Cleavage of X·C6H4·(CC)n·GeEt3compounds by aqueous methanolic perchloric acid. A remarkable constancy of the balance of inductive and resonance effects of substituents in reactions involving carbonium ion formation

Abstract

We have measured spectrophotometrically the rates of cleavage by aqueous methanolic perchloric acid of the compounds X·C6H4·(CC)n·GeEt3 in which n= 2 and 3 and X =p-OMe, m-Me, H, p-F, m-Br, and p-NO2. Good linear free-energy correlations are found between the effects of substituents X in these reactions and those involving the corresponding compounds with n= 1 and 0, which implies constancy of the balance between the inductive and resonance effects of the X groups in spite of the varying distance and efficiency of conjugation between the substituent site and the reaction centre. It is suggested that the effects of the substituents are exerted very largely on the positive charge delocalized into the aromatic ring in the transition state from the forming side-chain carbonium ion.The relative rates of cleavage of Ph·(CC)n·GeEt3 compounds are as follows: n= 0, ca. 1; n= 1, 3100; n= 2, 13; n= 3, 1.