Butanolysis of 2-methylbenzenediazonium ions: product distribution, rate constants of product formation, and activation parameters

摘要

We have determined the product distributions, the rate constants of product formation and substrate loss, and the activation parameters for the butanolysis of 2-methylbenzenediazonium, 2MBD, tetrafluoroborate in aqueous 1-Butanol (BuOH) solutions by combining UV-VIS spectroscopy, high performance liquid chromatography (HPLC), and a derivatization protocol that traps unreacted 2MBD as a stable azo dye. BuOH/H2O solutions are miscible over a narrow composition range, but in reverse micelles composed of sodium dodecyl sulfate, SDS, BuOH, and water, are miscible between 45-80%. Two major and two minor dediazoniation products are observed, 2-cresol, ArOH, 2-butyl-tolyl-ether, ArOBu, and small amounts of 2-chlorobenzene, ArCl (from HCl added to control solution acidity) and toluene, ArH (a reduction product). Product yields depend on experimental conditions, but quantitative conversion to products is achieved over the entire composition ranges investigated. The observed rate constants, k(obs), obtained by monitoring 2MBD loss or by monitoring ArOH or ArOBu formation, are the same and they are only modestly affected by changes in the solution composition. The activation parameters obtained from the effect of temperature on k(obs) show that the enthalpy of activation is relatively high compared to those found in bimolecular reactions and the entropy of activation is small but positive. The results suggest that 2MBD is mainly sampling in the BuOH-H2O rich interfacial region of the reverse micelle and are consistent with 2MBD decomposing through a D-N + A(N) mechanism, i.e., a rate limiting formation of an aryl cation that reacts immediately with nucleophiles.