A kinetic study of the dehydration of 9-hydroxy-10-methyl-c/s-decalone-2 (1) to form 10-methyl-A1 11'9octalone (2) in aqueous solution has been made using a variety of primary and secondary amines as catalysts. Evidence, including a large primary kinetic isotope effect with appropriately deuterated 1, has been found for catalysis involving rate-limiting -proton abstraction by general bases from an iminium derived from 1. When the general base is another molecule of the imine-forming amine, the bimolecular rate constant (Aab) varies less than a factor of 2 over an amine basicity range of 106. This invariance results from a balancing of two effects: an increase in the effectiveness of the amine as a general base for -proton abstraction with increasing (ß -0.5), and a decrease in the susceptibility of iminium ions to deprotonation with increasing pAa. These findings lead to the conclusion that a nucleophilic amine catalyst for a deprotonation will be most effective when pH = pAaN = pAaB, where pAaN is the pAa of the imine-forming amine and pAaB is the pAa of the general base. The fact that this condition for maximum catalytic effectiveness can be met at neutral pH elucidates why many enzymes have evolved which operate via iminium ion formation. The maximum rate of catalysis for an enzyme constrained to operate with pH = pAaN = pAaB is estimated and compared with experimental values.

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In order to establish the relative effectiveness of a variety of amines as catalysts for direct abstraction of a proton from a carbon acid with pJTa ~20, in the absence of possible competing nucleophilic catalysis via aminecarbonyl condensation, a kinetic study of amine-catalyzed elimination of hydrogen chloride from 9-fluorenylmethyl chloride (1) to form di'benzofialvene (2) in aqueous solution has been made. General base catalysis is observed, as expected, and the data clearly define three distinct Brpnsted relationships with ß = 0.5 for primary, secondary, and tertiary amines. Negative deviations from the Brpnsted lines are observed for the sterically hindered triethylamine and A-methylmorpholine, and for imidazole and A-methylimidazole.

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A kinetic study of the dehydration of 9-hydroxy-10-methyl-c/í-decalone-2 (1) to form lO-methyl-1 11•9octalone-2 (2) in aqueous solution has been made using hydroxide ion, hydronium ion, and a variety of tertiary amines as catalysts. Nonlinear dependence of k0b,d on catalyst concentration was found with hydroxide ion and unprotonated amines. For each catalyst a larger initial slope (fcB¡, A'ohO, proportional only to free base, was found at low catalyst concentrations, and a smaller final slope (fcBf, k0hO, also proportional to free base, was found at higher catalyst concentrations. The transition from initial to final slope occurred for all bases at ké = 8.1 X 10-7 sec-1. With lower pX, tertiary amines, additional terms in the final slope proportional to protonated amine