Equilibrium, kinetics, and mechanism of the malonic acid–iodine reaction

The equilibrium constant for the reaction CH 2 (COOH) 2 + I 3 − ⇆ CHI(COOH) 2 + 2I − + H + , measured spectrophotometrically at 25°C and ionic strength 1.00 M (NaClO 4 ), is (2.79 ± 0.48) × 10 −4 M 2 . Stopped‐flow kinetic measurements at 25°C and ionic strength 1.00 M with [H + ] = (2.09‐95.0) × 10 −3 M and [I − ] = (1.23‐26.1) × 10 −3 M indicate that the rate of the forward reaction is given by ( k 1 [I 2 ] + k 3 [I 3 − ]) [HOOCCH 2 COO − ] + ( k 2 [I 2 ] + k 4 [I 3 − ]) [CH(COOH) 2 ] + k 5 [H + ] [I 3 − ] [CH 2 (COOH) 2 ]. The values of the rate constants k 1 ‐ k 5 are (1.21 ± 0.31) × 10 2 , (2.41 ± 0.15) × 10 1 , (1.16 ± 0.33) × 10 1 , (8.7 ± 4.5) × 10 −1 M −1 ·sec −1 , and (3.20 ± 0.56) × 10 1 M −2 ·sec −1 , respectively. The rate of enolization of malonic acid, measured by the bromine scavenging technique, is given by k en [CH 2 (COOH) 2 ], with k en = 2.0 × 10 −3 + 1.0 × 10 −2 [CH 2 (COOH) 2 ]. An intramolecular mechanism, featuring a six‐member cyclic transition state, is postulated to account for the results on the enolization of malonic acid. The reactions of the enol, enolate ion, and protonated enol with iodine and/or triodide ion are proposed to account for the various rate terms.