HYDROLYSIS OF ORTHOCARBONATES - EVIDENCE FOR CHARGE IMBALANCE IN THE TRANSITION-STATE FOR THE GENERAL ACID-CATALYZED PROCESS

Catalytic constants have been measured for the hydrolysis of a range o f aryl orthocarbonates, in which both the leaving group and trioxocarb enium ion moiety have been systematically varied, by neutral carboxyli c acids (trichloroacetic, difluoroacetic, dichloroacetic, malonic, chl oroacetic, and acetic) at 70.0 degrees C in 60% water-40% acetonitrile , I = 1.0 M (KCI). Curvature cannot be detected in Bronsted plots invo lving carboxylic acids only, but inclusion of the point for H3O+ sugge sts downward curvature (i.e. p(x) greater than or equal to 0). beta(lg ), Plots are curved downward (i.e. p(y') = -partial derivative beta(lg )/partial derivative pK(lg) > 0). Substitutent effects in the tris(ary loxy)carbenium ion fragment were quantitated by use of the experimenta l aqueous pK(a) values of the phenol (pK(rc)), rather than Hammett sig ma values, since this gave better correlations for the spontaneous rea ctions (Kandanarachchi, P.; Sinnott, M. L. J. Am. Chem. Sec., precedin g paper in this issue). Cross coefficients are large and not constant: p(xy') (measured as partial derivative alpha/partial derivative pK(lg ) rather than partial derivative beta(lg)/partial derivative pK(HA)) v aries from 0.26 for (PhOC+ to 0.16 for (pMeOC(6)H(4)OC+. Likewise, p(x y) (measured as -partial derivative alpha/partial derivative pK(rc) ra ther than -partial derivative beta(rc)/partial derivative partial deri vative pK(HA)) experiences large changes with the leaving group pK. Da ta to estimate p(yy') (<partial derivative>beta(rc)/partial derivative K-lg) are more limited, but it too changes with the pK(a) of the cata lyzing acid. These data indicate that a two-dimensional More O'Ferrall -Jencks diagram, with one axis representing both C-O cleavage and the ability of substituents in the tris(aryloxy)carbenium ion moiety to se nse positive charge development, is inadequate to represent this react ion: separate axes are required for carbon-oxygen bond cleavage and de velopment of carbon-oxygen double-bond character.