KINETIC SOLVENT EFFECT ON HYDROLYSIS OF T-BUTYLCHLORIDE IN AQUEOUS MIXTURES OF SOME PROTIC, APROTIC AND DIPOLAR APROTIC COSOLVENTS IN THE LIGHT OF INITIAL AND TRANSITION-STATE SOLVATION ENERGETICS

Kinetic solvent effects (KSE) on a key S(N1) type reaction, such as hy drolysis/solvolysis of t-butylchloride (t-BuCl) in aqueous mixtures of some protic cosolvents, viz. methoxyethanol (ME), ethylene glycol (EG ) and glycerol (GL), aprotic cosolvents, viz. 1,2-dimethoxyethane (DME ) and dioxane (D) and dipolar aprotic cosolvents, like N,N-dimethylfor mamide (DMF) and dimethylsulphoxide (DMSO), have been analysed in the light of solvation energetics of the initial state (IS) and transition state (TS). The required rate constants (k(s)) at different temperatu res have been determined earlier by conductometric method and the rele vant activation parameters by use of the coefficients of the rate cons tant-temperature equation : log k(s) = AT-1 + B log T + C. Transfer en ergetics of IS and TS from water (w) to other solvents (s) were evalua ted by coupling the activation energetics (DELTAH(not-equal)) with the previously determined enthalpies of solution (DELTAH(s)oBAR) of IS : t-BuCl and by coupling the activation free energies (AG(not-equal)) wi th the solvation free energies of the ion-pair (CH3)3NH+Cl- a newly pr oposed model of the TS : (CH3)3C(delta+)...Cl(delta-) as obtained afte r due correction of the previously determined solvation free energies of the salt (CH3)3NHCl in the solvents by an indirect method. Each of the transfer energetic terms of IS and TS was dissected into cavity, d ipole-dipole, dispersion and the rest, hydrophobic-hydrophilic (HH) hy dration terms. The latter is the composite contributions of hydrophobi c hydration (H(b)H) arising from 3(CH3) groups of partially charged (C H3)3C(delta+)-group and the hydrophilic hydration (H1H) arising from p artially charged Cl(delta-) atom of the TS : (CH3)3C(delta+)...Cl(delt a+) of t-BuCl. DELTAX(t,HH)o(i) - and particularly TDELTAS(t,HH)o(i)-c omposition profiles (X = G, H and S and i = IS or TS) were then analys ed in terms of four-step transfer process of Kundu et al. and the invo lved physicochemical properties and especially the relative structured ness of solvents, which guide the HH hydration terms. This helped to u nderstand the observed unique minima in TDELTAS(not-equal) - and hance DELTAH(not-equal)-composition profiles in these and other aquo-organi c solvents, which proved baffling since the days of the classical work of Ingold and Hughes, so to say on this key S(N1)-type reaction.