General Base Catalysis of the Addition of Hydroxylic Reagents to Unstable Carbocations and Its Disappearance

General base catalysis of the addition of ROH is significant for 1-phenylethyl carbocations of moderate stability and is most important for weakly basic alcohols. The Brønsted slope for catalysis of the addition of trifluoroethanol to the l-(4-methoxyphenyl)ethyl carbocation is β = 0.08. The development of positive charge on the attacking alcohol decreases with decreasing stability of the carbocation; this may be described by the interaction coefficient P_yy = ∂β_nuc/−∂σ. The positive P_yy coefficient and small β correspond to a small component of proton transfer in the transition state for alcohol addition; in contrast, acetate catalysis of ROH addition to more stable electrophiles exhibits a negative P_yy coefficient that corresponds to a large component of proton transfer and a diagonal reaction coordinate on an energy contour diagram. It is concluded that the observed catalysis involves hydrogen bonding and represents the transition region between fully concerted, coupled general acid-base catalysis and specific acid catalysis with solvation of the proton in the transition state. Catalysis disappears with less stable cations when reaction with solvent and diffusion become competitive, with k_s ∼ 10⁹ s⁻¹, and the rate constant for base catalysis approaches a limiting value of ∼ 10⁷ M⁻² s⁻¹. The absence of general base catalysis of alcohol addition corresponds to specific acid catalysis of alcohol expulsion to form an unstable carbocation in the reverse direction.