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

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.

Phase transfer catalysis: Chemistry and engineering

“…However, in some cases, it might be important to consider the change in the solid-phase composition with reaction. Esikova and Yufit (1991a) explain the product hindrance as being due to the binding of the quat species Q'X--with the product M + Xleading to unproductive binding. On the other hand, adsorption complexes of Q ' X -with M ' Xlead to active catalysis or productive binding.…”

‘Reverse Micellar Aggregates’ in Phase Transfer Catalyzed Nucleophilic Displacement Reactions

“…Alternative explanations for the action of PTC's involve surface reactivity, and interfacial reactivity . In the former, reaction occurs via the adsorption of the organic substrate and the PTC on the insoluble solid reactant (usually an inorganic salt).…”

Solid−Liquid Phase-Transfer-Catalysed Methoxylation of Chloronitrobenzenes to Nitroanisoles

“…Naik and Doraiswamy have proposed different models of S−L PTC. There are other mechanistic aspects of S−L PTC discussed in some detail. , However, there is no information on the process development and kinetics of the preparation of nitroanisoles under S−L PTC. This paper deals with these aspects.…”