The excess acidity method. The basicities, and rates and mechanisms of enolization, of some acetophenones and acetone, in moderately concentrated sulfuric acid

The X-function method has been used to evaluate the basicities of six nuclear-substituted acetophenones and acetone, using a combination of new measurements and literature data; the protonation [Formula: see text] of acetone was found to be −5.37. The same method, which involves the excess medium acidity, when used to analyze the enolization rate constants obtained from the measured bromination rates, shows that most of the acetophenones enolize by an A-2 process involving two water molecules in the rate-determining step. Observed linear free energy relationships for the basicities and the enolization rates imply a relatively early transition state for the enolization. Acetone was found to enolize by a similar mechanism in sulfuric acid solutions more dilute than 81% w/w, but at higher acidities bisulfate ion was the preferred base. The mechanistic behaviour of 4-nitroacetophenone was found to be different, not A-2 but either A-1 or A-SE2.