Oxidation of L-rhamnose and D-mannose by Cr(VI) in perchloric acid. A comparative study

Abstract

LUIS F. SALA, SANDRA R. SIGNORELLA, MARCELA RIZZOT~O, M A R~A I. FRASCAROLI, and FABIO GANDOLFO. Can. J. Chem. 70, 2046Chem. 70, (1992.The oxidation of L-rhamnose and D-mannose by Cr(V1) in perchloric acid follows the rate law: M -~ S-I pour le L-rhamnose. Cette equation de vitesse correspond a la reaction conduisant a la formation de la L-1 ,4-rhamnonelactone et de la D-1 ,4-mannonelactone. I1 ne se produit pas de clivage en dioxyde de carbone lorsque la quantite d'aldose utilisee depasse de 20 fois ou plus la quantite de Cr(V1). On interprkte les valeurs relatives des constantes cinktiques en fonction d'une participation du groupe hydroxyle primaire dans 1.kster chromique qui se forme au cours de la premikre Ctape de la reaction. Les radicaux libres qui se forment au cours de la reaction reagissent avec le Cr(V1) pour former du Cr(V). On attribue deux signaux (gi,, = 1,977 et gi,, = 1,970) observes par resonance paramagnetique Clectronique (rpe) sur des solutions de 0,25 1 N de HC10, saturees en L-rhamnose et en c~,o,'-, aux complexes du Cr(V) intermediaire. A des pH supkrieurs (3-7), on n'a detecte que le signal a g = 1,977. Pour le D-mannose, on n'observe que le signal a g = 1,978 ti toutes les concentrations de HC104.[Traduit par la redaction]

Kinetics and mechanism of the oxidation of disaccharides by CrVI

“…[2]). This is based on the mechanism reported for aldoses, which are also selectively oxidized by Cr VI at the hemiacetalic group with an initial two-electron transfer slow step (21)(22)(23)(24)(25)(26)(27)(28). After the slow redox step, Cr IV is predicted to react with excess saccharide to yield Cr III and a disaccharide radical in a fast step (eq.…”

“…Our studies on the reduction of Cr VI and the intermediate Cr V by aldoses (21)(22)(23)(24)(25)(26), deoxyaldoses (27)(28), sugar acids (31,32,35), and methyl glycosides (29)(30) showed that the relative redox reactivities of these saccharides toward chromate are based on the relative rates of the oxidation vs. complexation processes. In view of some preliminary results obtained on the milk-Cr VI system suggesting the formation of long lived lactose-Cr V species generated by direct reaction of Cr VI with lactose (5), we decided to evaluate the kinetics of the reaction of Cr VI with four disaccharides: D-lactose (Lac), D-maltose (Mal), D-cellobiose (Cel), and D-melibiose (Mel), to determine the influence of the glycoside moiety attached to the C 6 or C 4 of glucose (glc) on the chelating and (or) redox reactivity of the saccharide with high oxidation states of chromium.…”

“…The EPR spectral parameters, g iso and A iso , together with the proton superhyperfine (shf) coupling, are useful in determining the binding modes of sugars to the Cr V center (1,22,24,26,27,57,58). An empirical relationship between the nature and number of donor groups and the EPR spectral parameters of Cr V complexes has been established (1,57).…”

“…We are studying the possible fate of Cr VI and Cr V in biological systems by examining reactions of Cr VI with low-molecular-weight molecules (21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37). Our studies on the reduction of Cr VI and the intermediate Cr V by aldoses (21)(22)(23)(24)(25)(26), deoxyaldoses (27)(28), sugar acids (31,32,35), and methyl glycosides (29)(30) showed that the relative redox reactivities of these saccharides toward chromate are based on the relative rates of the oxidation vs. complexation processes.…”

Kinetics and Mechanism of Micellar Catalyzed Oxidation of Dextrose by N-Bromosuccinimide in H2SO4 Medium

“…(ii) Paper Chromatography [18]. Generally, paper chromatographic technique is used to identify the oxidation products of carbohydrates.…”