| Summary: | Thesis (Ph.D.)--Boston University === This work was undertaken to extend the knowledge of chlorine water oxidations of carbohydrates in the acid range. The simplest models of cellulose, namely D-glucose and alkyl-β-D-glucosides, were used as reductants. The oxidation of glucono-δ-lactone has also been studied. Reductants were always present in great excess. The reaction vessel was a hypodermic syringe with an adapter. This vessel prevented the loss of chlorine through volatilization. Samples were periodically withdrawn, quenched and titrated. Specific rates of reaction were determined from the data. In all cases where the rate constant increased with time, the slope was determined from the initial portion of the run. The reaction is first order in oxidant and reductant for all substrates except methyl-α-D-glucoside. Methyl-α-D-glucoside exhibits a zero order dependence on the oxidant concentration and a variable order (0≤n≤1) in reductant.
The specific reaction rates for molecular chlorine and hypochlorous acid oxidations of all substrates have been obtained and are recorded below. The separation of catalytic (generalized base and nucleophilic) effects related to chlorine and hypochlorous acid was realized. Specific and generalized acid catalysis were proved to be absent in the case of D-glucose. In general, specific and generalized acid catalysis was assumed to be negligible in the oxidation of all substrates. Hypo-chlorite ion proved to be a potent catalyst - more effective than the Bronsted catalysis law would indicate. The sole exception to generalized base catalysis was obtained using glucono-δ-lactone as substrate. The oxidation of this substrate was inhibited by the presence of monohydrogen phosphate ion.
[Table 1: Stoichiometric data of chlorine water oxidations of carbohydrates in the acid range]
The pseudo zero order behavior in oxidant experienced with methyl-α-D-glucoside is explained on the basis of two consecutive
(1) liter/mole - second
(2) liters^2/mole^2 - second
reactions with the second reaction being very rapid relative to the first. Thus a rate expression was derived which explained the kinetics, i.e.,
v = k (Ox)o (G)
where (Ox)o = initial oxidant concentration
(G) =methyl-α-D-glucoside concentration.
The oxidation of this substrate is slow in comparison with D-glucose and the alkyl-β-D-glucosides. These data in addition to the stoichiometry data support the above conclusion that the second reaction is much faster than the first. The reaction was also found to be susceptible to chloride ion catalysis.
The presence of chloride ion was found to inhibit the oxidation of methyl-β-, isobutyl-β-, and isopropyl-β-D-glucoside by hypochlorous acid. This is a distinct difference from the α anomer and suggests a change in mechanism.
The differences in rate and catalytic constants for the β-glucosides are small and suggest that the structure of the aglycon does not influence the rate of oxidation.
The stoichiometry values determined under conditions of excess oxidant have been determined for D-glucose (pH 2.1, 2.8, 4.6 and 6.3), methyl-α-D-glucoside, methyl-β-D-glucoside and isopropyl-β-D-glucoside, all at pH 2.1, 4.6 and 6.3. Lowest values of equivalents of oxidant consumed per mole of reductant are obtained in the regions of highest acidity.
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