Relative Reactivity of Amines and Oxyanions toward Proton Abstraction from Nitroethane. Electrostatic Effects and the Reactivity-Selectivity Principle

Article abstract of DOI:10.1021/ja00332a034

Tertiary amines are more effective than primary and secondary amines which are more effective than oxyanions toward proton abstraction from nitroethane.The four homologous series of bases give rise to catalytic rate constants for nitroethane ionization that fall on three Broensted plots: for tertiary amines β=0.45, for primary and secondary amines β=0.60, and for oxyanions β=0.71.The increase in the Broensted β value with decreasing reactivity is in accord with the reactivity-selectivity principle.In nitroethane ionization a tertiary amine with conjugate acid of pK_a=10.0 is 13-fold and one of pK_a=6.2 is 130-fold more reactive than an oxyanion with a conjugate acid of the same pK_a.These are much smaller rate ratios than were observed in the enolization of oxaloacetate where tertiary amines were found to be 800-fold and 2200-fold more reactive than oxyanions of pK_a=10.0 and 6.2, respectively, giving further support to the previously proposed nucleophilic addition-elimination mechanism for tertiary amine catalyzed enolization of oxaloacetic acid.The greater reactivity of tertiary amines compared to oxyanions is ascribed to energy-lowering and energy-raising electrostatic interactions between the catalyst and the anionic charge developing on the substrate in the transition state.With primary and secondary amines the degree of transition-state electrostatic stabilization is decreased by solvation.Electrostatic stabilization can also be affected by certain functional groups in the amine.