Arenediazonium salts: new probes of the interfacial compositions of association colloids. 1. Basic approach, methods, and illustrative applications

Abstract

Product yields from the reactions of two different arenediazonium salts, z-ArN₂⁺ BF₄.>^-, bound to cetyltri' methylammonium halide ((CTA)X; X=Cl Br) micelles, and 10 aqueous three-ccmponent (CTA)X microemulsions containing an alcohol (R'OH), either 1· butanol (BuOH) or 1-hexanol (HexOH), are "snapshots" of the relative quantities of halide ion, water, and alcohol nucleophiles at the aggregates' interfaces. Yields of aryl ether, aryl halide, and phenol products measured simultaneously by HPLC arc consistent with high concentrations of these nucleophiles in the immediate vicinity of the aggregates' interfaces. The Interfacial concentration of each nucleophiles is estimated from the yield of its respective product over wide ranges of (CTA)X and R'OH concentrations by assuming that the selectivities of the long-chain (hexadecyl), water-insoluble, aggregate- bound arenediazonium ions, 16- ArN₂⁺, toward anionic or neutral nucleophiles compared to water are the same as the selectivities of the short-chain (methyl), water-soluble analogues, l- ArN₂⁺ toward the same nucleophiles in aqueous solutions. The suitability of dediazoniation reactions as interfacial probes and the basic assumptions used in our approach arc described. The observed rate constants for dediazoniation of the arenediazonium salts are almost completely independent of the salt. (CTA)X. and R'OH concentrations, consistent with rate-determining loss of N₂ to give an aryl cation which reacts at diffusion-controlled rates with available nucleophiles. Salt-induced spectral shifts indicate formation of ion pairs in tbe ground state, and all our data are consistent with a heterolytic dediazoniation mechanism in which product distributions ere determined by the equilibrium distribution of the ensemble of ground-state arenediazonium cation-anion and arenediazonium cation-molecule intimate pairs. Comparisons with previous results and potential applications are briefly discussed. The companion paper shows that ether product yields can also be used to estimate R'OH binding constants over a wide range of alcohol and surfactant concentrations.