Kinetics and mechanism for the iodination of methylmalonic acid

Abstract

The reaction CH(CH3)(COOH)z + 12 = CI(CHB)(COOH)~ + H+ + I-was followed spectrophotometrically a t 353 nm and 470 nm a t 25°C under various conditions of pH and methylmalonic acid concentration. The equilibrium constant for the reaction is 0.11 f 0.02. An iterative technique was used to integrate postulated rate equations. Agreement between experimental and calculated absorbance versus time curves was generally better than 0.005 A (approximately 5% of maximum) a t both wavelengths for a mechanism where the ratedetermining step is formation of an enolate (k = 1.63 X f 0.03 X sec-'). The enolate may be rapidly transformed to the enol or enol carboxylate anion depending on the pH. All three forms are rapidly iodinated. The mechanism of general base catalysis is supported by rate increases proportional to base concentration in buffer solutions. The bases, acetate ion, chloracetate ion, sulfate ion, dichloracetate ion, and water, follow a Br6nsted relationship with B = 0.7.

Benzylmalonic acid reaction with iodine and behavior in a Briggs–Rauscher oscillator

“…In the BzMA concentration range of 0.05–0.02 M, we found a linear relationship between the induction time and 1/[BzMA]: The induction time decreases as the initial concentration of BzMA increases. If, as claimed by several authors 19, 20, during the induction time a suitable amount of I − must be produced by the iodination of the substrate, then this production should be favored by an increase of [BzMA] 0 , consequently the induction time decreases. In the same range the dependence of the oscillation period on the inverse of [BzMA] 0 is also linear: This is in agreement with the observations of Cooke on the BR with acetone 8, and those of Furrow on MA, MMA, PhMA, and IMA BR oscillators 13.…”

Complex oscillations in the Belousov–Zhabotinsky batch reaction with methylmalonic acid and manganese(ii)

“…The rate constants used for the minimal bromate subsystem are based on previously published values. 11 Over many years, the kinetics of the enolization of MeMA as in step (8) has been studied by several investigators, including Furrow, 13 Ruoff et al, 9,12,14,15 Williams and Graham, 16 and Yoshimoto et al 17 Their results as summarized in Table 5 reect a disparity of nearly an order of magnitude in k 8 . Data from the latter two references mentioned above allow an estimate of the value of k À8 for the reverse of the enolization reaction, but they do not agree.…”