ESTIMATION OF MALONIC ACID AND METHYLMALONIC ACID ENOLIZATION RATE CONSTANTS BY AN ISOTOPIC-EXCHANGE REACTION USING 1H NMR SPECTROSCOPY

Article abstract of DOI:10.1021/j100320a048

In the oscillatory Belousov-Zhabotinsky (BZ) reaction, bromination of the organic substrate is an important component process.Because bromination of the organic substrate in the BZ reaction occurs via an enolization mechanism, which generally is considered to be rate determining, the enolization rate constants are important parameters.In this paper, we erport enolization rate constants of malonic and methylmalonic acid in 1 M sulfuric acid by following with 1H NMR spectroscopy the isotopic substitution of enol-exchangeable hydrogen atoms by deuterium.We find for the first enolization step of malonic acid a rate constant of (1.066+/-0.003)x1E-3 s^-1 and for the second enolization step a rate constant value of (4.2+/0.2)x1E-4 s^-1.The corresponding value for methylmalonic acid is (5.7+/-0.1)x1E-5 s^-1.While earlier literature data for malonic acid are difficult to compare with our present results, the determined enolization rate constant for methylmalonic acid is in excellent agreement with two other experimental determinations performed in 1 M sulfuric acid.When the sulfuric acid activity is taken into account, then methylmalonic acid enolization rates can be extrapolated to other sulfuric acid concentrations.