Interaction of NH₂ radical with alkylbenzenes

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 alkylbenzene 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 (Δ_rH°₂₉₈) and activation enthalpies (Δ^⧧H°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.