| The benzilic acid-type rearrangement of glyoxal to glycolic acid was followed in 0.25 to 75mM NaOH and from 5 to 80(DEGREES)C by spectrophotometric analysis of residual glyoxal. At essentially constant alkalinity (15 to 400(mu)M glyoxal), a first-order dependence on total glyoxal was found. The pseudo-first-order rate constant k(,obs) showed a complex dependence on OH('-) of the form: k(,obs) = (a(,1) OH('-) + a(,2) OH('-) ('2))/(1 + a(,3) OH('-) ). At 25(DEGREES)C and (mu) = 0.075M (NaCl), values of 0.144 s('-1)M('-1), 4.00 x10('2) s('-1)M('-2) and 40.6 M('-1) were obtained for a(,1), a(,2), and a(,3), respectively. Activation energies of 96, 57, and -8.0 kJ/mol were obtained for a(,1), a(,2), and a(,3), respectively; a value of 89.5 kJ/mol for a(,1) was obtained from experiments in bicarbonate/carbonate buffer. Integral rate experiments with 2 to 20mM initial glyoxal suggest that a(,3)K(,w) represents the effective acidity of monomeric glyoxal. The kinetic findings suggest parallel disproportionation pathways, and are consistent for rate-limiting, intramolecular hydride ion transfer via singly and doubly dissociated glyoxal monohydrate.;The rate of monomerization of glyoxal dimer was observed at (mu) = 0.060M and from pH 1.3 to 7.7 and 5 to 45(DEGREES)C by following the chromatographically observed concentration ratio of dimer to monomer. At constant pH, an essentially first-order dependence on glyoxal dimer was observed. Complex general acid-base catalysis was observed; after correction for catalysis by buffer salts, the forward pseudo-first-order rate constant k(,f(obs)) showed the dependence: k(,f(obs)) = b(,1) H(,3)O('+) + b(,2) + b(,3) OH('-) /(1 + b(,4) OH('-) ) + b(,5) OH('-) . At 25(DEGREES)C, values of 2.9 x 10('-3) s('-1)M('-1), 4.8 x 10('-7) s('-1), 7.9 x 10('5) s('-1)M('-1), 3.5 x 10('8) M('-1), and 5.7 x10('4) s('-1)M('-1) were obtained for b(,1)-b(,5), respectively; corresponding activation energies of 74.7, 70.4, 37.9, -18.2, and 59.6 kJ/mol were obtained. The observed, complex OH('-)-catalysis is consistent for parallel solvent and OH('-)-catalyzed pathways to and from a steady-state intermediate form.;A high pressure liquid chromatographic method, which employs low temperature elution on Aminex HPX-87H, was developed for the separation of dimeric from monomeric glyoxal forms. The equilibrium distribution of dimeric (G(,2)) and monomeric (G(,1)) glyoxal was observed from 5 to 85(DEGREES)C in solutions 0.05 to 1.0M in total glyoxal. A minimum value of 0.56 M('-1) at 25(DEGREES)C was obtained for the apparent dimerization constant, defined as K(,Dapp) = G(,2) / G(,1) ('2). A corresponding (DELTA)H(,Dapp) of +3.2 kJ/mol was determined. |
