ELECTROPHILIC SUBSTITUTION IN MALONAMIDE - EVIDENCE FOR REACTION VIA THE ENOL TAUTOMER

The kinetics of the nitrosation, bromination and iodination of malonam ide [MA] have been studied in water at 25-degrees-C. Throughout, react ion was first order in [MA]. At low acidity and relatively high [I2], reactions were fully zero-order in [I2] and acid-catalysed. At higher acidity and lower [I2] reaction was first-order in [I2] and not now ac id-catalysed. Similar first-order behaviour was found for bromination and nitrosation (by BrNO and ONSCN generated in situ). It was not poss ible by changing the reaction conditions to achieve the zero-order pat tern with either bromination or nitrosation. The results are consisten t with a mechanism involving reaction of the electrophile with an inte rmediate derived from MA which we suggest is the enol tautomer. For io dination either enolisation or reaction of the enol can be made rate-l imiting. For enolisation the value of the rate constant k(e) (in Rate = k(e)[MA] [H+]) was found to be 3.3 X 10(-3) dm3 mol-1 s-1. The resul ts also suggest that I2, Br2, BrNO and ONSCN react with the enol at or close to the encounter controlled limit, enabling a value for K(E) (t he equilibrium constant for enolisation) of (4 +/- 2) x 10(-10) to be obtained. The methylene protons in MA are readily exchanged with D fro m the solvent in an acid catalysed process. The enolisation of CD2(CON D2)2 is slower than that of CH2(CONH2)2 by a factor of 2.3 and the deu terium solvent isotope effect is close to 1. The mechanism of enolisat ion of MA is discussed.