Gas-phase basicities of halogenotoluenes

Abstract

Proton-transfer equilibria of the type XH⁺+ Y ⇌ X + YH⁺ between toluene and halogenotoluenes have been studied at 369 ± 2 K in a high-pressure, pulsed-electron-beam source. The equilibrium constants yield values of ΔG^°₃₆₉ for which a ladder of relative basicities can be constructed. The results are discussed and rationalised in terms of the combined effects of methyl and halogen substituents on the sites of protonation in the benzene ring. The results for fluorotoluenes are compared with a CNDO/2 calculation of protonated fluorotoluene structures which predicts that p-fluorotoluene is protonated on the methyl-bearing carbon atom.