Reactivities of organic poly-halogen compounds

Abstract

Two recent investigations have shown that an x-chlorine substituent increases S(_N)1 reactivity, but analysis of the trends in E(_A) and (_Δ)S* values in each case, shows that serious discrepancies exist between them. In view of this it was considered that the role of an x-chlorine substituent was still uncertain and it was decided to look into the matter further. Towards this end, the solvolyses of benzyl chloride, benzal chloride, benzotrichloride, diphenylmethyl chloride and diphenylmethylene chloride have been examined in various solvents, over as wide a range of temperatures as possible, consistent with accurate results, it being considered that the mechanism of solvolysis was S(_N)1 in all cases, with the possible exception of benzyl chloride. Analysis of the E(_A) and ΔS* values for the solvolyses of these compounds shows, in the first place, that the sol volysis of benzyl chloride is not straight forward S(_N)1. Its mechanism of solvolysis is discussed in chapter VII where it is concluded that it proceeds by a single, predominantly S(_N)2 mechanism. Further analysis of the E(_A) and ΔS* values (and in some cases AC*(_p) values) of the other compounds, in chapter VI, indicates that the primary role of an x-chlorine substituent in S(_N)1 reactions is electron release, although, as a second order effect it appears to cause an increase in the extent of solvation of the transition state. The disturbing effects in the hydrolysis of diphenyl methylene chloride in aqueous acetone are analysed in part II where it is shown that they can be quantitatively explained, over a wide range of experimental conditions, on the basis of the mass law and ionic strength effects, proposed by Hughes and Ingold, in S(_N)1 reactions. The exceptionally large mass law effect for diphenyl methylene chloride is ascribed to powerful electron release by the x-chlorine atom, so confirming the conclusions reached in part I.