Rates of base cleavage of X·C6H4·[CC]n·MEt3 compounds (n= 2 and 3, M = Si and Ge), and their significance for theories of substituent effects

Abstract

The rates of cleavage of X·C₆H₄·[CC]_n·MEt₃ compounds (n= 2 and 3, M = Si and Ge) by aqueous-methanolic alkali at 29·6 °C have been measured spectrophotometrically. The ease of cleavage increases as n is raised; e.g., for the C₆H₅·[CC]_n·SiEt₃ compounds with n= 1, 2, and 3 the approximate relative rates are 1 : 240 : 4100. Germanium compounds are less reactive than the corresponding silicon compounds.

The rates of cleavage of X·C₆H₄·[CC]₃·SiEt₃ compounds with X =p-OMe, H, p-F, m-Br, and p-NO₂ correlate well with the simple Hammett substituent constants, σ, which normally refer to reactions involving markedly smaller distances between the substituent X and the reaction site. This casts some doubt on the validity of treatments of substituent effects which involve separation into (a) field effects which depend only on the distance between the substituent site and the reaction site, and (b) resonance effects which depend on the effectiveness of conjugation between the sites. Substituent constants calculated according to one such treatment (that by Dewar and Grisdale) are less satisfactory than σ-constants for the cleavages studied.