Transition state enthalpies of transfer

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1971

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Abstract

Rates of nucleophilic displacement of tosyl ate from n-hexyl tosylate by azide, chloride, bromide, iodide and thiocyanate ions have been measured in methanol and dimethyl sulfoxide at three different temperatures. The activation parameters E[lowered a], [delta]H[plus-plus], [delta]S[plus-plus], and [delta]G[plus-plus] have been calculated. Transition state enthalpies of transfer, [delta][delta]H[lowered s], for the displacement of tosylate from n-hexyl tosyl ate by nucleophilic anions have been obtained from the appropriate enthalpies of transfer of reactants, aaHs, and difference in the enthalpies of activation of the reaction, [delta][delta]H[plus-plus], in the two solvents, methanol and dimethyl sulfoxide. The results show that the lower activation enthalpies, [delta]H[plus-plus], and faster rates of this reaction in dimethyl sulfoxide are due mainly to desolvation of the reactant anion in the case of Cl[raised -] and by stronger solvation of the transition state in the case of I[raised -]. The transition state of N[lowered 3, raised -] and SCN[raised -] appear to be only slightly more solvated by methanol while that of Br[raised -] is only slightly more solvated by dimethyl sulfoxide.

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