Gas-phase reaction of free isopropyl ions with phenol and anisole

Abstract

Unsolvated isopropyl ions, obtained in the dilute gas state from the radiolysis of propane, react with PhOH yielding isomeric isopropylphenols and isopropyl phenyl ether, in the ratio of ca. 3 : 1 at 320 Torr. At lower pressures, the ratio is further shifted in favour of ring alkylation, reaching a value in excess of 10 : 1 at 22 Torr. The isomeric composition of isopropylphenols passes from 83%ortho, 3%meta, 14%para at 720 Torr to 61%ortho, 3%meta, 36%para at 22 Torr. A similar pressure dependence characterizes the isomeric composition of products from the alkylation of PhOMe, which passes from 85%ortho, 5%meta, 10%para in C₃H₈ at 720 Torr to 64%ortho, 21%meta, and 15%para at 20 Torr. The substrate selectivity of isopropyl ion, referred exclusively to the alkylation, is measured by the apparent ratios k_PhOH:k_PhH= 1.0–1.6 and k_PhOMe:k_PhMe= 0.6–0.9 at 720 Torr. The results are consistent with a model involving kinetically predominant attack to the oxygen atom, i.e. to the n-type nucleophilic centre of the ambident substrate, and competition between proton transfer and alkylation channels, reflecting respectively the Brønsted acid and the Lewis acid reactivity of the alkyl cation. The role of isomerization processes in determining the orientation has been independently evaluated by protonating isopropyl phenyl ether with gaseous Brønsted acids, such as H₃⁺ and C_nH₅⁺(n= 1 or 2), thus obtaining via an independent route the same oxonium ion arising from the attack of isopropyl ion on PhOH. The results demonstrate dealkylation to PhOH and intramolecular isomerization to o- and p-isopropylphenols, whose ratio depends on the nature of the Brønsted acid, and the exothermicity of the proton transfer to the ether.