KINETICS OF GLUCOSE MUTAROTATION IN AQUEOUS UREA SOLUTIONS
1917
(
NH ) CO on solvation of the saccharide. Moreover,
entropy factor, since H increases. The reverse situa-
tion has been observed in mixtures of water with
hydrophobic tert-butanol and tetrahydrofuran [17].
2
2
water and urea molecules would be expected to com-
pete for place in the activated complex, acting as
difunctional catalysts.
To conclude, the quite an informative procedure of
separation of the effects of solvation of the initial and
transition states, we used previously [3, 4], fails with
anomerization, necessitating knowledge of individual
In the binary solvents studied in [12, 14, 15, 17],
the activated complexes could include water mole-
cules only. Therewith, the referees established forma-
tion of complex cyclic transition states but failed to
determine with assurance the number of water mole-
cules incorporated in the transition states and the
number of molecules in their solvation shells.
thermodynamic transfer functions for the
and
anomers which simultaneously present in solutions.
EXPERIMENTAL
A traditional approach to data treatment involves
division of the experimental rate constants into by the
Chemical grade -D-glucose was used as received.
The specific rotation of its aqueous solutions was
nicely consistent with published data. Urea was re-
crystallized from water isopropanol and thoroughly
dried. Water was twice distiiled.
molar concentration of water (c ) in different powers.
W
However, from the available evidence for various
solvent systems a contradictory picture emerges:
Reaction orders in water vary from 1 to higher than 4
[
15]. We performed similar calculations with the
Kinetic experiments were performed on an SU-4
polarimeter. The temperature was maintained to
within 0.05 K. The concentration of glucose in solu-
tions was 0.2 M. The results were treated by the
kinetic equation for a first-order reversible reaction.
densities of aqueous urea solutions taken from [24].
Division of k by c , c , and c gave functions
that steadily increased with increasing urea concentra-
tion. However, the experimental data could be fitted
to success by Eq. (7).
2
W
W
W
The activation energies were determined by a two-
parameter Arrhenius equation, the correlation coef-
ficient was no less than 0.998.
k = k c + k c .
(7)
W W
U U
5
Least-squares treatment gave k 0.75 10 and kU
W
.46 10 5 l mol s at a standard deviation
1
1
ACKNOWLEDGMENTS
6
0
of
.55 10 5 s . The resulting data provide supportive
1
The work was financially supported by the Com-
petitive Center for Basic Natural Science, Ministry of
Education of the Russian Federation (project no. E 00-
5.0-17).
evidence for the assumption that urea directly affects
the reaction kinetics.
Along with the above-considered effects of specific
intermolecular interactions, the observed kinetic re-
gularities of glucose mutarotation are also contributed
by the overall polarity of the medium. A plausible
explanation for this phenomenon is that the water
urea system is distinguished from previously studied
systems in that its dielectric constant increases with
increasing concentration of the organic component.
REFERENCES
1. Engberts, J.B.F.N., Water: A Comprehensive Treatise,
Franks, F., Ed., New York: Plenum, 1976, vol. 6,
p. 139.
2
. Belousov, V.P. and Panov, M.Yu., Thermodynamic
Properties of Aqueous Solutions of Organic Sub-
stances, Boca Raton: CRC, 1994.
Let us now dwell briefly on the activation param-
eters. As already mentioned, they are almost in-
dependent on the composition of the medium, imply-
ing preservation of the structure of the solutions in
the composition range studied. High negative activa-
tion entropies were previously observed in other
solvent systems [14, 15]. It is commonly accepted that,
on the one hand, they reflect stronger solvation of the
activated complex with -glucose and, on the other,
provide additional evidence showing that the me-
chanism of spontaneous anomerization is synchro-
nous and different from those characteristic of H O
and HO catalysis. Note, however, that in our case
small urea additives in water increase k due to the
3. Panov, M.Yu. and Sokolova, O.B., Zh. Obshch. Khim.,
1999, vol. 69, no. 6, p. 963.
4. Panov, M.Yu., Sokolova, O.B., and Nikolaeva, A.S.,
Zh. Obshch. Khim., 1999, vol. 69, no. 10, p. 1687.
5
6
. Franks, F., Phil. Trans. Roy. Soc. London B, 1977,
vol. 278, no. 959, p. 33.
. Franks, F., Pure Appl. Chem., 1987, vol. 59, no. 9,
p. 1189.
+
7. Galema, S., Engberts, J.B.F.N., Hoiland, H., and
Forland, G.M., J. Phys. Chem., 1993, vol. 97, no. 26,
p. 6885.
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RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 12 2003