The SRN1 reaction: An attempt to calculate the thermodynamic driving force for the addition of nucleophiles to phenyl radical

Abstract

Phenyl radical is formed in the propagation chain of the aromatic SRN1 reaction from cleavage of the radical anion of the precursor phenyl halide (PhX.-); subsequent addition of nucleophile Y- to Ph. yields the radical anion of the substitution product (PhY.-). In this addition step, transfer of an electron from the nucleophile to Ph. is concerted with bond formation, and the extra electron of the new two-center three-electron bond is, in general, located in a π* MO of the aromatic moiety. Three factors mainly affect the efficiency of this addition step: (i) the energy of the new Ph-Y bond; (ii) the stability of the radical anion of the substitution product; (iii) the oxidation potential of the nucleophile. In keeping with these points, the energy of some Ph-Y bonds has been calculated, and the oxidation potential of Y- species, along with the reduction potential of the related PhY substitution products, have been determined. Calculation of the thermodynamic driving force of the nucleophile/radical addition step is reported for some significant nucleophiles. Finally, comparison of the thermodynamic driving force with experimental reactivity of the same nucleophile in the addition step is attempted. © Acta Chemica Scandinavica 1998.