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A J CSIAN OURNAL OF HEMISTRYA J CSIAN OURNAL OF HEMISTRY
https://doi.org/10.14233/ajchem.2020.22646
INTRODUCTION
Potassium periodate is a versatile oxidant that can oxidize
a number of organic and inorganic compounds. Periodate is
used both in alkaline and acidic media [1-5]. In acidic medium,
the existing species is H5IO6 and its non-existence was reported
in the pH range of 5 to 9. In aqueous medium with a pH range
of 6.1 to 7.3, the possible species of periodate are IO4
−
, H4IO6
−
and H3IO6
2−
, whereas, the predominant species is H4IO6
−
near
pH = 7.0. In alkaline medium (0.005 to 0.1 M hydroxide concen-
tration), trihydrogen paraperiodate (H3IO6
2−
) and dihydrogen
paraperiodate(H2IO6
3−
)canbetheexpectedpredominantspecies.
Due to participation in the following reaction, the expected
species is H2IO6
3−
in highly alkaline medium.
2 3
3 6 2 6 2H IO OH H IO H O− − −
+ → +
H2I2O10
4−
is another species in alkaline medium. Periodate
existsinitsdimerformI2O9
4−
athigherconcentrationsofperiodate
[6]. Oxidation of pharmaceutically important polyhydric comp-
ounds like polyethylene glycols was studied in alkaline medium
[7,8]. In those PEG oxidations, transfer of one oxygen atom
or two electrons was observed for each periodate, i.e. periodate
was able to act as oxidant in these reactions till its conversion
to iodate. Among the four species of periodate (H2IO6
3−
, H3IO6
2−
,
Substrate Inhibition in Ruthenium(III) Catalyzed Oxidation of
Propane-1,3-diol by Periodate in Acidic Medium: A Kinetic Study
K.V.S. KOTESWARA RAO
1,
, R. VENKATA NADH
2,*,
and K. VENKATA RATNAM
2,
1
Department of Chemistry, G.V.S.M. Government Degree College, Ulavapadu-523292, India
2
Department of Chemistry, GITAM University-Bengaluru Campus, Bengaluru-562163, India
*Corresponding author: E-mail: doctornadh@yahoo.co.in
Received: 22 January 2020; Accepted: 12 March 2020; Published online: 27 June 2020; AJC-19914
Ruthenium(III) catalyzed oxidation of propane-1,3-diol by potassium periodate was studied in aqueous perchloric acid medium. Orders
of reaction with respect to concentrations of oxidant, substrate, acid and catalyst were determined. First order in oxidant and catalyst
concentrations, and inverse fractional order in acid medium were observed. In addition, substrate inhibition (i.e. a decrease in reaction rate
with an increase in substrate concentration) was observed. Effect of addition of salt and solvent was studied. Based on the studies of
temperature variation, Arrhenius parameters were calculated. Plausible mechanism was also proposed based on observed kinetics.
Keywords: Ruthenium(III), Propane-1,3-diol, Oxidation, Acidic medium, Potassium periodate, Kinetics.
Asian Journal of Chemistry; Vol. 32, No. 7 (2020), 1569-1575
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IO4
−
and H2I2O10
4−
) in alkaline medium, concentrations of the
first two species are considerable, whereas the concentrations
of the other two are trivial in the range of alkali from 0.05 to
0.5 M. Moreover, the predominant species is H2IO6
3−
at higher
concentration of alkali, however at low alkali concentration it
is H3IO6
2−
.
In addition to well-known trivalent, different oxidation
states of ruthenium catalyst were proposed in different reactions.
Ruthenium(IV) was the proposed reactive form of ruthenium
intheoxidationofdiolsbyperoxodiphosphateinacidicmedium.
Alteration of electrons availability at the oxidation site by the
positive inductive effect was used to explain the reactivity order
of ethane-1,2-diol < propane-1,2-diol < butane-1,2-diol. Some
researchers [4,9] reported the quick conversion of trivalent
ruthenium to octavalent by periodate in acidic medium.
Further reported the existence of Ru(VIII) in acidic medium
in the form of H2RuO5 or RuO3(OH)2.The former was assumed
to be active catalytic species in the kinetics of ruthenium(III)
catalyzed oxidation of methyl glycol by periodate in perchloric
acid [4] to form the corresponding monocarboxylic acid as
the product. Participation of more powerful oxidant species
Ru(V) was proposed for the observed catalysis in the oxidation
of alanine [10] and monoethanolamine [11] by N-bromosucci-
nimide (NBS) in aqueous acetic acid-perchloric acid medium.
Acceleration of NBS oxidation of monoethanolamine with the
involvement of Ru(V) was substantiated with the presence and
absence of catalysis with Ru(III) and Os(VIII), respectively.
It indicates the absence of complexation in these reactions,
but the formation of Ru(V) [11].
Oxidation of alcohols by various oxidants is well docu-
mented in literature. In the kinetics of oxidation of neopentyl
glycol by diperiodatocuprate(III) [5], protonated form of
H2IO6
3−
coordinates with central copper ion in alkaline medium
to form [Cu(H2IO6)2]3−
, which further reacts with OH−
to give
[Cu(HIO6)]−
. So formed Cu*
(III) was the active species to form
the corresponding aldol as the final product. In the oxidation
of aliphatic alcohols by iodate in acidic medium, reactions
were explained based on the involvement of halic acid (HIO3)
and hypoiodite ion (IO−
). Order of reaction rate in that study
was given as 1-hexanol < isobutyl alcohol < isopropyl alcohol
< 1-butanol <1-propanol. Length of chain and structural features
of the substrates were taken into consideration to explain the
reactivity. Compared to 2º aliphatic alcohols, the oxidation of
1º aliphatic alcohols was faster with an exception to 1-hexanol.
An inverse proportionality in reaction rate was observed with
respect to the chain length. Cadmium(II) was found to be the
effectivecatalystinthoseoxidations,amongthestudiedtransition
metal ions. Aldehydes and ketones were the products in the
oxidation of primary and secondary aliphatic alcohols [12].
In the oxidation of secondary cyclic alcohols by different
inorganic oxidants (potassium bromate, potassium iodate and
potassium periodate) in acidic medium, the corresponding halic
acids (HBrO3, HIO3 and HIO4) were reported as active oxidizing
species [13]. The oxidation products were the corresponding
ketones and same reaction order was reported with all oxidants
as menthol < isoborneol < borneol. Structural features (steric
hindrance and isomeric) were considered to be the responsible
for the observed sequence of secondary alcohols reactivity.
High and low vulnerability of oxidation in borneol and menthol
towards oxidation was explained based on the least and most
hindered α-hydrogen present in them respectively. The H2OBr+
wastheproposedactiveoxidisingspeciestoexplaintheenhanced
reaction rate by the addition of bromide ion in the oxidation
of n-butanol by p-methoxy-n-bromobenzamide in aqueous
acetic acid [14].
Higher rate of ditelluratoargentate(III) oxidation of 2-(2-
ethoxyethoxy)ethanol (EEE) compared to the oxidation of 2-
(2-methoxy ethoxy)ethanol (MEE) in alkaline medium was
described based on the higher electron-donating ability of the
former compared to the latter [15]. The observed experimental
results were explained based on a free radical mechanism.
Similarly, higher rate of oxidation of 2-ethoxyethanol by K2FeO4
in alkaline medium compared to the oxidation of methoxy-
ethanol was based on electron donating ability [16].
Propane-1,3-diol has a spectrum of applications as a mono-
mer in the production of different polymers like polyethers,
polytrimethylene terephthalate, polyurethanes and polyesters
[17,18]. It is also used as protective agent/antifreeze agent and
solvent [19,20]. Oxidation of vicinal diols in presence of ruthe-
nium catalyst was extensively studied and mechanistic pathways
were explored. But, the reaction pattern was not studied compl-
etely in departed hydroxyl groups of diols. Singh et al. [21]
studied the kinetics of oxidation of 1,3-propanediol by alkaline
N-bromoacetamide in presence of Os(VIII). Silver(I) catalyzed
oxidation was studied by Singh et al. [22] using peroxodi-
phosphate as an oxidant in acetate buffers to form 3-hydroxy
propanol.Agrawal et al. [23] carried out the Ru(III) catalyzed
oxidation of diols by ceric sulphate in aqueous sulphuric acid
medium.As the oxidation studies on non-vicinal diols are scanty,
the present study is aimed at understanding the mechanism
involved in ruthenium(III) catalyzed oxidation of propane-1,3-
diol by periodate in aqueous perchloric acidic medium.
EXPERIMENTAL
Allthechemicalsusedinthepresentstudywereofanalytical
grade. Double distilled water was used to prepare the solutions.
Kinetics of Ru(III) catalyzed oxidation of propane-1,3-diol by
periodate in aqueous perchloric acid medium was followed by
iodometry.Thereactiontookplacetilltheconversionofperiodate
toiodate.AseparatetestwasconductedwhereKIO3 didn'toxidize
propane-1,3-diol. However, Singh et al. [24] reported
aquachloro-rhodium(III) catalysis in oxidation of diol (1,2-
propanediol) by potassium iodate in acidic medium.
Generalprocedure:Thecatalystwasmaintainedonoxidant
side before mixing of oxidant and substrate. Both the oxidant
and substrate solutions (maintained along with desired acid
concentration and solvent composition) were kept in thermostat
at the required constant temperature for 1 h. Both of them were
mixed and then drawn at regular intervals of time. The kinetics
experiments were repeated in triplicate and the results were
reproducible within ± 4%.
Detectionmethod:Theformedproductinthepresentstudy
was converted to concerned phenylhydrazone by using acidic
2,4-DNP (2,4-dinitrophenylhydrazine). Then the hydrazone
was characterized by melting point and spectral studies.
RESULTS AND DISCUSSION
In the present study, an attempt is made to study the kinetics
of uncatalyzed and Ru(III) catalyzed oxidation of non-vicinal
diol (propane-1,3-diol) and vicinal diols (glycol, glycerol,
propane-1,2-diol) in aqueous perchloric acid medium. It is
observed that uncatalyzed oxidation of vicinal diols (glycol,
glycerol, propane-1,2-diol) by periodate in aqueous perchloric
acid medium was completed instantaneously. Whereas, the
uncatalyzed oxidation of propane-1,3-diol under similar
conditions was infinitesimally slow and Ru(III) catalyzed
oxidation of non-vicinal diol (propane-1,3-diol) by periodate
is a slow reaction with observable kinetics. The difference in
the reactivity of propane-1,3-diol from vicinal diols can be
understood from the fact that propane-1,3-diol is a simple
acyclic molecule and won't display any sort of steric strain.
Hence, kinetics of Ru(III) catalyzed oxidation of non-vicinal
diol (propane-1,3-diol) by periodate is carried out in the present
study. Effect of variation and hence reaction orders with respect
to various reactants are discussed below.
Variationof[periodate]:Whilekeepingtheconcentrations
of other reactants at constant concentrations, [periodate] was
1570 Koteswara Rao et al. Asian J. Chem.
varied in the range 0.00025 to 0.002 M. Constructed the pseudo
first order plots i.e. log(a-x) against time (Fig. 1). Linearity of
plots was observed upto completion of 85% of total reaction.
In addition, it was found that values of pseudo-first-order
constants (k1) were independent of the initial [oxidant] (Table-1).
TABLE-1
RATE CONSTANTS IN Ru(III) CATALYZED
OXIDATION OF 1,3-PROPANE-DIOL BY
POTASSIUM PERIODATE IN AN ACID MEDIUM
General conditions: [KIO4] = 0.0005 M; [Substrate] = 0.025 M;
[Ru(III)] = 2 × 10–7
M; [H+
] = 0.1 M; Temperature = 308 K
Variant Conc. of variant (M) k1 × 104
min–1
0.00025 46.52
0.00050 46.53
0.00100 47.63
[Periodate]
0.00200 49.63
0.01250 52.33
0.02500 46.53
0.05000 28.42
[Substrate]
0.10000 18.02
0.025 65.56
0.050 56.11
0.100 46.53
0.200 25.25
[Acid]
0.500 18.97
0 46.53
KCl (0.1 M) 10.74
KBr (0.1 M) 8.85
KI (0.1 M) No reaction
Salt
KNO3 (0.1 M) 44.88
0 M Infinitesimally slow
0.25 × 10-7
2.94
0.50 × 10-7
7.42
1.0 × 10-7
22.58
2.0 × 10-7
46.53
4.0 × 10-7
82.83
[Catalyst]
8.0 × 10-7
172.49
0 46.53
0.01 51.32
0.025 51.67
[Boric acid]
0.05 53.58
308 46.53
313 54.96
318 64.23
323 96.49
Temperature (K)
333 183.37
0:100 46.53
05:95 26.16
20:80 18.78
40:60 15.34
Acetic acid:water
(v/v)
60:40 14.15
Variation of [substrate]: Substrate inhibition is observed
in the present case i.e. an increase in concentration of propane-
1,3-diol from 0.0125 to 0.1 M has resulted in a decrease of
rate constant from 52.33 × 10-4
to 18.02 × 10-4
min-1
(Table-1).
The plot of log k1 vs. log[propane-1,3-diol] shows a slope of
-0.532 (Fig. 2), which indicates the inverse fractional order in
[substrate]. Literature [25,26] shows that substrate inhibition
0.40
0.39
0.38
0.37
0.36
0.35
0.34
0.33
0.32
4+log(a-x)
0 10 20 30 40
Time (min)
y = -0.00202x + 0.39962
R = 0.99448
2
Fig. 1. Plot of log(a-x) versus time at [KIO4] = 0.0025 M, [1,3-diol] =
0.025 M, [Ru(III)] = 2 × 10–7
M, [H+
] = 0.1 M and Temperature =
308 K
1.8
1.6
1.4
1.2
4+logk1
2.0 2.2 2.4 2.6 2.8 3.0
4 + log [S]
y = -0.5325x + 2.8811
R = 0.9483
2
Fig. 2. Effect of substrate concentration on oxidation of propane-1,3-diol
by KIO4
was reported in the oxidation of different sugar alcohols by
periodate in alkaline medium, which was assigned to the forma-
tion of periodate substrate complex which resists oxidation.
In the present case, an increase in substrate concentration
increases the chances of forming an activated complex between
substrateandthecatalyst.Atthesametime,theincreasednumber
of substrate molecules improves their complexation possibility
withtheoxidant.Andhence,decreasestheavailabilityofoxidant
to attack the substrate-catalyst complex. However, indepen-
dence of substrate concentration was reported in the periodate
oxidation of polyethylene glycols (PEGs) in alkaline medium
[27,28] which can be explained based on the fact that PEGs
possess high hydroxyl values [29].
Variation of [acid] and [catalyst]: A decrease of rate
constant was resulted from 65.56 × 10-4
to 18.97 × 10-4
min-1
by increasing the concentration of perchloric acid concen-
tration from 0.025 to 0.50 M (Table-1). An inverse fractional
order in [H+
] was noticed as the slope is -0.444 in the plot of
logk1 vs. log[perchloric acid] (Fig. 3). Literature survey shows
contrary results regarding acid effect on kinetics of oxidation
of propane-1,3-diol. Inverse order in acid concentration was
reportedbySinghetal.[22,24] inphosphotungsticacidcatalyzed
N-chlorosaccharin oxidation and silver ions catalyzed peroxy-
disulphate oxidation. However, an increase in rate constants
with increase in hydrogen ion concentration was noticed in the
oxidation of terminal hydroxyl compounds by chromium(VI)
Vol. 32, No. 7 (2020) Substrate Inhibition in Ru(III) Catalyzed Oxidation of Propane-1,3-diol by Periodate 1571
2.0
1.8
1.6
1.4
1.2
4+logk1
2.3 2.5 2.7 2.9 3.1 3.3 3.5 3.7
4 + log [H ]
+
y = -0.4447x + 2.9253
R = 0.9476
2
Fig. 3. Effect of acid concentration on oxidation of propane-1,3-diol by
periodate
[30,31]. In addition, present observation is contrary to that of
Bhamare et al. [32], who noticed an increase in rate of reaction
withanincreaseinacidity.Inpercholoricacidmedium,potassium
periodate predominantly exists as potassium metaperiodate
(KIO4) and paraperiodic acid (H5IO6) [33].According to Martin
et al. [33], in acidic medium, periodate exists in the following
equilibria:
H5IO6
K1
H+
+ H4IO6
- (1)
H4IO6
-
K2
2H2O + IO4
- (2)
H4IO6
-
K3
H+
+ H3IO6
-2 (3)
Increase in catalyst concentration has resulted in an increase
inreactionrate.Unitorderdependenceon[Ru(III)]wasobserved
from the slope value of the plot between log k1 and log [Ru(III)]
(Fig. 4). It shows the active involvement of catalyst species in
similar to the studies of Bhamare and Kulkarni [34].
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
4+logk1
0.3 0.8 1.3 1.8 2.3
8 + log [Ru(III)]
y = 1.1674x + 0.0761
R = 0.9874
2
Fig. 4. Effect of concentration of catalyst on oxidation of propane-1,3-
diol by KIO4
Effect of addition of solvent acetic acid: Acetic acid is
a commonly used dipolar aprotic solvent, which possesses lower
dielectric constant compared to water. In order to understand
the influence of dielectric constant on the reaction rate in the
present conditions, different compositions of acetic acid and
water were used. A decrease in reaction rate with an addition
of acetic was observed in this study (Table-1).A negative slope
was observed for the plot between log k1 versus 1/D (Fig. 5).
It indicates the reaction between negative ion and dipole [35].
However, the rate constant value decreased marginally with
an increase in percentage of acetic acid. It specifies the involve-
ment of dipole-dipole reactions too. It indicates that H5IO6 is
also an active oxidant species in addition to H4IO6
−
and IO4
−
.
In the oxidation of propane-1,3-diol and butane-1,4-diol by
N-chlorosaccharin, a decrease in rate constant with an increase
inphosphoricacidwasreportedbySingh etal.[24],whichcorro-
borates with the present trend.
2.8
2.7
2.6
2.5
2.4
2.3
2.2
2.1
2.0
5+logk1
y = -25.419x + 2.7899
R = 0.5987
2
0.010 0.015 0.020 0.025
1/D
Fig. 5. Effect of solvent composition on oxidation of propane-1,3-diol by
KIO4
Temperatureeffect:Anincreaseinfirstorderrateconstants
(k1) was observed with an increase in temperature (Table-1)
and yielded a straight line in the plot of ln k vs. 1/T (Fig. 6). By
using slope of the plot and Eyring equation, various activation
parameters were calculated (Table-2). The transition state is
highly solvated, in view of high positive values for ∆G#
. Further,
a reduction in the degree of freedom of molecule can be
observed in the so formed activated complex, as ∆S#
is negative.
2.4
2.2
2.0
1.8
1.6
4+logk1
y = -2.4964x + 9.7239
R = 0.9638
2
2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30
1000/T
Fig. 6. Effect of temperature on oxidation of propane-1,3-diol by KIO4
TABLE-2
ARRHENIUS PARAMETERS AT 308 K
∆E≠
(KJ/mol)
∆H≠
(KJ/mol)
−∆S≠
(JK–1
/mol)
log10 PZ
∆G≠
(KJ/mol)
47.79 45.23 177.07 3.99 99.77
1572 Koteswara Rao et al. Asian J. Chem.
Effect of boric acid, salts and ionic strength:A marginal
increase in reaction rate was observed with an addition of boric
acid. However, a substantial decrease in reaction rate with an
addition of halide ions is observed. Such an inhibition by
chloride ions was reported by Gupta et al. [30,36] in Cr(VI)
oxidation of diols, which was explained based on the formation
of less reactive CrO3Cl−
. Chloride inhibition was taken as
indirect evidence for the complexation between Cr(VI) and
substrate. Ionic strength variation didn't effect the rate of
reaction. The effect of salts on reaction rate was discussed by
Kulkarni et al. [37] based on their studies about Ru(III) cata-
lyzed oxidative degradation of azidothymidine by Mn(VII).
Stoichiometry and product analysis: The reaction was
allowed to proceed under experimental conditions (i.e.[propane-
1,3-diol] >> [periodate]) for overnight in a thermostatted water
bath in order to determine the reaction stoichiometry. Acidic
solution of 2,4-DNP was added to the reaction mixture and
maintained overnight at refrigerated temperature. Ice cold water
was used to wash the corresponding 2,4-dinitrophenylhyd-
razone, which was separated by centrifugation of the above
refrigerated mixture.The melting point of the air dried product
was found to be 425 K, which is in agreement with the earlier
reported values. IR and NMR spectral data confirmed the form-
ation of phenylhydrazone of 3-hydroxypropanal. Hence, the
corresponding reaction equation is shown below:
4 2 2 2 2 2 3 2IO HOCH CH CH OH HOCH CH CHO IO H O− −
+ → + +
It indicates the attacking of single hydroxy group whereas
the other hydroxyl group of the diol remain intact. It substan-
tiates the above observed stoichiometry of the reaction.
Variation in the product was reported based on either with
or without fission of C-C linkage in diols. Fission of C-C leading
totheformationofacetaldehydeandformaldehydewerereported
in the oxidation of propane-1,3-diol in presence ofAg+
/ peroxy-
disulphate[38] andF−
/acidpermanganate[39].However,instead
of C-C bond cleavage, one of the terminal hydroxyl groups
oxidizes in acid medium to yield hydroxyaldehyde [30,36].
In alkaline medium, 3-hydroxypropanal was reported as the
product in the oxidation of propane-1,3-diol in presence of
different combinations of catalyst/oxidant viz. Os(VIII)/chlor-
amine-T [40], Ru(III)/hexacyanoferrate(III) [41,42]. Same
product was reported in acidic medium in presence of different
catalyst/oxidant combinations viz. Ru(III)/Ce(IV) [23] andAg+
/
peroxodiphosphate [22]. In similar lines, 3-hydroxypropanal,
4-hydroxybutanal and 5-hydroxypentanal were the products
intheoxidationofpropane-1,3-diol, butane-1,4-diolandpentane-
1,5-diol, respectively [24,43].
Reaction mechanism: In aqueous medium, a range of
complexes [Ru(H2O)xCl6-x]x-3
were formed by ruthenium(III)
chloride [44] where x = 0 to 6. Based on structural and electro-
chemical studies, it was confirmed that x value varies from 4
to 6 in acidic medium [45-47]. However, perchlorate ion of
HClO4 has affinity towards ruthenium(III) to form a complex
[44,48]. Based on the spectral study carried out on RuCl3 in
acidic medium by Connick and Fine [49], [Ru(H2O)6]3+
is the
principal species. Literature survey shows that Ru(III) involves
in different mechanisms. In different acidic media (perchloric,
sulphuric, trifluoromethane sulphonic acid, trifluoro acetic acid
and p-toluenesulphonic acid), 0.239V was the redox potential
[45] for [Ru(H2O)6]3+
/[Ru(H2O)6]2+
. Ruthenium(III) is consi-
dered to be a weak oxidant in moderate concentrations of perch-
loric acid as the potential is about 0.1 V. Hence, instead of
trivalent ruthenium, octavalent ruthenium species is considered
to be active species which forms by the oxidation of Ru(III) in
acid medium [45,50]. For some of the substrates, ruthenium
forms a complex with substrate which decomposes to form final
products with recycling of catalyst [51-54]. In other reactions,
Ru(III) is converted to Ru(V) by the oxidant and the activated
catalyst participates in the reaction [55]. As per the previous
investigation, it was reported that at the higher concentration
of H+
or lower values of pH, catalyst Ru(III) is oxidized to its
higher oxidized forms Ru(VI) and Ru(VII) due to the higher
oxidation potential of oxidant. During the oxidation of Ru(III)
to higher oxidized forms by oxidant species [56], Ru(VI) is
found to be unstable species due to its rapid conversion into
hydroxide of ruthenium i.e. Ru(OH)4.
According to Menghani and Bakore et al. [38], propane-
1,3-diol complexes with [Ru(H2O)5OH]2+
to form a complex
in which one hydroxyl group coordinates with the active species.
Singh et al. [41,57] reported theestablishmentofRu(III)-alcohol
complex in alkaline medium, where hydride ion present on the
α-carbon (of alcohol) is abstracted by the catalyst to form
ruthenium ion and carbonium ion. A similar hydride abstrac-
tion from the α-carbon of propane-1,3-diol was proposed in
the Os(VIII) catalyzed oxidation by chloramine-T in alkaline
medium [40]. In such a case, an electron transfer was proposed
to the oxidant from the substrate via the catalyst.
Ionic [58] and free radical [59] pathways were proposed
in the Ru(III) catalyzed oxidations of alcohols. Free radical
mechanism was also adopted in the oxidation of 1,3-propane-
diol in the presence of different catalysts like silver (I) [6]. How-
ever, involvement of ionic pathway is proposed in the present
case taking into consideration of the above solvent effect on
rate of reaction and non-formation of insoluble precipitate in
free radical test by using acrylonitrile [56,60]. In addition,
propane-1,3-diol forms a complex with [Ru(H2O)6]3+
. In rate
determining step, the complex reacts with periodate to form a
geminal diol, which is unstable and hence undergoes loss of
water molecule in a fast step to form the concerned aldehyde
as a final product (Scheme-I).
Rate law:Propane 1,3-diol complexes with ruthenium(III)
catalyst.
1,3-diol + Catalyst
K4
Complex C
In rate determining steps, complex C reacts with different
species of periodate to form intermediate complexes, which
decompose quickly to final products.
C +
k1
IO4
- IP1
P
fast
slow
C + H5IO6 IP2 Pk2
slow
fast
C + H4IO6
- IP3
Pk3
slow
fast
Vol. 32, No. 7 (2020) Substrate Inhibition in Ru(III) Catalyzed Oxidation of Propane-1,3-diol by Periodate 1573
H2C
H2C
CH2OH
+ Ru
III CH2 O Ru
IIIH
H2C
CH2OH
OH
IO4
-
H
C
H2C
HOH2C
OHRuIII
+IO3
- + HO
H
C
H2C
HOH2C
OHHO
H2C
H2C
HC
O
OH
+ H2O
Scheme-I: Mechanism of Ru(III) catalyzed oxidation of propane-1,3-diol
by periodate
C + H3IO6
2– IP4 Pk4
slow
fast
From eqn. 1:
1 5 6
4 6
K [H IO ]
[H IO ]
[H ]
−
+
=
From eqn. 2:
1 2 5 6
4 2 4 6
K K [H IO ]
[IO ] K [H IO ]
[H ]
− −
+
= =
From eqn. 3:
2 3 4 6 1 3 5 6
3 6 2
K [H IO ] K K [H IO ]
[H IO ]
[H ] [H ]
−
−
+ +
= =
Total concentration of periodate [IO4
−
]T is given by:
2
4 T 5 6 3 6 4 6 4[IO ] [H IO ] [H IO ] [H IO ] [IO ]− − − −
= + + +
From eqn. 3, it is clear that H3IO6
2–
exists preferably at low
concentrations of acid. As [H+
] >> [IO4
−
], [H3IO6
2−
)] is negli-
gible. Hence, the above equation can be rewritten as:
4 T 5 6 4 6 4[IO ] [H IO ] [H IO ] [IO ]− − −
= + +
1 5 6 1 2 5 6
5 6
K [H IO ] K K [H IO ]
[H IO ]
[H ] [H ]+ +
= + +
{ }5 6
1 1 2
[H IO ]
[H ] K K K
[H ]
+
+
= + +
In percholoric acid medium, potassium periodate predo-
minantly exists in potassium metaperiodate (KIO4) and paraper-
iodic acid (H5IO6) [16]. By rearranging the above equation,
we get:
4 T
5 6
1 1 2
[IO ] [H ]
[H IO ]
[H ] K K K
− +
+
=
+ +
Total concentration of catalyst [Catalyst]T can be written
as:
[Catalyst]T = [Catalyst] + [Complex]
= [Catalyst] + K4 [1,3-diol][Catalyst]
= [Catalyst] {1+ K4 [1,3-diol]}
By rearranging the above equation:
T
4
[Catalyst]
[Catalyst]
1 K [1,3-diol]
=
+
1 4 2 5 6
3 4 6
Rate k [Complex][IO ] k [Complex][H IO ]
k [Complex][H IO ]
−
−
= + +
{ }1 4 2 5 6 3 4 6[Complex] k [IO ] k [H IO ] k [H IO ]− −
= + +
1 2 5 6
4 1
1 5 6
2 5 6 3
K K [H IO ]
K [1,3-diol][Catalyst] k
[H ]
K [H IO ]
k [H IO ] k
[H ]
+
+

= +


+ 

{ }4 5 6
1 1 2 2 3 1
K [1,3-diol][Catalyst][H IO ]
k K K k [H ] k K
[H ]
+
+
= + +
{ }
4 T
4
4 T
1 1 2 2 3 1
1 1 2
K [1,3-diol] [Catalyst]
[H ] 1 K [1,3-diol]
IO ] [H ]
k K K k [H ] k K
[H ] K K K
+
− +
+
+
 
=  
+ 
 
+ + 
+ + 
1 1 2 2 3 14 T 4 T
4 1 1 2
k K K k [H ] k KK [1,3-diol][Catalyst] [IO ]
1 K [1,3-diol] [H ] K K K
+−
+
 + +
=  
+ + + 
The above rate law explains the observed first order in
oxidant and ruthenium(III) and inverse fractional order with
the concentrations of acid and substrate.
Conclusion
The observed substrate inhibition in the ruthenium(III)
catalyzed periodate oxidation of propane-1,3-diol was expl-
ained based on the competitive formation of a complex between
propane-1,3-diol and periodate. Based on the inverse acid
dependence and solvent effect, H5IO6, H4IO6
−
and IO4
−
were
the active periodate species. Formation of 3-hydroxy propanal
as the final product indicates the attacking of single hydroxy
group, whereas the other hydroxyl group of substrate remain
intact.
ACKNOWLEDGEMENTS
One of the authors, KVSK is thankful to UGC, New Delhi,
IndiaforfinancialsupportintheformofMinorResearchProject,
F.No.4-4/2015-15(MRP-SEM/UGC-SERO), November 2014.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interests
regarding the publication of this article.
1574 Koteswara Rao et al. Asian J. Chem.
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Vol. 32, No. 7 (2020) Substrate Inhibition in Ru(III) Catalyzed Oxidation of Propane-1,3-diol by Periodate 1575

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Substrate Inhibition in Ruthenium(III) Catalyzed Oxidation of Propane-1,3-diol by Periodate in Acidic Medium: A Kinetic Study

  • 1. A J CSIAN OURNAL OF HEMISTRYA J CSIAN OURNAL OF HEMISTRY https://doi.org/10.14233/ajchem.2020.22646 INTRODUCTION Potassium periodate is a versatile oxidant that can oxidize a number of organic and inorganic compounds. Periodate is used both in alkaline and acidic media [1-5]. In acidic medium, the existing species is H5IO6 and its non-existence was reported in the pH range of 5 to 9. In aqueous medium with a pH range of 6.1 to 7.3, the possible species of periodate are IO4 − , H4IO6 − and H3IO6 2− , whereas, the predominant species is H4IO6 − near pH = 7.0. In alkaline medium (0.005 to 0.1 M hydroxide concen- tration), trihydrogen paraperiodate (H3IO6 2− ) and dihydrogen paraperiodate(H2IO6 3− )canbetheexpectedpredominantspecies. Due to participation in the following reaction, the expected species is H2IO6 3− in highly alkaline medium. 2 3 3 6 2 6 2H IO OH H IO H O− − − + → + H2I2O10 4− is another species in alkaline medium. Periodate existsinitsdimerformI2O9 4− athigherconcentrationsofperiodate [6]. Oxidation of pharmaceutically important polyhydric comp- ounds like polyethylene glycols was studied in alkaline medium [7,8]. In those PEG oxidations, transfer of one oxygen atom or two electrons was observed for each periodate, i.e. periodate was able to act as oxidant in these reactions till its conversion to iodate. Among the four species of periodate (H2IO6 3− , H3IO6 2− , Substrate Inhibition in Ruthenium(III) Catalyzed Oxidation of Propane-1,3-diol by Periodate in Acidic Medium: A Kinetic Study K.V.S. KOTESWARA RAO 1, , R. VENKATA NADH 2,*, and K. VENKATA RATNAM 2, 1 Department of Chemistry, G.V.S.M. Government Degree College, Ulavapadu-523292, India 2 Department of Chemistry, GITAM University-Bengaluru Campus, Bengaluru-562163, India *Corresponding author: E-mail: doctornadh@yahoo.co.in Received: 22 January 2020; Accepted: 12 March 2020; Published online: 27 June 2020; AJC-19914 Ruthenium(III) catalyzed oxidation of propane-1,3-diol by potassium periodate was studied in aqueous perchloric acid medium. Orders of reaction with respect to concentrations of oxidant, substrate, acid and catalyst were determined. First order in oxidant and catalyst concentrations, and inverse fractional order in acid medium were observed. In addition, substrate inhibition (i.e. a decrease in reaction rate with an increase in substrate concentration) was observed. Effect of addition of salt and solvent was studied. Based on the studies of temperature variation, Arrhenius parameters were calculated. Plausible mechanism was also proposed based on observed kinetics. Keywords: Ruthenium(III), Propane-1,3-diol, Oxidation, Acidic medium, Potassium periodate, Kinetics. Asian Journal of Chemistry; Vol. 32, No. 7 (2020), 1569-1575 This is an open access journal, and articles are distributed under the terms of the Attribution 4.0 International (CC BY 4.0) License. This license lets others distribute, remix, tweak, and build upon your work, even commercially, as long as they credit the author for the original creation. You must give appropriate credit, provide a link to the license, and indicate if changes were made. IO4 − and H2I2O10 4− ) in alkaline medium, concentrations of the first two species are considerable, whereas the concentrations of the other two are trivial in the range of alkali from 0.05 to 0.5 M. Moreover, the predominant species is H2IO6 3− at higher concentration of alkali, however at low alkali concentration it is H3IO6 2− . In addition to well-known trivalent, different oxidation states of ruthenium catalyst were proposed in different reactions. Ruthenium(IV) was the proposed reactive form of ruthenium intheoxidationofdiolsbyperoxodiphosphateinacidicmedium. Alteration of electrons availability at the oxidation site by the positive inductive effect was used to explain the reactivity order of ethane-1,2-diol < propane-1,2-diol < butane-1,2-diol. Some researchers [4,9] reported the quick conversion of trivalent ruthenium to octavalent by periodate in acidic medium. Further reported the existence of Ru(VIII) in acidic medium in the form of H2RuO5 or RuO3(OH)2.The former was assumed to be active catalytic species in the kinetics of ruthenium(III) catalyzed oxidation of methyl glycol by periodate in perchloric acid [4] to form the corresponding monocarboxylic acid as the product. Participation of more powerful oxidant species Ru(V) was proposed for the observed catalysis in the oxidation of alanine [10] and monoethanolamine [11] by N-bromosucci-
  • 2. nimide (NBS) in aqueous acetic acid-perchloric acid medium. Acceleration of NBS oxidation of monoethanolamine with the involvement of Ru(V) was substantiated with the presence and absence of catalysis with Ru(III) and Os(VIII), respectively. It indicates the absence of complexation in these reactions, but the formation of Ru(V) [11]. Oxidation of alcohols by various oxidants is well docu- mented in literature. In the kinetics of oxidation of neopentyl glycol by diperiodatocuprate(III) [5], protonated form of H2IO6 3− coordinates with central copper ion in alkaline medium to form [Cu(H2IO6)2]3− , which further reacts with OH− to give [Cu(HIO6)]− . So formed Cu* (III) was the active species to form the corresponding aldol as the final product. In the oxidation of aliphatic alcohols by iodate in acidic medium, reactions were explained based on the involvement of halic acid (HIO3) and hypoiodite ion (IO− ). Order of reaction rate in that study was given as 1-hexanol < isobutyl alcohol < isopropyl alcohol < 1-butanol <1-propanol. Length of chain and structural features of the substrates were taken into consideration to explain the reactivity. Compared to 2º aliphatic alcohols, the oxidation of 1º aliphatic alcohols was faster with an exception to 1-hexanol. An inverse proportionality in reaction rate was observed with respect to the chain length. Cadmium(II) was found to be the effectivecatalystinthoseoxidations,amongthestudiedtransition metal ions. Aldehydes and ketones were the products in the oxidation of primary and secondary aliphatic alcohols [12]. In the oxidation of secondary cyclic alcohols by different inorganic oxidants (potassium bromate, potassium iodate and potassium periodate) in acidic medium, the corresponding halic acids (HBrO3, HIO3 and HIO4) were reported as active oxidizing species [13]. The oxidation products were the corresponding ketones and same reaction order was reported with all oxidants as menthol < isoborneol < borneol. Structural features (steric hindrance and isomeric) were considered to be the responsible for the observed sequence of secondary alcohols reactivity. High and low vulnerability of oxidation in borneol and menthol towards oxidation was explained based on the least and most hindered α-hydrogen present in them respectively. The H2OBr+ wastheproposedactiveoxidisingspeciestoexplaintheenhanced reaction rate by the addition of bromide ion in the oxidation of n-butanol by p-methoxy-n-bromobenzamide in aqueous acetic acid [14]. Higher rate of ditelluratoargentate(III) oxidation of 2-(2- ethoxyethoxy)ethanol (EEE) compared to the oxidation of 2- (2-methoxy ethoxy)ethanol (MEE) in alkaline medium was described based on the higher electron-donating ability of the former compared to the latter [15]. The observed experimental results were explained based on a free radical mechanism. Similarly, higher rate of oxidation of 2-ethoxyethanol by K2FeO4 in alkaline medium compared to the oxidation of methoxy- ethanol was based on electron donating ability [16]. Propane-1,3-diol has a spectrum of applications as a mono- mer in the production of different polymers like polyethers, polytrimethylene terephthalate, polyurethanes and polyesters [17,18]. It is also used as protective agent/antifreeze agent and solvent [19,20]. Oxidation of vicinal diols in presence of ruthe- nium catalyst was extensively studied and mechanistic pathways were explored. But, the reaction pattern was not studied compl- etely in departed hydroxyl groups of diols. Singh et al. [21] studied the kinetics of oxidation of 1,3-propanediol by alkaline N-bromoacetamide in presence of Os(VIII). Silver(I) catalyzed oxidation was studied by Singh et al. [22] using peroxodi- phosphate as an oxidant in acetate buffers to form 3-hydroxy propanol.Agrawal et al. [23] carried out the Ru(III) catalyzed oxidation of diols by ceric sulphate in aqueous sulphuric acid medium.As the oxidation studies on non-vicinal diols are scanty, the present study is aimed at understanding the mechanism involved in ruthenium(III) catalyzed oxidation of propane-1,3- diol by periodate in aqueous perchloric acidic medium. EXPERIMENTAL Allthechemicalsusedinthepresentstudywereofanalytical grade. Double distilled water was used to prepare the solutions. Kinetics of Ru(III) catalyzed oxidation of propane-1,3-diol by periodate in aqueous perchloric acid medium was followed by iodometry.Thereactiontookplacetilltheconversionofperiodate toiodate.AseparatetestwasconductedwhereKIO3 didn'toxidize propane-1,3-diol. However, Singh et al. [24] reported aquachloro-rhodium(III) catalysis in oxidation of diol (1,2- propanediol) by potassium iodate in acidic medium. Generalprocedure:Thecatalystwasmaintainedonoxidant side before mixing of oxidant and substrate. Both the oxidant and substrate solutions (maintained along with desired acid concentration and solvent composition) were kept in thermostat at the required constant temperature for 1 h. Both of them were mixed and then drawn at regular intervals of time. The kinetics experiments were repeated in triplicate and the results were reproducible within ± 4%. Detectionmethod:Theformedproductinthepresentstudy was converted to concerned phenylhydrazone by using acidic 2,4-DNP (2,4-dinitrophenylhydrazine). Then the hydrazone was characterized by melting point and spectral studies. RESULTS AND DISCUSSION In the present study, an attempt is made to study the kinetics of uncatalyzed and Ru(III) catalyzed oxidation of non-vicinal diol (propane-1,3-diol) and vicinal diols (glycol, glycerol, propane-1,2-diol) in aqueous perchloric acid medium. It is observed that uncatalyzed oxidation of vicinal diols (glycol, glycerol, propane-1,2-diol) by periodate in aqueous perchloric acid medium was completed instantaneously. Whereas, the uncatalyzed oxidation of propane-1,3-diol under similar conditions was infinitesimally slow and Ru(III) catalyzed oxidation of non-vicinal diol (propane-1,3-diol) by periodate is a slow reaction with observable kinetics. The difference in the reactivity of propane-1,3-diol from vicinal diols can be understood from the fact that propane-1,3-diol is a simple acyclic molecule and won't display any sort of steric strain. Hence, kinetics of Ru(III) catalyzed oxidation of non-vicinal diol (propane-1,3-diol) by periodate is carried out in the present study. Effect of variation and hence reaction orders with respect to various reactants are discussed below. Variationof[periodate]:Whilekeepingtheconcentrations of other reactants at constant concentrations, [periodate] was 1570 Koteswara Rao et al. Asian J. Chem.
  • 3. varied in the range 0.00025 to 0.002 M. Constructed the pseudo first order plots i.e. log(a-x) against time (Fig. 1). Linearity of plots was observed upto completion of 85% of total reaction. In addition, it was found that values of pseudo-first-order constants (k1) were independent of the initial [oxidant] (Table-1). TABLE-1 RATE CONSTANTS IN Ru(III) CATALYZED OXIDATION OF 1,3-PROPANE-DIOL BY POTASSIUM PERIODATE IN AN ACID MEDIUM General conditions: [KIO4] = 0.0005 M; [Substrate] = 0.025 M; [Ru(III)] = 2 × 10–7 M; [H+ ] = 0.1 M; Temperature = 308 K Variant Conc. of variant (M) k1 × 104 min–1 0.00025 46.52 0.00050 46.53 0.00100 47.63 [Periodate] 0.00200 49.63 0.01250 52.33 0.02500 46.53 0.05000 28.42 [Substrate] 0.10000 18.02 0.025 65.56 0.050 56.11 0.100 46.53 0.200 25.25 [Acid] 0.500 18.97 0 46.53 KCl (0.1 M) 10.74 KBr (0.1 M) 8.85 KI (0.1 M) No reaction Salt KNO3 (0.1 M) 44.88 0 M Infinitesimally slow 0.25 × 10-7 2.94 0.50 × 10-7 7.42 1.0 × 10-7 22.58 2.0 × 10-7 46.53 4.0 × 10-7 82.83 [Catalyst] 8.0 × 10-7 172.49 0 46.53 0.01 51.32 0.025 51.67 [Boric acid] 0.05 53.58 308 46.53 313 54.96 318 64.23 323 96.49 Temperature (K) 333 183.37 0:100 46.53 05:95 26.16 20:80 18.78 40:60 15.34 Acetic acid:water (v/v) 60:40 14.15 Variation of [substrate]: Substrate inhibition is observed in the present case i.e. an increase in concentration of propane- 1,3-diol from 0.0125 to 0.1 M has resulted in a decrease of rate constant from 52.33 × 10-4 to 18.02 × 10-4 min-1 (Table-1). The plot of log k1 vs. log[propane-1,3-diol] shows a slope of -0.532 (Fig. 2), which indicates the inverse fractional order in [substrate]. Literature [25,26] shows that substrate inhibition 0.40 0.39 0.38 0.37 0.36 0.35 0.34 0.33 0.32 4+log(a-x) 0 10 20 30 40 Time (min) y = -0.00202x + 0.39962 R = 0.99448 2 Fig. 1. Plot of log(a-x) versus time at [KIO4] = 0.0025 M, [1,3-diol] = 0.025 M, [Ru(III)] = 2 × 10–7 M, [H+ ] = 0.1 M and Temperature = 308 K 1.8 1.6 1.4 1.2 4+logk1 2.0 2.2 2.4 2.6 2.8 3.0 4 + log [S] y = -0.5325x + 2.8811 R = 0.9483 2 Fig. 2. Effect of substrate concentration on oxidation of propane-1,3-diol by KIO4 was reported in the oxidation of different sugar alcohols by periodate in alkaline medium, which was assigned to the forma- tion of periodate substrate complex which resists oxidation. In the present case, an increase in substrate concentration increases the chances of forming an activated complex between substrateandthecatalyst.Atthesametime,theincreasednumber of substrate molecules improves their complexation possibility withtheoxidant.Andhence,decreasestheavailabilityofoxidant to attack the substrate-catalyst complex. However, indepen- dence of substrate concentration was reported in the periodate oxidation of polyethylene glycols (PEGs) in alkaline medium [27,28] which can be explained based on the fact that PEGs possess high hydroxyl values [29]. Variation of [acid] and [catalyst]: A decrease of rate constant was resulted from 65.56 × 10-4 to 18.97 × 10-4 min-1 by increasing the concentration of perchloric acid concen- tration from 0.025 to 0.50 M (Table-1). An inverse fractional order in [H+ ] was noticed as the slope is -0.444 in the plot of logk1 vs. log[perchloric acid] (Fig. 3). Literature survey shows contrary results regarding acid effect on kinetics of oxidation of propane-1,3-diol. Inverse order in acid concentration was reportedbySinghetal.[22,24] inphosphotungsticacidcatalyzed N-chlorosaccharin oxidation and silver ions catalyzed peroxy- disulphate oxidation. However, an increase in rate constants with increase in hydrogen ion concentration was noticed in the oxidation of terminal hydroxyl compounds by chromium(VI) Vol. 32, No. 7 (2020) Substrate Inhibition in Ru(III) Catalyzed Oxidation of Propane-1,3-diol by Periodate 1571
  • 4. 2.0 1.8 1.6 1.4 1.2 4+logk1 2.3 2.5 2.7 2.9 3.1 3.3 3.5 3.7 4 + log [H ] + y = -0.4447x + 2.9253 R = 0.9476 2 Fig. 3. Effect of acid concentration on oxidation of propane-1,3-diol by periodate [30,31]. In addition, present observation is contrary to that of Bhamare et al. [32], who noticed an increase in rate of reaction withanincreaseinacidity.Inpercholoricacidmedium,potassium periodate predominantly exists as potassium metaperiodate (KIO4) and paraperiodic acid (H5IO6) [33].According to Martin et al. [33], in acidic medium, periodate exists in the following equilibria: H5IO6 K1 H+ + H4IO6 - (1) H4IO6 - K2 2H2O + IO4 - (2) H4IO6 - K3 H+ + H3IO6 -2 (3) Increase in catalyst concentration has resulted in an increase inreactionrate.Unitorderdependenceon[Ru(III)]wasobserved from the slope value of the plot between log k1 and log [Ru(III)] (Fig. 4). It shows the active involvement of catalyst species in similar to the studies of Bhamare and Kulkarni [34]. 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 4+logk1 0.3 0.8 1.3 1.8 2.3 8 + log [Ru(III)] y = 1.1674x + 0.0761 R = 0.9874 2 Fig. 4. Effect of concentration of catalyst on oxidation of propane-1,3- diol by KIO4 Effect of addition of solvent acetic acid: Acetic acid is a commonly used dipolar aprotic solvent, which possesses lower dielectric constant compared to water. In order to understand the influence of dielectric constant on the reaction rate in the present conditions, different compositions of acetic acid and water were used. A decrease in reaction rate with an addition of acetic was observed in this study (Table-1).A negative slope was observed for the plot between log k1 versus 1/D (Fig. 5). It indicates the reaction between negative ion and dipole [35]. However, the rate constant value decreased marginally with an increase in percentage of acetic acid. It specifies the involve- ment of dipole-dipole reactions too. It indicates that H5IO6 is also an active oxidant species in addition to H4IO6 − and IO4 − . In the oxidation of propane-1,3-diol and butane-1,4-diol by N-chlorosaccharin, a decrease in rate constant with an increase inphosphoricacidwasreportedbySingh etal.[24],whichcorro- borates with the present trend. 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2.0 5+logk1 y = -25.419x + 2.7899 R = 0.5987 2 0.010 0.015 0.020 0.025 1/D Fig. 5. Effect of solvent composition on oxidation of propane-1,3-diol by KIO4 Temperatureeffect:Anincreaseinfirstorderrateconstants (k1) was observed with an increase in temperature (Table-1) and yielded a straight line in the plot of ln k vs. 1/T (Fig. 6). By using slope of the plot and Eyring equation, various activation parameters were calculated (Table-2). The transition state is highly solvated, in view of high positive values for ∆G# . Further, a reduction in the degree of freedom of molecule can be observed in the so formed activated complex, as ∆S# is negative. 2.4 2.2 2.0 1.8 1.6 4+logk1 y = -2.4964x + 9.7239 R = 0.9638 2 2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30 1000/T Fig. 6. Effect of temperature on oxidation of propane-1,3-diol by KIO4 TABLE-2 ARRHENIUS PARAMETERS AT 308 K ∆E≠ (KJ/mol) ∆H≠ (KJ/mol) −∆S≠ (JK–1 /mol) log10 PZ ∆G≠ (KJ/mol) 47.79 45.23 177.07 3.99 99.77 1572 Koteswara Rao et al. Asian J. Chem.
  • 5. Effect of boric acid, salts and ionic strength:A marginal increase in reaction rate was observed with an addition of boric acid. However, a substantial decrease in reaction rate with an addition of halide ions is observed. Such an inhibition by chloride ions was reported by Gupta et al. [30,36] in Cr(VI) oxidation of diols, which was explained based on the formation of less reactive CrO3Cl− . Chloride inhibition was taken as indirect evidence for the complexation between Cr(VI) and substrate. Ionic strength variation didn't effect the rate of reaction. The effect of salts on reaction rate was discussed by Kulkarni et al. [37] based on their studies about Ru(III) cata- lyzed oxidative degradation of azidothymidine by Mn(VII). Stoichiometry and product analysis: The reaction was allowed to proceed under experimental conditions (i.e.[propane- 1,3-diol] >> [periodate]) for overnight in a thermostatted water bath in order to determine the reaction stoichiometry. Acidic solution of 2,4-DNP was added to the reaction mixture and maintained overnight at refrigerated temperature. Ice cold water was used to wash the corresponding 2,4-dinitrophenylhyd- razone, which was separated by centrifugation of the above refrigerated mixture.The melting point of the air dried product was found to be 425 K, which is in agreement with the earlier reported values. IR and NMR spectral data confirmed the form- ation of phenylhydrazone of 3-hydroxypropanal. Hence, the corresponding reaction equation is shown below: 4 2 2 2 2 2 3 2IO HOCH CH CH OH HOCH CH CHO IO H O− − + → + + It indicates the attacking of single hydroxy group whereas the other hydroxyl group of the diol remain intact. It substan- tiates the above observed stoichiometry of the reaction. Variation in the product was reported based on either with or without fission of C-C linkage in diols. Fission of C-C leading totheformationofacetaldehydeandformaldehydewerereported in the oxidation of propane-1,3-diol in presence ofAg+ / peroxy- disulphate[38] andF− /acidpermanganate[39].However,instead of C-C bond cleavage, one of the terminal hydroxyl groups oxidizes in acid medium to yield hydroxyaldehyde [30,36]. In alkaline medium, 3-hydroxypropanal was reported as the product in the oxidation of propane-1,3-diol in presence of different combinations of catalyst/oxidant viz. Os(VIII)/chlor- amine-T [40], Ru(III)/hexacyanoferrate(III) [41,42]. Same product was reported in acidic medium in presence of different catalyst/oxidant combinations viz. Ru(III)/Ce(IV) [23] andAg+ / peroxodiphosphate [22]. In similar lines, 3-hydroxypropanal, 4-hydroxybutanal and 5-hydroxypentanal were the products intheoxidationofpropane-1,3-diol, butane-1,4-diolandpentane- 1,5-diol, respectively [24,43]. Reaction mechanism: In aqueous medium, a range of complexes [Ru(H2O)xCl6-x]x-3 were formed by ruthenium(III) chloride [44] where x = 0 to 6. Based on structural and electro- chemical studies, it was confirmed that x value varies from 4 to 6 in acidic medium [45-47]. However, perchlorate ion of HClO4 has affinity towards ruthenium(III) to form a complex [44,48]. Based on the spectral study carried out on RuCl3 in acidic medium by Connick and Fine [49], [Ru(H2O)6]3+ is the principal species. Literature survey shows that Ru(III) involves in different mechanisms. In different acidic media (perchloric, sulphuric, trifluoromethane sulphonic acid, trifluoro acetic acid and p-toluenesulphonic acid), 0.239V was the redox potential [45] for [Ru(H2O)6]3+ /[Ru(H2O)6]2+ . Ruthenium(III) is consi- dered to be a weak oxidant in moderate concentrations of perch- loric acid as the potential is about 0.1 V. Hence, instead of trivalent ruthenium, octavalent ruthenium species is considered to be active species which forms by the oxidation of Ru(III) in acid medium [45,50]. For some of the substrates, ruthenium forms a complex with substrate which decomposes to form final products with recycling of catalyst [51-54]. In other reactions, Ru(III) is converted to Ru(V) by the oxidant and the activated catalyst participates in the reaction [55]. As per the previous investigation, it was reported that at the higher concentration of H+ or lower values of pH, catalyst Ru(III) is oxidized to its higher oxidized forms Ru(VI) and Ru(VII) due to the higher oxidation potential of oxidant. During the oxidation of Ru(III) to higher oxidized forms by oxidant species [56], Ru(VI) is found to be unstable species due to its rapid conversion into hydroxide of ruthenium i.e. Ru(OH)4. According to Menghani and Bakore et al. [38], propane- 1,3-diol complexes with [Ru(H2O)5OH]2+ to form a complex in which one hydroxyl group coordinates with the active species. Singh et al. [41,57] reported theestablishmentofRu(III)-alcohol complex in alkaline medium, where hydride ion present on the α-carbon (of alcohol) is abstracted by the catalyst to form ruthenium ion and carbonium ion. A similar hydride abstrac- tion from the α-carbon of propane-1,3-diol was proposed in the Os(VIII) catalyzed oxidation by chloramine-T in alkaline medium [40]. In such a case, an electron transfer was proposed to the oxidant from the substrate via the catalyst. Ionic [58] and free radical [59] pathways were proposed in the Ru(III) catalyzed oxidations of alcohols. Free radical mechanism was also adopted in the oxidation of 1,3-propane- diol in the presence of different catalysts like silver (I) [6]. How- ever, involvement of ionic pathway is proposed in the present case taking into consideration of the above solvent effect on rate of reaction and non-formation of insoluble precipitate in free radical test by using acrylonitrile [56,60]. In addition, propane-1,3-diol forms a complex with [Ru(H2O)6]3+ . In rate determining step, the complex reacts with periodate to form a geminal diol, which is unstable and hence undergoes loss of water molecule in a fast step to form the concerned aldehyde as a final product (Scheme-I). Rate law:Propane 1,3-diol complexes with ruthenium(III) catalyst. 1,3-diol + Catalyst K4 Complex C In rate determining steps, complex C reacts with different species of periodate to form intermediate complexes, which decompose quickly to final products. C + k1 IO4 - IP1 P fast slow C + H5IO6 IP2 Pk2 slow fast C + H4IO6 - IP3 Pk3 slow fast Vol. 32, No. 7 (2020) Substrate Inhibition in Ru(III) Catalyzed Oxidation of Propane-1,3-diol by Periodate 1573
  • 6. H2C H2C CH2OH + Ru III CH2 O Ru IIIH H2C CH2OH OH IO4 - H C H2C HOH2C OHRuIII +IO3 - + HO H C H2C HOH2C OHHO H2C H2C HC O OH + H2O Scheme-I: Mechanism of Ru(III) catalyzed oxidation of propane-1,3-diol by periodate C + H3IO6 2– IP4 Pk4 slow fast From eqn. 1: 1 5 6 4 6 K [H IO ] [H IO ] [H ] − + = From eqn. 2: 1 2 5 6 4 2 4 6 K K [H IO ] [IO ] K [H IO ] [H ] − − + = = From eqn. 3: 2 3 4 6 1 3 5 6 3 6 2 K [H IO ] K K [H IO ] [H IO ] [H ] [H ] − − + + = = Total concentration of periodate [IO4 − ]T is given by: 2 4 T 5 6 3 6 4 6 4[IO ] [H IO ] [H IO ] [H IO ] [IO ]− − − − = + + + From eqn. 3, it is clear that H3IO6 2– exists preferably at low concentrations of acid. As [H+ ] >> [IO4 − ], [H3IO6 2− )] is negli- gible. Hence, the above equation can be rewritten as: 4 T 5 6 4 6 4[IO ] [H IO ] [H IO ] [IO ]− − − = + + 1 5 6 1 2 5 6 5 6 K [H IO ] K K [H IO ] [H IO ] [H ] [H ]+ + = + + { }5 6 1 1 2 [H IO ] [H ] K K K [H ] + + = + + In percholoric acid medium, potassium periodate predo- minantly exists in potassium metaperiodate (KIO4) and paraper- iodic acid (H5IO6) [16]. By rearranging the above equation, we get: 4 T 5 6 1 1 2 [IO ] [H ] [H IO ] [H ] K K K − + + = + + Total concentration of catalyst [Catalyst]T can be written as: [Catalyst]T = [Catalyst] + [Complex] = [Catalyst] + K4 [1,3-diol][Catalyst] = [Catalyst] {1+ K4 [1,3-diol]} By rearranging the above equation: T 4 [Catalyst] [Catalyst] 1 K [1,3-diol] = + 1 4 2 5 6 3 4 6 Rate k [Complex][IO ] k [Complex][H IO ] k [Complex][H IO ] − − = + + { }1 4 2 5 6 3 4 6[Complex] k [IO ] k [H IO ] k [H IO ]− − = + + 1 2 5 6 4 1 1 5 6 2 5 6 3 K K [H IO ] K [1,3-diol][Catalyst] k [H ] K [H IO ] k [H IO ] k [H ] + +  = +   +   { }4 5 6 1 1 2 2 3 1 K [1,3-diol][Catalyst][H IO ] k K K k [H ] k K [H ] + + = + + { } 4 T 4 4 T 1 1 2 2 3 1 1 1 2 K [1,3-diol] [Catalyst] [H ] 1 K [1,3-diol] IO ] [H ] k K K k [H ] k K [H ] K K K + − + + +   =   +    + +  + +  1 1 2 2 3 14 T 4 T 4 1 1 2 k K K k [H ] k KK [1,3-diol][Catalyst] [IO ] 1 K [1,3-diol] [H ] K K K +− +  + + =   + + +  The above rate law explains the observed first order in oxidant and ruthenium(III) and inverse fractional order with the concentrations of acid and substrate. Conclusion The observed substrate inhibition in the ruthenium(III) catalyzed periodate oxidation of propane-1,3-diol was expl- ained based on the competitive formation of a complex between propane-1,3-diol and periodate. Based on the inverse acid dependence and solvent effect, H5IO6, H4IO6 − and IO4 − were the active periodate species. Formation of 3-hydroxy propanal as the final product indicates the attacking of single hydroxy group, whereas the other hydroxyl group of substrate remain intact. ACKNOWLEDGEMENTS One of the authors, KVSK is thankful to UGC, New Delhi, IndiaforfinancialsupportintheformofMinorResearchProject, F.No.4-4/2015-15(MRP-SEM/UGC-SERO), November 2014. CONFLICT OF INTEREST The authors declare that there is no conflict of interests regarding the publication of this article. 1574 Koteswara Rao et al. Asian J. Chem.
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