Role of water and phase transfer catalysts in the kinetics of condensation of diaryl ketones with acetonitrile initiated by solid potassium hydroxide †

Abstract

The kinetics of reactions of benzophenone and substituted derivatives with acetonitrile in the presence of excess solid potassium hydroxide (KOH) to give 2,2-diarylacrylonitriles are reported. Reaction rates are strongly temperature dependent, and water, other cosolvents and quaternary ammonium salts all accelerate the reaction; also variation in substituents at one of the para-positions (from H to Cl, F and Me) gives a significant rate effect (ρ = 1.9). In contrast, variations in the amount of solid KOH pellets, the presence of crushed KOH, or an increase in the speed of stirring of the refluxing solution do not have significant kinetic effects. The results are consistent with a mechanism in which the anion [CH₂CN]^– is transported from the KOH surface before it reacts with the ketone. The rate of disappearance of benzophenone during most of the reactions is predominantly zero order, but variation in the initial concentrations of benzophenone shows that the initial rate is first order in benzophenone; water formed during the reaction does not remain in the acetonitrile (Karl Fischer titration), and the increasingly moist KOH further catalyses the reaction, so the observation of zero order kinetics is fortuitous. The role of solvents as phase transfer catalysts is discussed.

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