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Etude de la composition de solutions aqueuses d'acide glyoxylique en RMN de 13C
Chastrette, Francine,Bracoud, Corinne,Chastrette, Maurice,Mattioda, Georges,Christidis, Yani
, p. 66 - 74 (2007/10/02)
In order to determine the composition of aqueous solutions of glyoxylic acid, we studied the 13C nmr spectra of variously concentrated solutions (20 to 60 percent).The two well-separated regions (acids and esters on one side, acetals, hemiacetals and hydrates on the other) display not directly related intensities owing to the relaxation times of carbonyl carbons.Nevertheless, by plotting the relative intensity of each line in its own region, we were able to associate signals coming from the same species.Aside from the two prominent lines of monomeric hydrate, we identified two other pairs with intensity increasing with total concentration.Apart from a small line due to glyoxal and some other very small lines, we observed four lines of equal and slightly increasing intensity, three of acetal type and one of acid or ester type.In order to establish the structures of the species present, we determined the chemical shifts of a series of related molecules to be used as models (reported on table 2): first esters, acids, acetals, hemiacetals and hydrates of monomeric strukture, then dimers and trimers of acetaldehyde, ethyl glyoxylate and glyoxal.We were thus able to establish increments and correlations which allowed us to estimate the values of chemical shifts for possible dimers and trimers of glyoxylic acid: it was easy to see that two of the entities present in the solution are dimeric hemiacetals-acids, the erythro and threo isomers (figure 3).The fourth species is necessarily a combination of acid with glyoxal and the inspection of models led us to identify it as dihydroxy-4,5 dioxolan-1,3 carboxylic acid (figure 4).Its concentration was shown to increase with that of glyoxal in glyoxylic acid, and furthermore, some of its derivatives could be isolated.The acetalization and esterification of glyoxylic acid by ethanol in acidic medium, effected with remowal of water, provided a mixture of esters-acetals which was analysed by coupled VPC-mass spectrometry and VPC-chemical ionization (with ammonia).The analysis confirmed the existence of the above monomer and dimers in proportions related to the composition of the aqueous solution; moreover a small amount of a cyclic dimer is observed.No more than 5 percent of higher oligomers was evidenced.As for the combinations between glyoxal and glyoxylic acid, the principal one can be accompanied by many others, of various stoechiometries, when more glyoxal is initially present in the glyoxylic acid solution.Thus it is shown that glyoxylic acid in aqueous solution is mostly in the form of monomeric hydrate (69 to 88 percent)) and dimeric hemiacetals-acids (2,5 to 12 percent).Higher oligomers never exceed 5 percent in concentrated solutions, but glyoxal, if present, easily combines with glyoxylic acid, to form various compounds, the major one accounting for up to 12 percent of the material in some concentrated mixture of glyoxal and glyoxylic acid.
