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
M. Datta et Kk. Kundu, 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, Journal of the Indian Chemical Society, 70(4-5), 1993, pp. 467-480
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.