Solvolysis isopropyl sulfonate

The methyl sulfonates and ethyl halides are on the Sn2 side of the borderline, so /ch/ d is nearer unity. The ethyl sulfonates are nearer SnI and the isopropyl halides still more so, so fcn is increasingly larger than ku. When, with the isopropyl sulfonates, the mechanism has become essentially dissociative, we get an isotope effect (ftn/fen = 1.13 — 1.14) characteristic of SnI solvolysis in non-aqueous media (cf. Table VIII). 

The required investigation in mixed solvents has yet to be carried out, but /3-effects in solvent water, at the extreme end of the scale, have been reported by Leffek et al. (150). Their results, shown in Table XI, are quite compatible with the considerations of See. VA, 2(c) in which water solvolyses were classified as essentially SnI, borderline, or Sn2 on the basis of the a-isotope effect. Solvolysis of terf-butyl chloride, which is certainly dissociative, shows an effect close to that in non-aqueous media. The isopropyl sulfonates, which we concluded also solvolyze by an essentially SnI mechanism, are subject to effects almost as large whereas those on solvolysis of the isopropyl halides. 

Secondary isotope effects, as we have seen particularly in cormec-tion with eqs. (III-9) and (III-15), are regarded as being predomi nantly zero-point enei y effects. It is thus something of a shock to learn, from the careful investigation of the temperature dependence of the j3-effect on water solvolysis of the isopropyl sulfonates and bromide, performed by Leffek et al. (151), that virtually all of the effect is on the entropy of activation. At 50 C., AAF /n = 46, 47, and 29 cal., respectively, for the tosylate, methanesulfonate, and bromide, of which 45, 50, and 35 cal. are due to This is temperature... 

These effects, determined with tertiary chlorides, unquestionably refer to 8n1 reactions so also do Lewis and Boozer s effects on acetolysis and formolysis—-if we can rely on the constancy of the effect of a-deuteration (Table VIII). The /3-effects then suggest that formolysis of the secondary bromide and solvolysis in aqueous ethanol of the tosylate are also SnI—or nearly so, unless bimolecular displacement reactions are also subject to similar isotope effects. Shiner (117) had however already shown that this was not so, since deuteration in the two methyl groups of isopropyl bromide did not lead to an experimentally significant effect on the displacement reaction with ethoxide ion in ethanol. It is thus also reasonable to interpret the very small effect (AAF" = 6 cal.) cited by Lewis (74f) for acetolysis of ethyl-2d8 brosylate as evidence that acetolysis of primary sulfonate esters is borderline if not Sn2. This conclusion, already suggested by the abnormally low a-effect for unassisted acetolysis of phenylethyl tosylate [Table VIII and text of Sec. VA, 2(b) 1 is supported by a similarly... 

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