KINETICS OF HYDROXIDE AND NITROMETHIDE ION ADDITION TO SUBSTITUTED BETA-NITROSTYRENES IN ME2SO WATER MIXTURES - SOLVENT DEPENDENCE OF PI-DONOR SUBSTITUENT EFFECTS AND OF INTRINSIC RATE CONSTANTS

The kinetics of nucleophilic addition of hydroxide (k1OH) ion and nitromethide (k1NM) ion to phenyl-substituted beta-nitrostyrenes (Z = 4-Me2N, 4-MeO, H, 3-Cl, 4-Br, 4-CN, and 4-NO2) have been measured in water, 50% Me2SO-10% water. Equilibrium constants, K(eq), for the addition of piperidine to the same beta-nitrostyrenes, to form ArCH(NC5H10)CH = NO2- + H+, have also been determined. The latter were measured in lieu of the equilibrium constants of hydroxide ion addition (K1OH) which are experimentally difficult to come by; they are used to model the substituent dependence of K1OH. Hammett plots of log k1OH and log k1NM show positive deviations for Z = 4-Me2N in water, but increasingly strong negative deviations as the Me2SO content of the solvent increases. A similar trend is observed for Z = 4-MeO although the magnitude of the deviations, positive and negative, is much smaller. The 4-Me2N- and 4-MeO-derivatives also show positive deviations from a Bronsted-type plot of log k1OH vs log Keq in water, while a similar plot in 90% Me2SO-10% water may or may not show small negative deviations. The negative deviations from the Hammett plot are caused by the strong resonance stabilization of the pi-donor-substituted beta-nitrostyrenes which also depresses the equilibrium constants for these substrates. The positive deviations from the Hammett and Bronsted-type plots are attributed to a preorganization of the electronic structure towards that of the product, induced by the through resonance that occurs with pi-donor substituents. This preorganization facilitates the delocalization of the negative charge into the nitro group at the transition state and enhances the intrinsic rate constant (k0 = k when K = 1) of the reaction.