Kinetics of the addition of anilines to methyl vinyl ketone

Abstract

Rates of the addition of aniline (A) to methyl vinyl ketone (MVK) to form 4-anilinobutan-2-one (I) have been measured by ultraviolet spectrophotometry at 31·0°. The reaction in aqueous buffers follows second-order kinetics, and involves both general-acid and general-base catalysis. The rates of addition in unbuffered 40 vol. % aqueous dioxan also satisfy a similar rate equation for general-acid and -base catalysis, where aniline itself acts as a catalysing base. Mono- and di-basic phosphate ions show catalysis much more strongly than acetic acid or acetate ion probably because of bifunctional catalysis through a cyclic transition state. Electron-withdrawing group on aniline lower the rate, the substituent effect giving a Hammett ρ-value of –1·60. A mechanism which involves an activated complex of free aniline or aniline associated with base and hydrogen-bonded or free methyl vinyl ketone is postulated and discussed.