Cyclisation of some bz-substituted anils of acetylacetone in sulphuric acid

Abstract

Previous work on cyclodehydration reactions leading to the formation of an aromatic ring is reviewed. further investigations are reported on the cylodehydration of anils of acetylacetone in aqueous sulphuric acid and perchloric acid to give 2:4-dimethylquinolines. It has now been shown that in the range of media where cyclisation occurs, the anils are probably completely mono-protonated and stable. from the relationship found between rate coefficient and the acidity function H0 it is concluded that cyclisation is an Internal electrophilic substitution reaction which proceeds through a diprotonated anil foiled in relatively small amount and not through the monoprotonated form as previously assumed. In more aqueous acids, where the acid is not strong enough to bring about cyclisation, hydrolysis of the anil to acetyl-acetone and the aromatic amine occurs. A correlation has been found between the rate coefficient of hydrolysis, the acidity function Ho and the activity of the water present and a satisfactory mechanism has been put forward to interpret this result. The rates of cyolisation of anils with both one and two substituents in the benzene nucleus have been measured and the results examined by means of the Hammett equation log k/k0 =rho6. The additivity of substituent effects which is observed in the pKa values of the parent anilines Is only found to be operative in a limited number of the cyolisation reactions. The m-halogeno anils are all found to cyclise faster than the unsubstituted anil, i.e., a halogen atom is activating in this electrophllic substitution of the benzene ring. There is a definite parallelism between these rates and the partial rate factors for p-nitration and p-halogenation of the corresponding halogenobenzenes, indicating that the same electronic effect occur in all three reactions. The constants of the Arrhenius equation have been evaluated for the cyclodehydration of aniline anil and p-toluidine anil in different media and the results are discussed.