Interaction of NH2 radical with alkylbenzenes

The Abstraction of a hydrogen atom from the alkyl side chain, attached to a benzene ring, by the amidogen radical (NH2), plays a critical importance in thermal processes that involve the presence of alkyl benzene species and NH2-containing species, as in the pyrolysis of biomass. Yet, literature provides no thermo-kinetic account of this important category of reactions. In this contribution, we compute standard reaction (Delta H-r degrees(298)) and activation enthalpies (Delta double dagger H degrees(298)) for H removal from the alkyl side chains in toluene, ethylbenzene and n-propylbenzene, as well as addition of NH2 at the four possible sites of the phenyl ring in toluene and ethylbenzene. Abstraction of the benzylic H atom in toluene constitutes the sole feasible channel at all temperatures. The same finding applies to ethylbenzene, albeit with a gradual increase of the contribution from the channel of abstraction of primary's H with increasing temperatures. The rate constant of the abstraction of benzylic H in n-propylbenzene dominates that of the primary and secondary H atoms. Computed branching ratios confirm the dominance of H abstraction corridors over the addition channels, even at low temperatures. For primary's H abstraction reactions, comparing reaction rate constants of alkylbenzenes with those of the analogous sites in alkanes indicates a noticable influence of the aromatic ring on the reaction rate constants. The results of the present calculations apply to any branched aromatic hydrocarbon interacting with the NH2 radical. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.