The cyclisation of anils

Previous work on cyclisation reactions in acidic media, is reviewed, particular attention being paid to those reactions which take place in concentrated sulphuric acid, especially the synthesis of quinoline compounds from anils of beta-diketones by the Combes method. A method of analysis of dilute aqueous sulphuric acid solutions containing acetylacetone and full experimental details for the measurement of the rate-constant of the cyclisation of the anil, beta-p-tcluidinopropenylmethyl ketone in concentrated sulphuric acid to 2:4:6-trimethyl-quinoline are given. The analysis of the aliquots taken from the reaction mixture depends on the Hydrolysis of the anil into p-toluidine and acetyl acetone and the subsequent estimation of the acetyl acetone. The cyclisation reaction is found to be first order with respect to the anil and the rate-constant is found to increase with increasing strength of sulphuric acid. Unlike most nitration reactions, which have a maximum rate at about sulphuric acid, the rate of this cyclisation reaction increases with increasing acidity and does so very rapidly as absolute sulphuric acid is approached. The tautomerism of anils and the nature of beta-p-toluidinoprop-enylmethyl ketone in sulphuric acid is discussed. The rate-constant k, is found to be related to Hammett's acidity function Hc and the anil is therefore assumed to take up a single proton in concentrated sulphuric acid. A mechanism of cyclisation is suggested involving the uptake of a proton, ring-closure and subsequent loss of molecule of water. A note is made of the failures of certain anils to cyclise and a reason for this failure is suggested.