Gas-phase acidities of o-, m- and p-dehydrobenzoic acid radicals. Determination of the substituent constants for a phenyl radical site

Abstract

Reaction of CO 2 with o-, m-, and p-benzyne radical anions in the gas phase produces o-, m-, and p-dehydrobenzoate radical anions, respectively. The (oxygen) gas-phase basicities of these ions, which are equivalent to the gas-phase acidities of the corresponding dehydrobenzoic acid radicals, ΔG acid( o-, m-, or p-C 6H 4CO 2-H), have been determined with a flowing afterglowtriple quadrupole apparatus by means of the kinetic method. The measured values are (in kcal mol −1): ΔG acid( o-C 6H 4CO 2H) = 330.4 ± 0.4, ΔG acid( m-C 6H 4CO 2H) = 330.2 ± 0.4, and ΔG acid( p-C 6H 4CO 2H) = 331.6 ± 0.4 kcal mol −1. All three radicals are more acidic than benzoic acid ( ΔG acid = 333.1 ± 2.0 kcal mol −1). The measured gas-phase acidities for the meta and para isomers suggest values for the resonance-effect substituent constant, σ R, and the field/inductive effect substituent constant, σ F, for a phenyl radical site of - 0.47 and 0.57, respectively. This classifies a phenyl radical site as a strong inductive withdrawing, and strong resonance donating substituent. Density functional calculations of the gas-phase acidities of dehydrobenzoic acids are in good agreement with the experimental results. The increased acidities of the dehydrobenzoic acids are shown to arise from a balance between the electron withdrawing effect of the electronegative radical site, and a compensating polarization of the π system which mimics the effect of a resonance donor group located at the radical carbon.