Reinvestigation of the linear free energy relationship on the solvent effects in the solvolysis of arylalkyl tosylates

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The Winstein-Grunwald Equation, log (k/k_o) = mY + 1N, has been widely used as a mechanistic probe for analyzing the solvolytic processes. Despite the nucleophilically limiting nature of aryl-assisted and benzylic solvolyses, the solvent effect on these solvolyses failed to give a single linear correlation with the adamantyl Y_<OTs> parameter but showed characteristic dispersion for various binary aqueous solvent series. The pattern of dispersion can not be explained by simply Invoking the nucleophilic solvent assistance. Developing charge at the reaction center in the transition state of benzylic k_c carbocationic process is highly delocalized through effective PI-overlapping with aryl group, and this may cause a significant loss of the highly oriented specific solvation at the reaction center In comparison with that for aliphatic localized 2-adamantyl cation.Detailed examination of ionizing power scale most suitable to the aryl-assisted solvolysis as well as to the benzylic solvolyses, has been carried out in the comparison with the Y_<OTs> solvent scale based on the solvolysis of 2-adamantyl tosylate. The solvent polarity scale Y_<DELTA> based on p-methoxyneophyl solvolysis combined with the Y_<OTs> based on 2-adamantyl one has been found to show wide applicability to the solvolyses of benzylic arylalkyl tosylates involving varying degree of PI-delocalization interaction with the incipient carbocation charge ; log (k/k_o) = aY_<OTs> + bY_<DELTA>. Benzylic solvolyses can be treated to an extremely high precision R>0.995 and SD<0.15 with linear combination of Y_<OTs> and Y_<DELTA>. Application of revised equation, aY_<OTs> and bY_<DELTA> Including cN_<OTs> term has also been extended to the nucleophilic solvolyses of alkyl and arylalkyl tosylates, with limited success, indicating no precise additivity for the nucleophilic solvent involvement.