Stabilities and Reactivities of Carbocations

This chapter examines the various aspects of stabilities and reactivities of carbocations. The choice of an equilibrium constant for measuring the stability of a carbocation depends partly on experimental accessibility and partly on the choice of solvent. A desire to relate measurements to the majority of existing equilibrium constants implies the use of water as a solvent. The majority of equilibrium constants measured for carbocation formation refers to the ionization of alcohols or alkenes in acidic aqueous solution and corresponds to pKR or pKa. The application of kinetic methods for determining pKa and pKR for carbocations, by combining rate constants for their formation from an alcohol or alkene with a rate constant for the reverse reaction of the carbocation with water, has provided the most important development in the measurements of these equilibrium constants. Oxygen substitution has a radical effect on the stability of a carbocation, which is manifested in the chemistry of carbohydrates, benzopyran pigments, and the extensive acid-dependent reactions of carbonyl compounds. The greatest effects are from α-oxygen substituents, but the effects of substituents in the aromatic ring of benzylic carbocations are also large. © 2010 Elsevier Ltd