Aromatic substitution in the gas phase. A comparative study of the alkylation of benzene and toluene with C3H7+ ions from the protonation of cyclopropane and propene.

Gas-phase alkylation of benzene and toluene has been used as a probe to sample the isomeric population of C3H7+ ions obtained from the protonation of cyclopropane and of propene, respectively, with H3+ ions in H2 gas at pressures from 50 to 720 torr. The reagent from cyclopropane promotes extensive (over 40%) n-propylation, in addition to isopropylation of the arenes, in contrast to the reagent from propene that yields only traces of n-propylated products. The n-propylating reagent is identified as protonated cyclopropane, whose lifetime must exceed s in the gaseous systems investigated. The mechanism and the selectivity of the gas-phase alkylation are discussed and compared to relevant mass spectrometric and kinetic results.