Protiodeacylation of 4-substituted 1-acetyl-2,6-dimethylbenzenes in sulphuric acid: kinetics and mechanism

Abstract

In 89.8%(w/w) sulphuric acid rate coefficients (10⁴k₁/s^–1) for the protiodeacetylation of 4-substituted (X) 1-acetyl-2,6-dimethylbenzenes (1) were, at 25 °C; X = Br, 0.468; I, 0.509; Cl, 0.635; H, 1.345; F, 2.52; Ph, 31.5; Bu^t, 38.6; and Me, 47.2. At 25 °C the reaction of the ketone (1; X = OMe) was too fast, and at 80 °C reactions of the ketones (1; X = CONH₂ or CO₂H) were too slow, for convenient measurement. The carboxy nitrile (X = CN) underwent hydration to the amide (X = CONH₂) rather than protiodeacylation. The rate coefficients at 25 °C of eight ketones (1) gave an accurate Hammett correlation, with σ⁺ constants, giving ρ=–4.64 ± 0.05. The reaction of 1-acetyl-2,4,6-trimethyl-benzene (1; X = Me) was studied of a range of acidity (73.6–99.9% sulphuric acid), and a maximum was found near 86.0% acid. Rates were also measured in D₂SO₄–D₂O. The mechanism is discussed.