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

The equilibrium constant for the reaction CH2(COOH)2 + I3− ⇆ CHI(COOH)2 + 2I− + H+, measured spectrophotometrically at 25°C and ionic strength 1.00M (NaClO4), is (2.79 ± 0.48) × 10−4M2. Stopped‐flow kinetic measurements at 25°C and ionic strength 1.00M with [H+] = (2.09‐95.0) × 10−3M and [I−] = (1.23‐26.1) × 10−3M indicate that the rate of the forward reaction is given by (k1[I2] + k3[I3−]) [HOOCCH2COO−] + (k2[I2] + k4[I3−]) [CH(COOH)2] + k5[H+] [I3−] [CH2(COOH)2]. The values of the rate constants k1‐k5 are (1.21 ± 0.31) × 102, (2.41 ± 0.15) × 101, (1.16 ± 0.33) × 101, (8.7 ± 4.5) × 10−1M−1·sec−1, and (3.20 ± 0.56) × 101M−2·sec−1, respectively. The rate of enolization of malonic acid, measured by the bromine scavenging technique, is given by ken[CH2(COOH)2], with ken = 2.0 × 10−3 + 1.0 × 10−2 [CH2(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.