Kinetics and mechanism of the cyclic‐hemiacetal ring opening in 2‐keto‐L‐gulonic acid. A polarographic study

Abstract

The Occurrence of two consecutive cathodic waves, both with absolute rate control, in the case of the polarographic reduction of 2-keto-~-gulonic acid in a buffer solution of malonic acid (pH between 4.8 and 6.3) is de-scribed. The theory of this case is based on the assumption that 2dketo-gulonic acid or its anion predominantly exists in the cyclic-hemiacetal form, which is non-reducible, but which produces the free keto-acid or its anion by a very rapid reaction in the zone around the dropping mercury electrode. The latter substances are reducible, with different half wave potentials. Expressions in the relevant rate constants are derived for both the total current and the current of the first wave. This does not permit the calculation of all the rate constants involved. However, it could be shown that the wave heights are related to pH in the way predicted by the theory. Definite conclusions with respect to the relative magnitude of the rate constants could be drawn. The single wave in the lower pH range of 0.2 to 3.5 was also studied as a function of pH. The reaction mechanism is discussed on the basis of the relationships found and in connection with the expected influence of the ionization of the carboxyl group upon the reactivity of the catalytic species present. The process of ring closure or opening was shown to be second-order, rather than third.In 1947, Brdicka* reported on the polarography of pyruvic acid and phenylglyoxylic acid. In the pH range of 5 to 8 (Mclluaine buffer) the solmutions of these compounds were shown to give two consecutive waves. The first of these waves, at the more positive potential, was attributed to the reduction of the acid, the second to the reduction of its anion. Although the total wave height was that expected for complete diffusion control, corresponding to the analytical concentration of the acid, the height of the first wave was much too large to be accounted for in terms of diffusion of the acid. The first wave thereupon was posbulated to be of a kinetic nature, i.e. caused