Gas Phase Aromatic Substitution. Reactivity of (trifluoromethoxy)benzene toward Charged Electrophiles

The reactivity of C6H5OCF3 toward gaseous cations, including C2H5+, i-C3H7+, (CH3)2F+, and CH3O(H)NO2+, has been investigated with mass spectrometric and radiolytic techniques. Two distinct kinetic patterns emerge, depending on whether formation of an early electrostatic adduct between the reactants is rate determining. In this case, which occurs in ethylation and isopropylation, the reactant displays positional but not substrate selectivity, e.g., i-C3H7+ is characterized by a k(C6H5OCF3)/k(C6H6) ratio of 1.0 and by a 77% ortho, 12% meta, and 11% para orientation. By contrast, methylation and nitration display both substrate and positional selectivity, e.g., CH3O(H)NO2+ is characterized by a k(C6H5OCF3)/k(C6H6) ratio of 0.096 and by a 35% ortho, 5% meta, and 60% para orientation. Only the latter reactions lend themselves to construction of free-energy correlations, e.g., a sigma(p)+ value of +0.12 has been calculated for the nitration of C6H5OCF3 by CH3O(H)NO2+ in CH4 at 37.5-degrees-C, compared with the +0.07 value from the nitration by NO2+BF4- in solution. The gas-phase reactions are compared with the solution-chemistry counterparts, and their distinctive features are discussed.