides. The assumption that the Baker-Nathan effect in this system is due to a CH hyperconjugative order of electron release by alkyl groups does not satisfactorily account for the results. On the other hand, the results are completely consistent with the viewpoint that the Baker-Nathan effect is due to steric hindrance to solvation of electron-deficient sites in the near vicinity of the alkyl substituent.