Solvent effect correlations and the mechanism of solvolysis of alkyl chlorosulfates

Abstract

. Can. J. Chem. 55. 1401 (1977).The rate of hydrolysis of the series of alkyl chlorosulfates ROS0,Cl (R = ethyl, n-propyl, isobutyl, neopentyl) has been measured as a function of solvent conlposition in dioxane-mater media. The solvent effect is examined from the viewpoint of dielectric constant, sol~ation number, and solvent ionizing power. The results point to a contirluous variation of solvolytic mechanisms (from S,2, through borderline, to S,1) along the series. The correlations are also considered with respect to the 'entropy criterion' for multiple bond scission previously advanced for solvolysis of the alkyl chlorosulfates. Solute-solvent interactions are identified as a possible alternative origin of the abnormally large AS" values observed in chlorosulfate solvolysis. Although many discussions of solvent effects o n reaction rates of nucleophilic substitution processes have been reported, systematic investigations of reaction series are relatively few in number (1-5). Previously we reported on the mechanism of solvolysis of a series of primary alkyl chlorosulfates, ROS02Cl, in aqueous dioxane (6-8). A normal mode of solvolysis, that is bimolecular or unilnolecular displacement o n carbon with -OS02Cl as the leaving group, was indicated by (i) the effect of nucleophiles on the rate of hydrolysis of rz-propyl chlorosulfate; (ii) the observation of a large lyate ion effect (ko,-lk,,, = 380) for n-propyl chlorosulfate but an absence of a lyate ion effect (ko,-lk,,, bimolecular mechanisms, respectively, with N u as the nucleophilic species). However, the observation of 'abnormally' large entropies of activation suggested that ternary fission (fragmentation (9)) -as occurring, i.e. transition states 3 and 4 with both carbon-oxygen and sulfur-chlorine bond weakenings.Further work has been undertaken in order lo obtain additional information on the nature of the solvolytic mechanism of alkyl chlorosulfates, and t o shed light on the validity of the entropy criterion of fragmentation. The present paper describes the results of the application of solvent Can. J. Chem. Downloaded from www.nrcresearchpress.com by 54.191.190.102 on 05/10/18For personal use only.

Kinetics and mechanism of unimolecular heterolysis of cage-like compounds: XXI. Solvent effect on the realtive rate of heterolysis of 2-methyland 2-phenyl-2-haloadamantanes. Role of activation parameters

“…As a result, its solvation number increases by a factor of 2 to 3 [17,18]. Therefore, interaction of ion pair A solvate with a solvent void leads to its partial desolvation [5,19].…”