The kinetics of cyclization of beta-anilinopropenyl-methylketone to 2:4-dimethylquinoline

Abstract

The role of the sulphuric acid medium in aromatic nitration reactions is described briefly and a review is made of the previous work on acid catalysed aromatic cyclodehydration reactions, with special reference to those carried out in concentrated sulphuric acid. Of these, the Combes' synthesis of quinoline compounds from the anils of beta-diketones is discussed more fully. The general method for the preparation of the anils of acetylacetone is described and details given of the preparation of beta-2:4;6-trideutero-anilinopropenyl-methyl-ketone. The experimental method used to measure the rate of cyclization of the anils is recorded. This involves hydrolysis of the unchanged anil present in the aliquots removed from the reaction mixture to the corresponding amine and acetylacetone, followed by estimation of the acetylacetone. The cyclization reaction is first order with respect to the anil and the values of the rate constants obtained for the cyclization of beta-anilinopropenyl methylketone and beta-(p-toluidino-)propenyl-methyIketone in H20/H2so4 and (NH4) SO4/H2SQ4 media and of the deutero-anil in H2O/H2SO2 media are tabulated. The rate constants, kl, for the cyclizations investigated are found to be related to Hammett's acidity function, Ho, from which it is concluded that the anils take up a single proton in sulphuric acid, and a relationship is also shown to exist between k1 and the ratio [H2SO4] /[HSO4] for both the anilino- and p-toluidino-anils. The implications of the value found for the isotopic rate ratio kH/kD for the anilino and deutero-anils is discussed, and possible mechanisms for the reaction are considered. The most acceptable appears to be one involving an intermediate addition compound, the formation of this addition compound being the rate-determining step.