Direct Observation of the Kinetic Acidities of Transient Aromatic Cation Radicals. The Mechanism of Electrophilic Side-Chain Nitration of the Methylbenzenes

Article abstract of DOI:10.1021/ja00188a048

The transient cation radicals ArCH3(.+) are spontaneously generated by the 532-nm excitation of the charge-transfer complexes with a 10-ns laser pulse.The decay kinetics of the spectral transients in the presence of added base establish the kinetic acidities (k_H) for various methylarene cation radicals with different pyridines and trinitromethide.Such a proton transfer from ArCH3(.+) proceeds with a deuterium kinetic isotope effect of k_H/k_D ca. 3.Side-chain nitration of hexamethylbenzene (HMB) is shown to proceed in high yields via the intimate triad of reactive fragments II, , that is produced upon the charge-transfer excitation.The subsequent annihilation of the reactive triad II occurs via a rapid succession of bimolecular steps involving either (i) the initial ion-pair collapse of by proton transfer, as shown in Scheme VI, or (ii) the alternative sequence with the initial ion-radical collapse of by homolytic coupling, as shown in Scheme VII.The marked variations of k_H/k_D with solvent polarity and added innocuous salt (Bu4N(+)ClO4(-)), as reflected in ion-pair separation and the "special" salt effect, serve to effectively distinguish these pathways.The direct bearing of Schemes VI and VII on the mechanism of the thermal (adiabatic) nitration of methylarene side chains with nitric acid is delineated.