A flash photolytic investigation of the photosolvolysis of α-(2,6-dimethoxyphenyl)vinyl chloride. Characterization of the 2,6-dimethoxyacetophenone keto-enol system

Article abstract of DOI:10.1139/v93-243

Photosolvolysis of α-(2,6-dimethoxyphenyl)vinyl chloride in trifluoroethanol solution at 25°C was found to produce the α-(2,6-dimethoxyphenyl)vinyl cation as a transient species with the lifetime τ = 1 μs. Addition of water reduced this lifetime markedly, until, at compositions greater than 80% water, it was too short to measure (τ < 20 ns); extrapolation of the data suggests τ = 9 ns in pure water. Hydration of this cation gave the much longer lived enol of 2,6-dimethoxyacetophenone whose rates of ketonization were measured in dilute perchloric acid and sodium hydroxide solutions. These data, when combined with rates of enolization of the ketone determined by iodine scavenging, give pK(E) = 6.98 for the keto-enol equilibrium constant in the 2,6-dimethoxyacetophenone system, pK(a)(E) = 10.72 for the acidity constant of the enol ionizing as an oxygen acid and pK(a)(K) = 17.70 for the acidity constant of the ketone ionizing as a carbon acid; these constants refer to wholly aqueous solution at 25°C and ionic strength = 0.10 M.