Proton-Transfer and Electron-Transfer Processes in Reaction of p-Nitrotoluene with Bases. A Spectrophotometric Study

Article abstract of DOI:10.1021/ja00530a032

The interaction of p-nitrotoluene (PNT) with a variety of basic systems has been studied spectrophotometrically.The bases employed, in decreasing order of base strength, were potassium hydride, dimsyl potassium, triphenylmethylpotassium, fluorenylpotassium, and potassium p-nitroanilide, while the solvents used were dimethyl sulfoxide, tetrahydrofuran, and dimethoxyethane, with 18-crown-6 ether present in the latter two cases.The major species produced in the reactions were p-nitrobenzyl anion (PNT^-), p-nitrotoluene radical anion (PNT^-.), and p,p'-dinitrostilbene radical anion (DNS^-.).The principal processes in the PNT-base systems are believed to be (i) PNT + B^- <=> PNT^- + BH; (ii) PNT + B^- <=> PNT^-. + B^.; (iii) PNT^- + PNT <=> PNT^. + PNT^-..DNS^-. is formed through secondary processes via p,p'-dinitrobibenzyl.Discussion of process (i) is given in terms of the relative pK^as of PNT and the respective BH species, while (ii) is analysed on the basis of the reduction potential of PNT and the oxidation potential of B^-.For the case of the PNT/Ph3C^- system, the electron-transfer process (ii) is kinetically preferred but this equilibrium is rapidly established.On the other hand the proton-transfer process (i) is greatly favored thermodynamically and is effectively irreversible; as a result the proton transfer predominates.This is in accord with observation of PNT^- as the initially formed species in this system.The processes occurring in the other base systems are similarly analysed.The assignment of the absorption due to the p-nitrobenzyl anion is at variance with an earlier literature report deduced from examination of the PNT/t-BuOK/t-BuOH system.Electron-transfer and proton-transfer processes in the PNT/t-BuO^- system are also discussed.