When Does Hammond's Postulate Predict Stabilities of Carbocations?

Abstract

AbstractHammond's postulate, which relates the rate of formation of an unstable intermediate to the energy of this intermediate, has widely been used to determine stabilities of carbocations. However, the rates of formation of highly stabilized carbocations R+ by ionization of R−X are only weakly or not at all correlated with their stabilities, because the corresponding activation energies are strongly affected by intrinsic barriers. In this article we will show that consideration of the rate of the reverse reaction allows one to define the range in which Hammond's postulate holds and when its predictive power fades as the role of intrinsic barriers is gaining importance. The ambiguity of the term “carbocation stability” is discussed. Kinetic data and quantum chemical calculations show that vinyl cations are not extraordinarily high energy intermediates, but that high intrinsic barriers account for their slow formation from vinyl halides and their rather slow reactions with nucleophiles.