Formation of adsorption complexes in phase‐transfer nucleophilic substitution. Kinetic analysis of solid‐phase system

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Abstract
The kinetics of the nucleophilic substitution between n‐hexyl bromide and solid potassium chloride (KCl) in toluene were studied in the presence of catalytic amounts of tetra‐n‐butylammonium bromide at 84°C. This reaction is characterized by an induction period that disappears on activation of the solid salt. The substitution with the activated salt constitutes a reversible pseudo‐first‐order reaction with variable orders in the substrate and the catalyst equal to 0 ≤ n ≤ 1. A quantitative increase in the solid KCl at first accelerates the substitution reaction, but when the level reaches KCl/RBr ≥ 7 the rate constant is no longer influenced by the amount of salt. Comparing data obtained with kinetic equations for several possible substitution schemes, the mechanism of the reaction can be derived, including the formation of a ternary adsorption complex (TC) from the substrate, catalyst and solid salt. The data obtained allow the evaluation of the equilibrium constant of TC formation (KΣ= 20.48 l2 mol−2) and the rate constant of its disintegration into substitution products (k+ = 0.75 × 10−2 s−1). Through kinetic analysis the adsorption sequence on the surface of the solid salt was determined, including primary formation of the binary complex KCl.QX and subsequent TC formation. The concentration of active KCl molecules, estimated on the basis of the primary kinetic data is 10−2 M and is commensurate with the catalyst concentration. This concentration of active KCl molecules is achieved owing to the formation of subcolloidal‐sized particles in the process of the solid salt activation.

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