Specific salt effects upon the rates of SN1 solvolyses

Abstract

Specific kinetic salt effects upon the solvolyses of ferf-butyl bromide, 1-and 2-methyl-exo-2-chloronorbornane, isobornyl chloride, and camphene hydrochloride have been examined in methanol, aqueous methanol, acetone, 1,2-dimethoxyethane, and methanol-1,2-dimethoxyethane. Anion effects are important but cation effects are small (for Li+, Na+, and Et,N+). The anion order is CIO,-> OTos-» NO.i-» Br-> Cl-» no salt > F-> OH-. Isotopic and azide trapping experiments show that carbonium ions or ion pairs can return in solvolyses of camphene hydrochloride and ieri-butyl chloride, but return is not large enough to explain the salt effects. This conclusion is supported by the observation of specific salt effects upon solvolyses of isobornyl chloride and l-methyl-e$o-2-chloronorbornane. Retention of configuration in the methanolysis of isobornyl chloride and camphene hydrochloride shows that methyl or hydride shifts do not occur during the lifetime of the carbonium ions. Experiments on isobornyl chloride in aqueous methanol and acetone show that chloride and perchlorate ions have little effect upon the activity coefficient of the substrate. The transition state effects appear to be related, at least in part, to solvent structure induced interactions between the carbonium-like transition state, especially with a large anion such as perchlorate.

The ion-pair mechanism and bimolecular displacement at saturated carbon. VI. Racemization and radio-bromide exchange for substituted 1-phenylbromoethanes; solvent effects

“…In acetone at 30°C, for example, where any unimolecular component was very small, the second order rate constant for the 4'-methyl compound was slightly more than double that of the unsubstituted compound, in acetonitrile at 45°C it is triple. Still, as Bunton et al have shown, there is most likely a positive salt effect on the unimolecular component, a contribution which probably increases linearly with Bu4NBr concentration (15). Just how large a fraction of the k2 value should be assigned to that salt effect remains to be established.…”

Concentrated salt effects on solvolysis rates in 1,4-dioxane–H2O mixed solvent and the expansion of the use of the Hammett equation for salt effects

“…In previous studies, 8, 9 we have observed that, for a typical S N 1 substrate, such as 1-adamantyl halides or tert-butyl chloride, cation effects increase as Et 4 N ϩ Ӷ Li ϩ , Na ϩ < Mg 2ϩ , Ba 2ϩ as perchlorate salts, in methanol-H 2 O and acetone-H 2 O mixed solvents. We have pointed out that the cation effect proposed by Bunton et al 6 would have been reduced by the interaction between M ϩ (the metal ions) and the anions (except for ClO 4 Ϫ ) in modified media in the presence of concentrated salts. For the incredibly strong interaction between the metal ions and simple anions in "aqueous" solutions, we have proposed a model for water containing highly concentrated salts (cf., ref.…”

“…It is known that the rates of S N 1 solvolysis of alkyl halides and related compounds are subject to positive salt effects. 5 Bunton et al 6 reported an exponential increase in the solvolysis rate of tert-butyl bromide with increasing concentration of LiClO 4 . They concluded that anion effects are important but cation effects are small (for Li ϩ , Na ϩ and Et 4 N ϩ ) and that the order of the effect of the anions is ClO 4 Ϫ > OTs Ϫ ÑO 3 Ϫ ~Br Ϫ > Cl Ϫ ~no salt > F Ϫ > OH Ϫ .…”

The SN1 hydrolysis of isothioureas. 2

“…Selenium is larger and more polarizable than sulfur, and it is reasonable to assume that energetically it should be more difficult for selenourea to depart from the developing carbonium ion. This greater energy requirement is seen in the 8 kcal/mol higher activation energy for the isoselenourea (5) over the equivalent isothiourea (8). initial solvation for 5, contributing to the high activation entropy when hydration water is released in the activated state.…”

“…JV-Methyl-Substituted 9-Anthrylmethylisothiourea Hydrochlorides (2)(3)(4). IV-Methyl, jV.TV-dimethyl, and N, N'-dimethyl analogues of 9-anthrylmethylisothiourea hydrochloride were synthesized in a manner similar to 9-anthrylmethylisothiourea hydrochloride (8) reported in our previous paper. 1 The appropriate N-substituted thiourea was used in place of thiourea.…”