Kinetics of Hydroxide and Nitromethide Ion Addition to Substituted β-Nitrostyrenes in Me2SO-Water Mixtures. Solvent Dependence of ?-Donor Substituent Effects and of Intrinsic Rate Constants.

Article abstract of DOI:10.1021/jo00030a019

The kinetics of nucleophilic addition of hydroxide (k₁^OH) ion and nitromethide (k₁^NM) ion to phenyl-substituted β-nitrostyrenes (Z = 4-Me2N, 4-MeO, H, 3-Cl, 4-Br, 4-CN, and 4-NO2) have been measured in water, 50percent Me2SO-50percent water, 70percent Me2SO-30percent water ((OH(1-) reaction only), and 90percent Me2SO-10percent water.Equilibrium constants, K_eq, for the addition of piperidine to the same β-nitrostyrenes, to form ArCH(NC5H10)CH=NO2(1-) + H(1+), have also been determined.The latter were measured in lieu of the equilibrium constants of hydroxide ion addition (K₁^OH) which are experimentally difficult to come by; they are used to model the substituent dependence of K₁^OH.Hammett plots of log k₁^OH and log k₁^NM 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 Broensted-type plot of log k₁^OH vs log K_eq in water, while a similar plot in 90percent Me2SO-10percent 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 ?-donor-substituted β-nitrostyrenes which also depresses the equilibrium constants for these substrates.The positive deviations from the Hammett and Broensted-type plots are attributed to a preorganization of the electronic structure towards that of the product, induced by the through resonance that occurs with ?-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.