GAS-PHASE ACIDITIES OF O-DEHYDROBENZOIC, M-DEHYDROBENZOIC AND P-DEHYDROBENZOIC ACID RADICALS - DETERMINATION OF THE SUBSTITUENT CONSTANTS FOR A PHENYL RADICAL SITE

Reaction of CO2 with o-, m-, and p-benzyne radical anions in the gas p hase produces o-, m-, and p-dehydrobenzoate radical anions, respective ly. The (oxygen) gas-phase basicities of these ions, which are equival ent to the gas-phase acidities of the corresponding dehydrobenzoic aci d radicals, Delta G(acid)(o-, m-, or p-C6H4CO2-H), have been determine d with a flowing afterglow-triple quadrupole apparatus by means of the kinetic method. The measured values are tin kcal mol(-1)): Delta G(ac id)(o-C6H4CO2H) = 330.4 +/- 0.4, Delta G(acid)(m-C6H4CO2H) = 330.2 +/- 0.4, and Delta G(acid)(p-C6H4CO2H) = 331.6 +/- 0.4 kcal mol(-1). All three radicals are more acidic than benzoic acid (Delta G(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, sigma(R), and the field/inductive effect substituent constan t, sigma(F), for a phenyl radical site of - 0.47 and 0.57, respectivel y. This classifies a phenyl radical site as a strong inductive withdra wing, and strong resonance donating substituent. Density functional ca lculations of the gas-phase acidities of dehydrobenzoic acids are in g ood agreement with the experimental results. The increased acidities o f the dehydrobenzoic acids are shown to arise from a balance between t he electron withdrawing effect of the electronegative radical site, an d a compensating polarization of the pi system which mimics the effect of a resonance donor group located at the radical carbon. (C) 1998 El sevier Science B.V. All rights reserved.