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A.K. Singh et al. / Carbohydrate Research 337 (2002) 345–351
of HCl was maintained at 0.1N, and the strength of
RuCl3 was 9.64×10−3 mol dm−3. A standard aqueous
solution of mercuric acetate (E. Merck) was acidified
with 20% AcOH. Perchloric acid (E. Merck), diluted
with double-distilled water, was standardized via acid–
base titration. All other standard solutions of KCl,
NaClO4, and acetamide (E. Merck) were prepared with
double-distilled water.
Methods.—The reactions were studied at constant
temperature 40 °C (90.1 °C). The requisite volume of
reactants, i.e., NBA, Ru(III), HClO4 and Hg(OAc)2,
were taken in a black-coated (Jena glass) vessel and
equilibrated at 40 °C. An appropriate volume of sugar
solution, also equilibrated at 40 °C, was rapidly poured
into the reaction mixture to initiate the reaction. The
progress of the reaction was followed by estimating the
amount of unconsumed NBA iodometrically in aliquots
withdrawn from the reaction mixture at regular time
intervals.
Stoichiometry and product analysis.—Various sets
of experiments were performed with different
[NBA]:[sugar] ratios, under the conditions of [NBA]ꢀ
[sugar]. Determination of unreacted NBA indicated that
the 2 mol of NBA were consumed to oxidize 1 mol of
each of the sugars. Accordingly, the following stoichio-
Fig. 1. Plots of (−dc/dt) versus [NBA] at 40 °C for the
oxidation of -galactose (G) and -ribose (R) under the
experimental condition of Table 1.
D
D
indicating first order in each sugar. The order of reaction
with respect to hydrogen ions (obtained from HClO4)
was determined as −0.37 (Gal) and −0.62 (Rib) from
metric equations could be formulated.
+
the slope of the plot between log k% and log[H+] (Fig.
RuCl /H
3
1
C6H12O6+2CH3CONHBr+2H2O ꢀꢀꢀꢀꢀꢁ HCOOH
D
-galactose
NBA
2). A log k% versus log[Ru(III)] plot gives a straight line
1
+2CH3CONH2+2HBr+C4H9O4COOH
with a slope of nearly one (0.90 for Gal and 0.95 for
Rib), which confirms the first order dependence of the
reactions on [Ru(III)] (Fig. 2). First-order kinetics, with
respect to [Ru(III)], is further verified by the plot of
(−dc/dt) values against [Ru(III)], where a straight line
passing through the origin is obtained (Fig. 3).
D
-lyxonic acid
RuCl /H
+
3
C5-Hrib1o0sOe 5+2CH3CONHBr+2H2O ꢀꢀꢀꢀꢀꢁ HCOOH
D
NBA
+2CH3CONH2+2HBr+C3-Hery7tOhr3oCnicOaOcidH
D
The main products of the oxidation, formic, D-lyxonic
and L
-erythronic acids were detected17 chromatographi-
Table 2 shows the effect of the addition of Hg(OAc)2,
Cl− and acetamide. A perusal of the results in Table 2
shows the positive effect of the addition of Hg(OAc)2,
indicating the involvement of Hg(II) as a co-catalyst in
addition to its role as a Br− ion scavenger.20–22 Succes-
sive addition of acetamide (NHA) shows a negative
effect on the rates of reaction in oxidation of both Gal
and Rib. A positive effect of the addition of chloride ions
and the negligible effect of ionic strength ‘v’ of the
medium varied by addition of a required amount of
NaClO4, i.e., from 0.62×10−3 to 5.62×10−3 mol
dm−3 for (Gal) and 1.65×10−3 to 5.65×10−3 mol
dm−3 for (Rib), on the rate constant were also observed.
The reactions were studied at different temperatures,
and the rate constants at 35, 40 and 45 °C used to
compute activation parameters in the oxidation of Gal
and Rib (Table 3). Identical kinetic results obtained for
the oxidation of Gal and Rib suggest that both reactions
follow a common mechanism.
cally (TLC) and by conventional (spot test) methods.18,19
3. Results and discussion
The oxidation of Gal and Rib were investigated at
different initial concentrations of reactants. The rate
(i.e., −dc/dt) of reaction in each kinetic run was
determined by the slope of the tangent drawn at fixed
concentration of NBA, which is written as [NBA]*. The
first-order velocity constant (k%) was calculated as
1
k% = −dc/dt/[NBA]*
1
The first-order dependence of reaction on NBA at its
lower concentrations tends to zeroth order at its higher
concentrations as demonstrated by the plot of −dc/dt
versus [NBA] (Fig. 1), which is also verified by the
decreasing values of k% obtained at various initial con-
1
centrations of NBA (Table 1). A close examination of
the data in Table 1 shows that the rate of reaction is
directly proportional to the concentration of sugars,
It has been reported2–12 that NBA exists in the
following equilibria in acidic medium.