Stability and selectivity trends in the addition of atomic radicals to xylenes

Abstract

In this work, the reaction of the atomic radicals H, O( 3P), F and Cl with xylenes has been studied using ab initio theoretical methods. The stability of the isomeric adducts is analyzed and rules for the selectivity of addition of the different radicals are discussed. Ipso addition is found to be clearly favored for fluorine. In all cases, positions that are either ortho or ipso to a methyl group yield the most stable adducts, while carbon atoms that are in para positions with respect to the methyl groups seem to be the least susceptible of attack. These preferences contrast with normal addition rules for electrophilic reactants. Estimates of the enthalpy changes at 298 K for the reaction C 6 H 4 ( CH 3 ) 2 + X → C 6 H 4 ( CH 3 ) 2 X are determined for the first time. Values for Δ H r are compared with estimates obtained using Benson's additivity rules. They correlate very well with the atomic radius, the ionization potential, hardness and polarizability of the atoms.