Rates and Mechanism of Proton Transfer from Transient Carbon Acids. The Acidites of Methylbenzene Cations

Article abstract of DOI:10.1021/ja00336a029

The fast rates of proton transfer from various methylbenzene cation radicals to a series of substitued pyridine bases are successfully measured in acetonitrile solutions.The technique utilizes the production of the cation radical as a transient intermediate during the electron-transfer oxidation of the methylbenzene with an iron(III) oxidant.Complete analysis of the complex kinetics affords reliable values of the deprotonation rate constants k₂ which span a range from 3 x 10² to more than 2 x 10⁷ M^-1 s^-1.The relative acidities of the cation radicals of hexamethylbenzene, pentamethylbenzene, durene, and prehnitene can be obtained from the Broensted correlation of the deprotonation rate constants with the pyridine base strengths and the standard oxidation potentials of the methylarenes.An estimate of the acidity constant for the hexamethylbenzene cation radical is based on several empirical extrapolations to that of the toluene cation radical previously evaluated by Nicholas and Arnold on thermochemical grounds.The kinetic acidities of the various methylarene cation radicals are also examined in the context of the Marcus equation, as applied to proton transfer.The mechanism of proton transfer from these labile carbon acids is discussed with regard to the electronic effects relevant to the methylarene oxidation potential and the pyridine base strength, the kinetic isotope effects with deuterated methyl groups, the salt effects in acetonitrile, and the steric effects of ortho substituents on pyridine.