Kinetic behaviour and relative reactivities of some aldoses, amino sugars, and methylated sugars towards platinum(IV) in alkaline medium

Article abstract of DOI:10.1016/S0008-6215(98)00136-0

The kinetic behaviour and relative reactivities of some carbohydrates (aldoses, amino sugars and methylated sugars) towards platinum(IV) in alkaline medium have been investigated. The reactions are first order with respect to [substrate] and PtIV. The rates increase with the increase in OH^-. The reactions show pseudo-first-order dependence on OH^-. The oxidation rates in alkaline medium follow the order triose>tetrose>pentose>hexose. Activation parameters of the reactions have been calculated. Mechanisms have been proposed for the reactions. Copyright (C) 1998 Elsevier Science Ltd.

Full text of DOI:10.1016/S0008-6215(98)00136-0

Carbohydrate Research 309 (1998) 303±310
Kinetic behaviour and relative reactivities of
some aldoses, amino sugars, and methylated
sugars towards platinum(IV) in alkaline medium
Kalyan Kali Sen Gupta *, Bilkis Ara Begum, Biswajit Pal
Department of Chemistry, Jadavpur University, Calcutta 700 032, India
Received 30 December 1997; accepted 14 May 1998
Abstract
The kinetic behaviour and relative reactivities of some carbohydrates (aldoses, amino sugars and
methylated sugars) towards platinum(IV) in alkaline medium have been investigated. The reac-
tions are ®rst order with respect to [substrate] and ‰PtꢀIV†Š. The rates increase with the increase in
‰OH Š. The reactions show pseudo-®rst-order dependence on ‰OH Š. The oxidation rates in alka-
line medium follow the order triose>tetrose>pentose>hexose. Activation parameters of the
reactions have been calculated. Mechanisms have been proposed for the reactions. # 1998
Elsevier Science Ltd. All rights reserved
1. Introduction
2. Experimental
The mechanism for the oxidations of some
aldoses by chromium(VI) [1], vanadium(V) [2],
cerium(IV) [3], manganese(III) [4], iridium(IV)
[5], gold(III) [6] and periodic acid [7] have been
investigated in acidic media. The kinetics of the
oxidation of aldoses by halogens [8,9], chloramine-T
[10], bromamine-T [11] (sodium N-bromo-p-toluene-
sulfonamide), osmium tetroxide [12] and hexacyano-
ferrate(III) [13] have been reported in alkaline
media. However, no data are available on the
reactivities of aldoses, amino sugars, and methy-
lated sugars with hexachloroplatinate(IV) in alkaline
medium. The reactions of platinum(IV) with some
aldoses represented by the general formula CnH2nOn
(n=3±6), as well as reactions with some amino
sugars, have been carried out in alkaline medium.
Reagents.ÐThe sugars and hydrochlorides of
the amino sugars were obtained from BDH, E.
Merck, or Sigma Chemical Co. Reagent grade
sodium hexachloroplatinate (Johnson-Matthey,
London) was used as supplied. The platinum(IV)
content of the salt was determined by an iodo-
metric method [14].
Instrumentation.ÐSpectral measurements were
recorded in the UV-vis region using a Systronics
(India) spectrophotometer. IR spectra were recorded
on KBr discs using a Perkin±Elmer 783 ratio-
recording infrared spectrophotometer. Thermo-
gravimetric analysis was performed in a Shimadzu
Corporation (Japan) a TG 50 instrument at normal
atmospheric environment by heating a sample
(21.39 mg) at the rate of 15 ꢁC min 1 up to 200 ꢁC.
Kinetic measurements.ÐThe platinum(IV) solu-
tion has a maximum absorption at 400 nm. Beer's
* Corresponding author.
0008-6215/98/$Ðsee front matter # 1998 Elsevier Science Ltd. All rights reserved
PII S0008-6215(98)00136-0

304
K.K. Sen Gupta et al./Carbohydrate Research 309 (1998) 303±310
law is valid in the concentration range (1.00±
3
carried out in the descending mode with Whatman
No.1 paper in 4:1:5 (upper layer) butanol±AcOH±
H2O, and the sugars were detected by alkaline sil-
ver nitrate. The PC of the syrup obtained by oxi-
dation of aldoses indicated the presence of aldonic
acids. Similar experiments when carried out with
the oxidation products of the amino sugars gave
yellow colourations with the above iron(III)
reagent. It may be mentioned [17] that amino
sugars are also oxidised by HgO to give aldosa-
minic acids, and that when iron(III) chloride solu-
tion coloured violet with phenol is added to an
aldosaminic acid solution, a yellow colouration is
also obtained. However, the addition of alkaline
hydroxylamine solution to the reaction mixtures
and subsequent addition of 2% FeCl3 and 1% Hcl
failed to give any blue colouration, which indicated
that no lactone formation was taking place during
the reaction [18]. It is to be mentioned that the
primary products in the oxidation of cyclic aldoses
are aldonic acids or ꢀ- or ꢁ-lactones in acidic solu-
tions, but at pH>2.5 the proportion of lactones is
reduced because of the formation of aldonic acid
anions that displace the equilibrium away from the
lactones [19]. Consequently, the lactones which are
formed in the rate-determining steps are hydro-
lyzed to form the aldonic acid anions in alkaline
media in a fast step. The black product obtained in
the oxidations by platinum(IV) is hydrated PtO as
indicated by thermogravimetric and IR spectral
analysis [20]. There is also literature evidence [21]
to indicate that platinum(II) oxide exists as
Pt(OH)2. This is supported by the thermogravi-
metric analysis of the solid product, which indi-
cates that loss of water takes place in one step. The
weight loss calculation based on Pt(OH)2 con-
®rmed the loss of only one water molecule. The IR
spectrum of the thermogravimetric residue identi-
®ed it as PtO by the presence of ꢂ(Pt±O) at around
10.0)Â10 3 mol dm . The kinetic studies were
carried out under pseudo-®rst-order conditions
with the concentration of substrates in large excess
3
(2Â10 2 mol dm ) relative to that of platinum(IV)
concentrations. Hence, the rate of disappearance of
platinum(IV) could be followed spectro-
photometrically at 400 nm. The spectrophotometer
had a cell compartment kept at constant tempera-
ꢁ
ture (‹0.1 C) by circulating water from a ther-
mostat. The reactants were previously equilibrated
to bath temperature. In each case, alkali was added
to the substrate immediately before the kinetic
experiment in order to minimise [15] isomerisation,
degradation into substances with fewer carbon
atoms than the original sugar, and internal oxida-
tions and reductions. Platinum(IV) was then added
to the mixture of aldose and alkali, the reaction
mixture was transferred to the cell of path length
1 cm, and 8±10 experimental readings were taken in
each run. Good straight lines were obtained for at
least two half-lives. The pseudo-®rst-order rate
constants, kobs, were calculated from the plots of
log A (A=absorbance) versus time. The rate con-
stants were reproducible to within ‹5%.
Stoichiometry and product analysis.ÐThe reac-
tion mixtures containing a known excess of [Pt(IV)]
over [glucose] at ®xed [OH ] were made. After
completion of the reactions the unreacted platinu-
m(IV) was determined, showing that one mole of
glucose consumed one mole of Pt(IV). The reaction
stoichiometry may thus be represented as follows:
After the kinetic experiments, the reaction mixtures
of the aldoses and the amino sugars were acidi®ed
and treated with 2,4-dinitrophenylhydrazine
hydrochloride. The absence of any yellow pre-
cipitate indicated that neither the ±CH2OH nor the
ˆ CHOH group was oxidised. Iron(III) chloride
solution that had been coloured violet with phenol
when added to the reaction mixture gave a bright-
yellow colouration [1], indicating that aldonic acids
are formed in the oxidation of the aldoses. The
aldoses were oxidized separately by the oxidant
under kinetic conditions and then acidi®ed. After
puri®cation and concentration, the products were
con®rmed [16] by paper chromatography (PC) by
comparison with the products of the respective
aldoses from bromine-water oxidation. PC was
1
570 cm and the absence of ꢂ(O±H) in the 3600±
3400 cm 1 region. The IR spectrum of an authentic
sample of PtO was compared with that of the PtO
obtained as a reaction product.
3. Results and discussion
The reactions were investigated at di€erent
[Pt(IV)] and at constant [substrate], [OH ], and
temperature. The pseudo-®rst-order rate constants
remain unaltered in the region of [Pt(IV)] of (1±
3
5)Â10 3 mol dm (Table 1). These indicate that

K.K. Sen Gupta et al./Carbohydrate Research 309 (1998) 303±310
305
3
the rate is ®rst order with respect to [Pt(IV)]. At
constant [Pt(IV)], [OH ], and temperature, the rate
increases with increase in [substrate]. The results
plotted in Fig. 1 indicate that each reaction is ®rst-
order with respect to [substrate] and that the slope
of each line is one.
The in¯uence of hydroxide ion on the reaction
rates was investigated at di€erent hydroxide ion
concentrations but at constant [substrate] and
temperature. The rate increased with increase in
[OH ]. Plots of log kobs versus log [OH ] gave a
slope of each line that was one, which indicates
that the reaction is ®rst order (Fig. 2).
(0.6±4Â10 2 mol dm ), which was varied by the
addition of NaCl. The values of kobs were found to
be independent of sodium chloride concentrations.
The in¯uence of temperature on the rate was
determined for each reaction (Table 2). The values
of k3 (k3 ˆ kobs=‰substrateŠ‰OH Š) were calculated
(Table 3) at di€erent temperatures from 298±318 K.
Least-squares treatment of the log k3=T against 1/T
plots were used to obtain the best straight lines
ˆ
from which the enthalpies of activation (ÁH ) and
ˆ
entropies of activation (ÁS ) were calculated using
the theory of absolute reaction rate, the equation
ˆ
ˆ
being k3 ˆ ꢀRT=Nh†e ÁH =RT:eÁS =R. The activa-
tion parameters are recorded in Table 4. The acti-
vation enthalpies are in the order hexose>
The reactions were studied at constant [Pt(IV)],
[substrate], [OH ], and 303 K at di€erent [Cl ]
ˆ
pentose>tetrose>triose. The values of ÁS for
aldohexoses, which are higher than those of aldo-
pentoses, are also in keeping with the idea of
Table 1
E€ect of ‰PtꢀIV†Š on the rate of oxidation rate at 303 K;
3
3
‰PtꢀIV†Š ˆ ꢀ1 5†  10 mol dm
,
‰OH Š ˆ 1:0  10 2mol
dm 3, [substrate]=2.0Â10 2 mol dm
Substrate
3
ˆ
Hammett and Price [22] who showed that ÁS is
kobs  104ꢀs
†
1
related to the rigidities of the transition states. The
enthalpy of activation is linearly related with the
entropy of activation [23] (r ˆ 0:9939, Fig. 3), and
the isokinetic temperature is 333 K. The isokinetic
behaviour supported [24] by the linear plot of log
k02 versus log k2 (r ˆ 0:9925) where k20 and k2 are
the second-order rate constants at the temperatures
308 and 303 K, respectively (Fig. 3). The isokinetic
temperature was calculated from the relation
ꢃ ˆ T1T2ꢀ1 f †=ꢀT1 T2f †, where f is the slope of
the Exner plot, and ꢃ is 348 K. Since the value of ꢃ
d-Glucose
0.54‹0.01
0.61‹0.01
0.69‹0.02
0.96‹0.02
1.08‹0.01
1.28‹0.02
3.3‹0.02
40.1‹0.03
1.27‹0.01
4.8‹0.02
9.6‹0.01
d-Mannose
d-Galactose
d-Xylose
l-Arabinose
d-Ribose
d-Erythrose
d-Glyceraldehyde
d-Glucosamine
d-Mannosamine
d-Galactosamine
Fig. 1. Variation of the reaction rate with [substrate]. Plots of
2
Fig. 2. Dependence of the reaction rate on ‰OH Š. Plots of
3
3
log kobs versus log [substrate] at ‰OH Š ˆ 1  10 mol dm
log kobs versus log ‰OH Š at [substrate]=5Â10 2 mol dm
and temperature=303 K. (*) d-glucose, (*) d-ribose, (~)
d-erythrose, [&] dl-glyceraldehyde, ( ) d-glucosamine.
and temperature=303 K. (*) d-glucose, (*) d-ribose, (~)
d-erythrose, [&] dl-glyceraldehyde, ( ) d-glucosamine.

306
K.K. Sen Gupta et al./Carbohydrate Research 309 (1998) 303±310
is above the experimental temperature (305 K), it
can be concluded that enthalpy is a controlling
factor [11] of the reaction.
forms [27]. Since the experiments were performed
3
at low concentrations (ꢂ5.0Â10 2 mol dm ) of
substrates, it is likely that the hydrated monomeric
forms should be preponderant, although the oxi-
dation via the nonhydrated form cannot be ruled
out.
Cyclic aldoses exist [25] mainly in their cyclic
hemiacetal forms in equilibrium with the open-
chain forms. Again aldoses which predominantly
exist as a ꢃ-anomer of the pyranoid form are con-
verted to anions in alkaline medium. Erythrose is
known to exist in acyclic form, although literature
evidence [26] indicates that it can also exist in 88%
cyclic furanoid form with the ꢃ-anomer pre-
dominating. dl-Glyceraldehyde in aqueous solu-
tion exists in acyclic monomeric form that remains
in equilibrium with hydrated and nonhydrated
Again, there is literature evidence [14] to indicate
that in alkaline medium (pH>8) PtCl26 changes to
2
PtCl5ꢀOH† in a fast step.
2
PtCl26 ‡ OH ! PtCl5ꢀOH† ‡ Cl
The addition of Cl to the reaction mixture failed
to inhibit the rate, which indicates that the above
Table 2
The values of pseudo-®rst-order rate constants (kobs  104 s 1) of the oxidations of some aldoses and amino sugars at di€erent
temperatures; ‰PtꢀIV†Š ˆ 2  10 mol dm 3, ‰OH Š ˆ 1  10 mol dm 3, [substrate]=2Â10 2 mol dm
3
2
3
Temperature (K)
a
b
c
d
e
f
g
h
i
j
k
298
303
308
313
318
0.25
0.54
1.10
2.22
0.80
1.28
1.99
3.34
2.21
3.30
4.84
7.02
29.2
40.1
54.2
73.5
0.61
1.23
2.50
4.94
0.69
1.28
2.30
4.10
0.96
1.74
3.14
5.66
1.08
1.86
3.29
5.54
1.27
1.79
2.53
3.48
4.80
7.25
10.9
16.7
9.60
15.3
24.9
39.2
(a) d-glucose, (b) d-mannose, (c) d-galactose, (d) d-xylose, (e) l-arabinose, (f) d-ribose, (g) d-erythrose, (h) dl-glyceraldehyde,
(i) d-glucosamine, (j) d-mannosamine, (k) d-galactosamine.
Table 3
In¯uence of temperature on the oxidations of some aldoses and amino sugars in alkaline medium (k3 in mol 2 dm6 s
1
)
Temperature (K)
k3a
k3b
k3c
k3d
k3e
k3f
k3g
k3h
k3i
k3j
k3k
298
303
308
313
318
0.127
0.27
0.546
1.11
0.395
0.640
0.995
1.67
1.107
1.65
2.42
3.51
14.5
20.0
27.1
36.7
0.300
0.614
1.25
0.350
0.640
1.15
0.480
0.868
1.57
0.540
0.930
1.64
0.633
0.893
1.27
2.40
3.63
5.458
8.38
4.80
7.67
12.5
19.6
2.47
2.05
2.83
2.77
1.733
(a) d-glucose, (b) d-mannose, (c) d-galactose, (d) d-xylose, (e) l-arabinose, (f) d-ribose, (g) d-erythrose, (h) dl-glyceraldehyde,
(i) d-glucosamine, (j) d-mannosamine, (k) d-galactosamine.
Table 4
Values of activation parameters of the oxidations of aldoses and amino sugars
1
1
1
ˆ
ˆ
1
ˆ
Substrates
ÁH ꢀkJ mol
†
ÁS ꢀJK mol
†
ÁG ꢀkJ mol
†
d-Glucose
113‹8
107‹4
91‹4
90‹4
86‹4
73‹8
56‹4
47‹4
55‹2
64‹2
68‹2
117‹27
99‹13
47‹13
46‹13
31‹12
8‹26
56‹13
65‹13
67‹7
77.55‹0.1
77.00‹0.06
76.76‹0.06
76.06‹0.06
76.6 ‹0.3
75.42‹0.1
72.97‹0.06
66.70‹0.06
75.30‹0.1
72.20‹0.1
70.4 ‹0.1
d-Mannose
d-Galactose
d-Xylose
l-Arabinose
d-Ribose
d-Erythrose
dl-Glyceraldehyde
d-Glucosamine
d-Mannosamine
d-Galactosamine
27‹7
8‹7

K.K. Sen Gupta et al./Carbohydrate Research 309 (1998) 303±310
307
Fig. 3. Isokinetic plots for the oxidations of aldoses and
amino sugars by platinum(IV) in alkaline medium. (a,a0), plot
Fig. 4. Dependence of the reaction rate on the number of
asymmetric centres (n). Plot of log kobs against `n'.
of ÁS versus ÁH ; (b,b0), plot of log k20 versus log k2. (1) d-
glucose, (2) d-mannose, (3) d-galactose, (4) d-xylose, (5) l-
arabinose, (6) d-ribose, (7) d-erythrose, (8) dl-glyceraldehyde,
(9) d-glucosamine, (10) d-mannosamine, (11) d-galactosa-
mine.
ˆ
ˆ
®rst-order rate constants thus follow the order
triose>tetrose>pentose>hexose. The presence of
di€erent substitutents at C-5 (i.e., H or CH2OH)
a€ects the rate appreciably. The much lower rate
for the aldohexoses in comparison to that for
aldopentoses may be due to the inductive e€ect
( I), as well as the stabilising e€ect of the hydro-
xymethyl group in the former. The di€erence in the
rates of oxidation of aldoses having the same sub-
stituents at C-5 seems to be due to the con®gura-
tional variation at positions other than C-1 and
C-5 [35]. There is literature evidence [10] to indicate
that aldoses exist as enolate ions in alkaline
medium. The dependence of the rate on the con-
centration of OH was explained as resulting from
the anion of aldoses being much more reactive than
the unionized form, which would require speci®c
OH catalysis [36].
Again, aldoses may react [11,15] with hydroxide
ion to form an enediol, followed by the reaction
with the oxidant to form lower aldoses and formic
acid. Under the pseudo-®rst-order conditions of a
large excess of substrate over oxidant used in this
study, the possibility of oxidation via the inter-
mediate formation of an enediol has been dis-
counted. This is supported by the formation of
aldonic acid anions, which are formed [10] under
kinetic conditions. It is believed that the anion of
the aldose reacts with the oxidant to form products
of oxidation. Attempts have also been made to
determine rate constants for the oxidations of both
step is not reversible. The ligand replacement reac-
and OH to give
2
tion between PtCl5ꢀOH†
2
PtCl4ꢀOH† and Cl , followed by the oxidation
2
2
of aldose by PtCl4ꢀOH† , can be ruled out since
2
the dihydroxy platinum(IV) complex is unstable in
aqueous solution and readily disproportionates
[28] according to the following equation:
2
2
2
4
2 PtCl4ꢀOH† ! PtCl26 ‡ PtCl2ꢀOH†
The path by which platinum(IV) is reduced by dif-
ferent reductants has been the subject of much
discussion [29,30], which was mainly concerned
with the possibility of platinum(III) as an inter-
mediate. The reductions of some platinum(IV)
complexes by di€erent reductants have been shown
to proceed via a free-radical mechanism [31,32].
However, the alternative mechanistic paths
whereby platinum(IV) undergoes a two-electron
reduction have also been reported [20,33,34].
The oxidation reactions of all the aldoses and
the amino sugars that we have studied are kineti-
cally similar. It appears that oxidation rates for
substrates existing mainly in acyclic forms are
higher than those existing in cyclic forms. The
decrease in the rate with increase in asymmetric
centre (n) is shown in Fig. 4. The plot of log kobs
versus `n' is hyperbolic in character. The pseudo-

308
K.K. Sen Gupta et al./Carbohydrate Research 309 (1998) 303±310
3
methyl ꢄ-d-glucopyranoside and methyl ꢃ-d-glu-
copyranoside at high hydroxide ion concentrations
and temperature. As it is known [37] that hydro-
lysis of methyl ꢄ-d-glucopyranoside and methyl ꢃ-
d-gꢁlucopyranoside occurs in 2.0 M HCl and at
60 C, it is unlikely that hydrolysis of the methyl
glucosides takes place under the present experi-
mental conditions. The values of kobs obtained for
the oxidations of the two methyl glucosides are
[OH ]=0.10 mol dm and T=328 K. The oxida-
tions of methyl ꢄ- and methyl ꢃ-d-glucopyranoside
are too slow to be studied under the conditions at
which other substrates are oxidised. Consequently,
C-1 of the aldoses appears to be more reactive [38]
than C-6. The H-1 bonds of the cyclic and acyclic
forms are ruptured, and two electrons are trans-
ferred to platinum(IV) to give the ꢁ-lactone and
the ketene, respectively. The latter compounds
undergo fast alkaline hydrolysis to give aldonic
acid anions [10]. The rate expression may be
deduced as follows:
5
1
7.68Â10 and 10.9Â10 5 s , for methyl ꢄ- and
methyl ꢃ-d-glucosides, respectively, at [substrate]=
3
3
2.0Â10 2 mol dm , [Pt(IV)]=1.0Â10 3 mol dm ,
Scheme 1.
Scheme 2.

K.K. Sen Gupta et al./Carbohydrate Research 309 (1998) 303±310
309
Scheme 3.
Scheme 4.
K
!
Aldose ‡ OH
Enolate ‡ H2O
which is formed in alkaline medium and is trans-
formed into hydrated PtO.
k
Enolate ‡ PtꢀIV† ! products
PtCl24 ‡ 2OH ! PtꢀOH†2# ‡4Cl
Thus irrespective of whether the substrates are in
cyclic or acyclic forms, oxidations of the aldoses
take place to form aldonic acid anions. All the
above-mentioned evidence provides adequate sup-
port to the proposed mechanism.
d‰PtꢀIV†Š
:Á:
ˆ k‰PtꢀIV†Š‰EnolateŠ
dt
d ‰PtꢀIV†Š
dt
or
ˆ k3‰PtꢀIV†Š‰AldoseŠ‰OH Š
The rate of oxidation of the substrates by plati-
num(IV) increases with the ®rst power of the con-
centration of [platinum(IV)], [substrate], and
‰OH Š. Oxidations of these substrates by other
oxidants [8±11] in alkaline medium are kinetically
dissimilar, and hence, the present rate constant
values cannot be compared with those obtained
previously with other oxidants. However, from the
where k3 ˆ kK.
The reactions leading to the formation of aldo-
nic acid anions may be expressed according to
Schemes 1±4. When the reaction mixtures were
allowed to stand for several hours, a black pre-
cipitate was obtained, presumably due to PtCl24 ,

310
K.K. Sen Gupta et al./Carbohydrate Research 309 (1998) 303±310
results it appears that even in alkaline medium,
oxidations of the substrates by platinum(IV) pro-
ceed at appreciable rates, and thus platinum(IV)
can also be used as an e€ective oxidant in the oxi-
dation of carbohydrate molecules.
[15] W. Pigman, The Carbohydrates: Chemistry, Bio-
chemistry, and Physiology, Academic Press, New
York, 1957, pp. 60±69.
[16] S. Signorella, R. Lafarga, L. Ciullo, and L. F. Sala,
Carbohydr. Res., 259 (1994) 35±43.
[17] W. Pigman, The Carbohydrates: Chemistry, Bio-
chemistry, and Physiology, Academic Press, New
York, 1957, p. 472.
[18] M. Abdel Akher and F. Smith, J. Am. Chem. Soc,
73 (1951) 5859±5860.
Acknowledgements
Thanks are due to C.S.I.R. for ®nancial assis-
tance to K.K.S.G. and also to Indian Government
for an I.C.C.R. fellowship to B.A.B. The latter
author also thanks the authorities of Bangladesh
Atomic Energy Commission for granting leave.
[19] B. Capon, Chem. Rev., 69 (1969) 407±498.
[20] K.K. Sen Gupta, A.B. Bilkis, and S. P. Ghosh,
Transition Met. Chem. (1998), in press.
[21] J.C. Bailar, Jr., H.J. Emeleus, R. Nyholm, and A.F.
Trotman-Dickenson, Comprehensive Inorganic
Chemistry, Pergamon Press, Oxford, 1973, pp. 1330.
[22] F.P. Price and L.P. Hammett. J. Am. Chem. Soc.,
63 (1941) 2387±2393.
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