Solvolyses of 4-methylbenzenediazonium ions in trifluoroethanol/water mixtures. Effects of [TFE], acidity and temperature on the kinetics and mechanism of the reaction.
Résumé
We have studied the solovolyses of 4-methylbenzenediazonium, 4MBD, ions in 2,2,2-trifluoroethanol, TFE, - water mixtures by employing a combination of spectrometric (UV-VIS) and chromatographic (high performance liquid chromatography, HPLC) techniques. HPLC analyses of the reaction mixtures show that only very small amounts (< 8%) of the reduction product toluene, ArH, are obtained, suggesting that the reaction mainly takes place through an ionic mechanism. Kinetic results show that kobs values do not change significantly on changing solvent composition, which means that the nucleophilic attack of the solvent is not involved in the rate-limiting step of the reaction. On lowering the acidity, both kobs values and the product distribution remained basically unchanged, in contrast with the observed behaviour found in solvolytic dediazoniations in other alcohols such as MEOH, EtOH or BuOH, where formation of diazo ethers of the type Ar-N=N-O-R was evidenced, suggesting that in TFE the formation of diazo ethers is not important. To seek for further evidence supporting the ionic mechanism, we also analyzed the effects of temperature on kobs at different acidities and determined the corresponding activation parameters. H‡ values are high compared to those of bimolecular and radical reactions, and S‡ values are small but positive. Results are therefore suggestive of a heterolytic DN + AN dediazoniation mechanism comprising a rate-determining formation of a highly reactive aryl cation that traps any nucleophile available in its solvation shell.
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