Solvolytic studies of some organic chlorides and fluorides

Abstract

Rates, (with mass-law constants where appropriate), and activation parameters have been determined for the reactions of triphenylmothyl chloride, its p-nitro derivative, the corresponding fluorides and methoxymethyl chloride with aqueous acetone. Thus information has been obtained on the effects of replacing chlorine by fluorine as leaving group in S(_N)1 reactions. The rapid S(_N)1 reactions of the triphenylraethyl chlorides with 85% aqueous acetone at low temperatures showed the expected large 1mass-law effects. Mass-law constants and rates of ionisation at zero ionic strength were obtained for each experimental temperature, account being taken of the influence of changing ionic strength during reaction. The substituent effects on the rate of ionisation and the activation parameters, attending the introduction of the p-nitrogroup into triphenylmethyl chloride, were consistent with earlier results for diphenylmethyl chlorides, (^2) although both reactions showed extremely negative entropies of activation. The relatively slow reactions of triphenylraethyl fluoride and its p-nitro derivatives with 70% aqueous acetone have been studied. As in the case of the chlorides, the p-nitro group reduced the rate almost entirely though an increase in the energy of activation. The entropies of activation when compared to those obtained for the chlorides provide support for the hypothesis (^3) that the transition states for the reactions of the fluorides are stabilised by hydrogen bonding between a water molecule and the incipient fluoride ion. The ΔC(^+)(_p)/ΔS(^+) ratio for the hydrolysis of triphenylmethyl fluoride can also be interpreted in terms of this hypothesis. Contrary to expectations,(^4) several features of the reaction of methoxyraethyl chloride suggested that the reaction does not proceed by mechanism 8(_N)1. The observed values of the entropy and heat capacity of activation were consistent with a transition state involving covalent attachment of a water molecule(^5). Further, the reaction although rapidv/as not subject to mass-law effects, indicating that the carbonium ion(Me(^+)OCH(_2)←→MeO(^+)=GH(_2)) is not a reaction intermediate. Thus as the purpose of the present work was to study the effect of a change in leaving group on S(_N)1 reactions, no attempt was made to prepare methoxymethyl fluoride.