Enols as intermediates in nitrosation and halogenation of malonamide and malonic acid

Abstract

Catalysed nitrosation of malonamide(MA) was carried out in aqueous acidic solution at 25ºC. In the presence of bromide ion as a catalyst, the reaction was always first order in [HNO(_2)] despite the changes in acidity. However, the kinetic behaviour of SCN(^-) catalysed reaction varied from first order to zero order with [SCN(^-)], suggesting that the reaction occurs via an enol intermediate and the rate limiting step can move from enolisation to nitrosation under certain conditions. lodination and bromination of malonamide were investigated in aqueous acidic solution at 25ºC. At low acidity, the iodination of malonamide was zero order in [I2], which is in good agreement with the proposed mechanism where the reaction occurs via an enol intermediate and enolisation is the rate limiting step. The deuterium isotope effect on iodination of malonamide was also examined and the result strongly confirmed the existence of enol form in the reaction. The acid catalysed enolisation can be expressed as follows: At higher acidity and lower [I2] reaction was first order in [I2], when now iodination of the end is rate limiting. When enolisation is rate limiting the rate equation was established as Rate=k(_obs)=k(_1)[H(^+)][MA] The values of enolisation rate constant k(_1) (3.72(±0.4)xl0(^-3) l.mol(^-1).s(^-1)) is significantly larger than that measured for simple ketones such as acetone (3.8xl0(^-5) 1-mol(^-1).s(^-1)), reflecting the greater acidity of the central hydrogen atom in malonamide structure brought about by the second –CONH(_2) group or possibly the greater concentration of the O-protonated intermediate. Bromination was perfectly first-order in [Br(_2)] at all acidities. Attempts to achieve rate-limiting enolisation by increasing [Br(_2)] were only partially successful. Perfect zero order behaviour could not be obtained. A possible explanation is that dibromination occurs in the reaction. By combination of the above three reactions, Kg the equilibrium constant of enolisation of malonamide was estimated as 4(±2)xl0(^-10), with an assumption that the rate limiting step of electrophilic addition occurs at the encounter limit. The bromination of malonic acid (MAL) was studied under pH 1.0-4.65 at 25ºC. The reaction was zero order in [Br(_2)] and the plots of k(_o)/[MAL](_T) vs [MAL](_T) were linear, implying that the enolisation reaction is via the malonate ion catalysed pathway.