Base-Catalyzed Reactions of Triarylmethyl Carbocations with Water

Article abstract of DOI:10.1021/ja00312a067

The general base catalyzed addition of water to a series of triarylmethyl carbocations has been studied in aqueous solution with a series of substituted quinuclidines as base catalysts.Broensted β values for general base catalysis range from 0.33 for trianisylmethyl carbocation, TAM (pK_R(1+)=0.82), to 0.52 for Malachite Green, MG (pK_R(1+)=6.94).There is a slight tendency for the β values to decrease with decreasing carbocation stability.The rate constants for the "water"-catalyzed reactions show a large positive deviation (800-fold in the case of Malachite Green) and the rate constants for the hydroxide ion catalyzed reactions a small negative deviation from Broensted plots based on quinuclidine catalysis.These results are consistent with a concerted mechanism for the buffer-catalyzed reactions, a stepwise mechanism that proceeds through an oxonium ion intermediate for the "water"-catalyzed reactions (this mechanism corresponds to an A-1 mechanism in the microscopic reverse direction), and either a concerted or preassociation mechanism for the hydroxide ion catalyzed reactions.The positive deviation of water and the negative deviation of the hydroxide ion from the Broensted relationship are opposite the deviations that are normally observed for these species in related reactions: compared to these related reactions, the pH range where buffer catalysis is strongest is shifted to higher pH values.These results also explain why the early attempts to find general base catalysis of the addition of water to triarylmethyl carbocations with acetate buffers proved unsuccessful.