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

In order to investigate competition between radical substitution and additi on reactions, the gas-phase reactivity of phenyl radicals bearing a chemica lly 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 cy clotron resonance mass spectrometer. The radicals undergo hydrogen atom abs traction from the substituent and/or addition to the phenyl ring of benzene selenol, thiophenol, benzaldehyde, toluene, aniline, and phenol. The presen ce of an electron-withdrawing substituent Cl or Br on the phenyl ring of th e radical slightly increases the rates for both hydrogen atom abstraction a nd addition due to favorable polarization of the reactions' transition stat es. The observation of a stable ion-molecule addition product in most react ions 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 he re are low in energy [addition is exothermic by 24-30 kcal/mol; B3LYP/6-31G d+ZPVE] and hence are long lived enough to become stabilized by infrared em ission. The extent to which the charged radicals are able to abstract a hyd rogen atom from the aromatic substrate and form stable products via additio n to the aromatic ring was found to vary greatly. The outcome of this compe tition can be rationalized by reaction exothermicities only in extreme case s, i.e. for benzeneselenol and thiophenol, that predominantly react by hydr ogen atom abstraction due to their especially weak heteroatom-hydrogen bond s and aniline that undergoes almost exclusive addition due to particularly stable resonance-stabilized addition products. For the other substrates, co mpetition between the two reaction pathways is controlled by a complex inte rplay of polar effects that affects the energies of both transition states but to different extents. (C) 2001 Elsevier Science B.V.