Stoichiometry of protonation of aromatic hydrocarbon radical anions by weak proton donors. A marked discrepancy between the number of protons used and those incorporated into the aromatic structure

Article abstract of DOI:10.1016/0022-328X(95)05959-S

The stoichiometries of the reaction between alkali metal radical anions of biphenyl, naphthalene, phenanthrene and anthracene, and methanol and/or other proton donors have been determined by the magnetic titration technique. In the case of naphthalene radical anion and, for example, methanol as the proton source, the stoichiometry was found to be cation-dependent: Li, 2: 1; Na, 1.75: 1; K, 1.33: 1. The reaction products using the experimentally determined stoichiometric conditions were ca. 95% naphthalene and 5% dihydronaphthalene(s). Thus, a marked discrepancy is observed between the protons used and those incorporated into the naphthalene molecule. Radical anions, at concentrations comparable with those of preparative reactions, react with carbon acids or amines according to the first-order kinetic law, although the initial concentrations of the two reactants were of the same order of magnitude or even equal. Lithium anthacene radical anion reacts with phenylacetylene and diethylamine at comparable rates, although the two "acids" differ in their acidities by ca. 10 orders of magnitude. A deuterium isotope effect of 2.49 ± 0.05 was observed in the reaction between lithium anthracene radical anion and diethylamine. A general reaction scheme is proposed that involves electron transfer to the proton donor and hydrogen-atom attack on the neutral hydrocarbon as the key reaction steps.