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

Phenyl radical is formed in the propagation chain of the aromatic SRN 1 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 pi* 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.