Halogenation of malonic acid and malonic acid monoethyl ester. Enolisation pathways and enol reactivity

Abstract

Reactions of iodine (2 × 10^–5 mol dm^–3) and bromine (5 × 10^–5 mol 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 limit and the results enable a value for K_E(the equilibrium constant for enolisation) of 7.4 × 10^–9 to be deduced. At a much higher bromine concentration (ca. 2 × 10^–3 mol dm^–3) the reaction is zero-order in [Br₂], consistent with a rate-limiting enolisation. Values of k_e(the rate constant for enolisation) for MAL and for malonic acid monoethyl ester (EHM) are reasonably similar (4.0 × 10^–3 and 1.1 × 10^–3 s^–1, respectively) and are about 10² greater than the value for diethyl malonate, supporting a mechanism of enolisation involving internal H⁺ transfer for both MAL and EHM. Under conditions zero-order in [Br₂], the dependence of the observed rate constant upon the total stoichiometric concentration of malonic acid [MAL]_T over the pH range 1.0–4.3 enables the kinetic identification of a reaction pathway involving base catalysis by malonate ion in addition to the solvent promoted reaction which is dominant at lower pH. At higher pH values there is evidence of a third pathway involving malonate ion catalysis of enolisation of the malonate ion.