Hydrogen atom abstraction and addition reactions of charged phenyl radicals with aromatic substrates in the gas phase

Abstract

In order to investigate competition between radical substitution and addition reactions, the gas-phase reactivity of phenyl radicals bearing a chemically inert, positively charged group and a neutral substituent (CH3, Cl, or Br), both at a meta position with respect to the radical site, was examined toward several aromatic substrates in a dual-cell Fourier transform ion cyclotron resonance mass spectrometer. The radicals undergo hydrogen atom abstraction from the substituent and/or addition to the phenyl ring of benzeneselenol, thiophenol, benzaldehyde, toluene, aniline, and phenol. The presence of an electron-withdrawing substituent Cl or Br on the phenyl ring of the radical slightly increases the rates for both hydrogen atom abstraction and addition due to favorable polarization of the reactions’ transition states. The observation of a stable ion-molecule addition product in most reactions was unexpected since in a low-pressure gas-phase environment, adducts are typically unable to release their excess energy before dissociation to products or back to reactants. However, the addition products discussed here are low in energy [addition is exothermic by 24–30 kcal/mol; B3LYP/6-31Gd+ZPVE] and hence are long lived enough to become stabilized by infrared emission. The extent to which the charged radicals are able to abstract a hydrogen atom from the aromatic substrate and form stable products via addition to the aromatic ring was found to vary greatly. The outcome of this competition can be rationalized by reaction exothermicities only in extreme cases, i.e. for benzeneselenol and thiophenol, that predominantly react by hydrogen atom abstraction due to their especially weak heteroatom-hydrogen bonds and aniline that undergoes almost exclusive addition due to particularly stable resonance-stabilized addition products. For the other substrates, competition between the two reaction pathways is controlled by a complex interplay of polar effects that affects the energies of both transition states but to different extents.