HALOGENATION OF MALONIC-ACID AND MALONIC-ACID MONOETHYL ESTER - ENOLIZATION PATHWAYS AND ENOL REACTIVITY

Reactions of iodine (2 x 10(-5) mol dm(-3)) and bromine (5 x 10(-5) mo l dm(-3)) in water with malonic acid (MAL) (in large excess) are first -order in [halogen] and in [MAL] and are independent of acidity in the range 0.02-0.16 mol dm(-3) H+. These results are consistent only with a rate-limiting reaction between the halogen and the enol form of MAL . Evidence is presented that these reactions occur at the diffusion li mit and the results enable a value for K-E (the equilibrium constant f or enolisation) of 7.4 x 10(-9) to be deduced. At a much higher bromin e concentration (ca. 2 x 10(-3) mol dm(-3)) the reaction is zero-order in [Br-2], consistent with a rate-limiting enolisation. Values of k(e ) (the rate constant for enolisation) for MAL and for malonic acid mon oethyl ester (EHM) are reasonably similar (4.0 x 10(-3) and 1.1 x 10(- 3) s(-1), respectively) and are about 10(2) greater than the value for diethyl malonate, supporting a mechanism of enolisation involving int ernal H+ transfer for both MAL and EHM. Under conditions zero-order in [Br-2], the dependence of the observed rate constant upon the total s toichiometric concentration of malonic acid [MAL](T) over the pH range 1.0-4.3 enables the kinetic identification of a reaction pathway invo lving base catalysis by malonate ion in addition to the solvent promot ed reaction which is dominant at lower pH. At higher pH values there i s evidence of a third pathway involving malonate ion catalysis of enol isation of the malonate ion.