408
A. K. Singh et al. / Carbohydrate Research 341 (2006) 397–409
From the slopes of the straight lines obtained in logk
versus 1/D plots (Fig. 9), the value of dAB has been cal-
culated and found as 6.10 and 5.01 A for Glc and Fru,
respectively. Positive slopes obtained for the oxidation
of both Glc and Fru clearly support the interaction
between two oppositely charged species resulting in the
formation of an activated complex of the type
whereas in the reported Ir(III)-catalysed oxidation of
reducing sugars by NBA, it varies from two to one
and in the Pd(II)-catalysed oxidation of reducing sugars
by NBS, its role is limited to that of a Brꢀ ion scavenger
only. First-order kinetics with respect to [Pd(II)]
throughout its variation in the oxidation of D-glucose
and D-fructose by NBA, as well as in the oxidation of
reducing sugars by NBS, is certainly different from the
first-order kinetics not usually observed at low concen-
tration of Ir(III), which changes to zero-order kinetics
at its high concentration in the oxidation of reducing
sugars by NBA in the presence of perchloric acid.
Although the reported negative fractional order in
[H+] for the Pd(II)-catalysed oxidation of reducing sug-
ars by NBS is similar to the observed negative fractional
order at low concentration of H+, a distinguishing fea-
ture in the present study is that at very high [H+], almost
no effect of [H+] on the rate of reaction is observed. Neg-
ative fractional order in Clꢀ ions and negligible effect of
l on the rate of reaction were observed in the present
case, as well as in the reported Pd(II)- and Ir(III)-catal-
ysed oxidation of reducing sugars.
˚
in the rate-determining step of the pro-
posed reactions in Scheme 1.
3.7. Primary salt effect
The negligible effect of l observed in Pd(II)-catalysed
oxidation of Glc and Fru also supports the rate-deter-
mining step of Scheme 1, where reaction is taking place
between two ions and a neutral molecule (step III).
3.8. Comparative studies
Efforts have also been made to compare the findings of
this paper with the results reported10,23 for the Ir(III)-
catalysed oxidation of reducing sugars by NBA in the
presence of perchloric acid and the Pd(II)-catalysed oxi-
dation of reducing sugars by N-bromosuccinimide
(NBS) in acidic solution. The present paper on the one
hand shows similarity with Ir(III)-catalysed10 oxidation
of reducing sugars being first to zero order with respect
to [NBA] in each case, and on the other hand it differs in
order with respect to [NBS], indicating first-order kinet-
ics throughout the variation of [NBS] in the Pd(II)-catal-
ysed23 oxidation of reducing sugars by NBS in the
presence of perchloric acid. In the present study order
with respect to reducing sugar has been found to be first
order at its lower concentration and zero order at its
very high concentration, whereas in the reported
Ir(III)-catalysed oxidation of reducing sugars by NBA,
order with respect to reducing sugar was found to be
zero, and as a result, participation of the reducing sugar
molecule was not shown in the proposed mechanism
before the rate-determining step. Due to order being
fractionally positive, with respect to reducing sugar con-
centration in the Pd(II)-catalysed oxidation of reducing
sugars by NBA or NBS, the participation of a reducing
sugar molecule in or before the rate-determining step
and its contribution to the reaction rate is in contrast
to the Ir(III)-catalysed oxidation of reducing sugars
where the derived rate law does not relate the rate of
reaction with the concentration of reducing sugar.
The present study entirely differs with the other two
studies10,23 as far as the order with respect to Hg(II) is
concerned. In the Pd(II)-catalysed oxidation of D-glu-
cose and D-fructose by NBA, order with respect to
[Hg(II)] has been found to be fractionally positive,
Positive entropy of activation observed in the present
case supports the interaction between two oppositely
charged species in the rate-determining step and shows
similarity with the reported Ir(III)-catalysed oxidation
of reducing sugars by NBA. But this is in contrast to
the negative entropy of activation observed in Pd(II)-
catalysed oxidation of reducing sugars by NBS. The dis-
tinct feature of the present study is that the proposed
mechanism is consistent with kinetic data and is sup-
ported by the spectra collected for the formation of
complexes during the course of reaction and also by
multiple regression analysis.
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