Oxidation of some α-hydroxy acids by pyridinium fluorochromate in aqueous acetic acid media - A kinetic and mechanistic study

Article abstract of DOI:10.14233/ajchem.2013.13142

Kinetics and mechanism of oxidation of some α-hydroxy acids i.e., glycolic acid, lactic acid or mandelic acid by pyridinium fluorochromate have been studied in aqueous acetic acid medium. The main product of oxidation is oxo acids. The reaction is first order with respect to pyridinium fluorochromate, hydroxy acids and [H⁺]. The reaction is acid catalyzed. The oxidation of α-deuterio mandelic acid exhibits a substantial primary kinetic isotope effect (k_H/k_D = 5.58 at 303 K). The reaction has been studied in different percentage of acetic acidwater mixture. A suitable mechanism has been proposed.

Full text of DOI:10.14233/ajchem.2013.13142

Asian Journal of Chemistry; Vol. 25, No. 2 (2013), 921-925
http://dx.doi.org/10.14233/ajchem.2013.13142
Oxidation of Some α-Hydroxy Acids by Pyridinium Fluorochromate
in Aqueous Acetic Acid Media-A Kinetic and Mechanistic Study
1
2,*
3
S. ZAHEER AHMED , S. SYED SHAFI and S. SHEIK MANSOOR
1
2
3
Department of Chemistry, Islamiah College, Vaniyambadi-635 752, India
Department of Chemistry, Thiruvalluvar University, Serkadu, Vellore-632106, India
Department of Chemistry, C. Abdul Hakeem College, Melvisharam-632 509, India
*
Corresponding author: E-mail: suban_shafi@yahoo.com
(
Received: 30 November 2011;
Accepted: 25 August 2012)
AJC-12010
Kinetics and mechanism of oxidation of some α-hydroxy acids i.e., glycolic acid, lactic acid or mandelic acid by pyridinium fluorochromate
have been studied in aqueous acetic acid medium. The main product of oxidation is oxo acids. The reaction is first order with respect to
+
pyridinium fluorochromate, hydroxy acids and [H ]. The reaction is acid catalyzed. The oxidation of α-deuterio mandelic acid exhibits a
H D
substantial primary kinetic isotope effect (k /k = 5.58 at 303 K). The reaction has been studied in different percentage of acetic acid-
water mixture. A suitable mechanism has been proposed.
Key Words: Pyridinium fluorochromate, Hydroxy acids, Kinetics, Oxidation.
INTRODUCTION
EXPERIMENTAL
Chromium(VI) reagents are widely used in organic
Pyridine and chromium trioxide were obtained from Fluka
(Buchs, Switzerland). The hydroxy acids used were glycolic
acid, lactic acid and mandelic acid. Acetic acid was purified
by standard method and the fraction distilling at 118 ºC was
collected.
chemistry for oxidation of primary and secondary alcohols to
carbonyl compounds. A variety of compounds containing
chromium(VI) have proved to be versatile reagents capable
1
of oxidizing almost every oxidiazable functional group . A
number of new chromium(VI) containing compounds, with
Preparation of pyridinium fluorochromate: Pyridinium
fluorochromate has been prepared from pyridine, 40 %
hydrofluoric acid and chromium trioxide in the molar ratio
1:1.3:1 at 0 ºC. Pyridinium fluorochromate is obtained as
yellow orange crystals. It is non-hygroscopic, light insensitive
2
heterocyclic bases, like pyridinium chlorochromate ,
3
pyridinium bromochromate , quinoliium chlorochromate ,
4
5
benzimidazolium fluorochromate , quinolinium bromochro-
6
7
mate , imidazolium fluorochromate , pyridinium fluorochro-
8
mate (PFC) , tributylammonium chlorochromate (TriBACC) ,
9
8
on storage . The purity of pyridinium fluorochromate was
checked by the iodometric method.
1
0
tripropylammonium fluorochromate (TriPAFC) , benzyl-
1
trimethyl ammonium fluorochromate (BTMAFC) and
1
Kinetic measurements:The pseudo-first-order conditions
were attained by maintaining a large excess (×15 or more) of
α-hydroxy acids over pyridinium fluorochromate. The solvent
was 50 % acetic acid + 50 % water (v/v), unless specified other-
wise. The reactions were followed, at constant temperatures
(± 0.01 K), by monitoring the decrease in [PFC] spectrophotomet-
12
imidazoliun dichromate have been developed to improve the
selectivity of oxidation of organic compounds. The kinetics
and mechanism of oxidation of hydroxyl acids by various
1
3-19
oxidants have been reported . However, the kinetics of
oxidation of hydroxyl acids by pyridinium fluorochromate, a
Cr(VI) reagent has not yet been studied. This prompted us to
undertake the present investigation. The present work reports
the kinetics of oxidation of α-hydroxy acids by pyridinium
fluorochromate and evaluates the reaction constants.
rically at 366 nm. The pseudo-first-order rate constant. kobs
,
was evaluated from the linear (r = 0.990 to 0.999) plots of log
[PFC] against time for up to 80 % reaction. The second order
2 2
rate constant k , was obtained from the relation k = kobs/[HA].

922 Ahmed et al.
Asian J. Chem.
Data analysis: Correlation analysis were carried out using
TABLE-1
+
EFFECT OF VARIATION OF [HA], [PFC] AND [H ]
ON THE RATE OF THE REACTION AT 303 K
Microcal origin (version 6) computer software. The goodness
of the fit was discussed using the correlation coefficient (r in
the case of simple linear regression and R in the case of multiple
linear regression) and standard deviation (SD).
3
0 [PFC] 10 [HA]
2
+
5
10 k (s )
-1
1
(
[H ]
-3
mol dm ) (mol dm ) (mol dm )
1
-
3
-3
GA
9.12
9.04
9.00
9.08
9.10
4.38
6.62
11.12
13.42
5.28
7.08
10.64
12.48
LA
MA
0.6
0.8
1.0
1.2
1.4
1.0
1.0
1.0
2.0
2.0
2.0
2.0
2.0
1.0
1.5
2.5
3.0
2.0
2.0
2.0
2.0
0.1
0.1
13.00
13.30
13.50
13.60
13.50
6.80
22.56
22.72
22.90
22.96
22.62
11.08
16.70
28.00
33.40
13.40
18.40
27.18
31.60
RESULTS AND DISCUSSION
0.1
Product analysis: Product analysis was carried out
under kinetic conditions i.e. with excess of the reductant over
pyridinium fluorochromate. In a typical experiment, mandelic
acid (15.2 g, 0.l mol), perchloric acid (0.1 mol) and pyridinium
fluorochromate (0.01 mol) were dissolved in acetic acid-
water mixture (50% + 50%) and the solution was allows to
stand in the dark for about 24 h to ensure completion of the
reaction. The residue was treated with an excess (200 mL) of
a saturated solution of 2,4-dinitro phenylhydrazine in 1 mol
0.1
0.1
0.1
0.1
10.29
17.35
20.90
7.92
0.1
1.0
1.0
0.1
0.06
0.08
0.12
0.14
1.0
1.0
1.0
10.40
15.96
18.90
-3
dm HCl and kept overnight in a refrigerator. The precipitated
,4-dinitro phenyl hydrozone (DNP) was filtered off, dried
Solvent composition = 50 % AcOH- 50 % H O (v/v)
2
GA = Glycolic acid, LA = Lactic acid, MA = Mandelic acid.
2
and recrystallized from ethanol. The product was identical (mp
and mixed mp) to an authentic sample of the DNP of phenyl
glyoxylic acid.
1
1
1
1
1
.6
.5
.4
.3
.2
MA
LA
Glycolic acid (GA)
Lactic acid (LA)
Mandelic acid (MA)
Stoichiometric studies: The stoichiometric studies for
the oxidation of hydroxy acids by pyridinium fluorochromate
were carried out with oxidant in excess. The solvent composition
+
5
0 % acetic acid + 50 % water (v/v) and [H ] were maintained
as in the corresponding rate measurements. The temperature
was maintained at 303 K. The hydroxy acids and pyridinium
fluorochromate were mixed in the ratio 1:4, 1:5, 1:6 and were
allowed to react for 24 h at 303 K. The concentration of
unreacted pyridinium fluorochromate was determined. ∆[PFC]
was calculated. The stoichiometry was calculated from the
ratio between [HA] and [PFC].
GA
1.1
.0
0.9
1
0.8
0.7
0.6
Stoichiometric analysis showed that the following overall
reaction.
+
CrFO-PyH + H →
+
RCH(OH)COOH + O
2
0.0
0.1
0.2
0.3
0.4
0.5
+
RCOCOOH + H O + OCrFO-PyH
2
(1)
2 + log [HA]
Effect of varying pyridinium fluorochromate concen-
Fig. 1. Showing order plot of α-hydroxy acids for the oxidation of hydroxy
acids by PFC
tration: The concentration of pyridinium fluorochromate was
-
3
-3
varied in the range of 0.6 × 10 to 1.4 × 10 mol dm at constant
-3
-3
+
HA], [H ] at 303 K and the rates were measured (Table-1,
to 0.14 mol dm keeping all other reactant concentration as
constant at 303 K and the rates were measured (Table-1). The
[
Fig. 1). The near constancy in the value of kobs irrespective of
the concentration confirms the first order dependence on
pyridinium fluorochromate.
acid catalyzed nature of this oxidation is confirmed by an
+
increase in the rate on the addition of H . The plot of log k
1
+
versus log [H ] is a straight line with the slope of 1.0 (r =
.999), 0.998 (r = 0.996) and 1.0 (r = 0.996) respectively for
glycolic acid, lactic acid and mandelic acid respectively.
Effect of varying α-hydroxy acid concentration: The
concentration of the substrates glycolic acid, lactic acid,
0
-2
mandelic acid were varied in the range of 1.0 × 10 to 3.0 ×
1
+
-2
-3
0 mol dm at 303 K and keeping all other reactant concen-
Therefore, order with respect to H is one for glycolic acid,
lactic acid and mandelic acid respectively. Pyridinium
fluorochromate may become protonated in the presence of
acid. The protonated pyridinium fluorochromate may function
as an effective oxidant.
trations as constant and the rates were measured (Table-1).
The rate of oxidation increased progressively on increasing
1
the concentration of hydroxy acids. The plot of log k versus
log [HA] gave the slope of 1.05 (r = 0.997), 1.02 (r = 0.998)
and 0.994 (r = 0.996) respectively for glycolic acid, lactic acid
and mandelic acid respectively (Fig. 1). Under pseudo-first-
Induced polymerization of acrylonitrile: Vinyl mono-
mers like acrylonitrile are added to the reaction mixture under
nitrogen atmosphere to find out whether the reaction under
investigation involves the formation of free radicals as the
reaction intermediates. In the present study, freshly distilled
acrylonitrile free from inhibitor is added to the reaction mixture
containing 0.1 M perchloric acid. After the completion of the
reaction, the reaction mixture is diluted with methanol to
1
order conditions, the plot of 1/k versus 1/[HA] were linear
with a negligible intercept indicating that the intermediate
formed in a slow step got consumed in a subsequent fast step.
Effect of varying perchloric acid concentration: Per-
+
chloric acid has been used as a source of H in reaction
+
medium. The concentration of H was varied in the range 0.06

Vol. 25, No. 2 (2013)
Oxidation of Some α-Hydroxy Acids by Pyridinium Fluorochromate 923
observe the formation of polymer. It is observed that the
oxidation reaction does not induce the polymerization (Table-
TABLE-4
KINETIC ISOTOPE EFFECT ON THE OXIDATION OF
MANDELIC ACID BY PYRIDINIUM FLUOROCHROMATE
2
). Thus, a one-electron oxidation giving rise to free radicals
is unlikely.
Effect of varying ionic strength on reaction rate: The
5
0 × k (s )
-1
1
1
Substrate
MA
DMA
2
98 K
303 K
308 K
313 K
17.20
3.22
5.34
22.90
4.10
5.58
30.46
5.26
5.79
38.46
6.45
5.96
ionic strength of the reaction medium is changed by the
addition of anhydrous sodium perchlorate and the influence of
ionic strength on the reaction rate has been studied. The values
of the rate constants at different ionic strength of the reaction
medium has no significant effect on the reaction rate (Table -3).
k /k
H
D
2
-3
3
-3
+
0 [MA] = 2.0 mol dm ; 10 [PFC] = 1.0 mol dm ; 10 [H ] = 1.0 mol
1
dm ; Solvent composition = 50 % AcOH – 50 % H O (v/v)
-
3
2
TABLE-5
EFFECT OF VARYING SOLVENT POLARITY ON
THE RATE OF REACTION AT 303 K
TABLE-2
EFFECT OF ACRYLONITRILE (AN) ON
THE OXIDATION OF HYDROXY ACIDS BY
PYRIDINIUM FLUOROCHROMATE AT 303 K
5 -1
10 × k (s )
%
Acetic acid- Dielectric
constant
1
1/D
Water (v/v)
GA
LA
MA
3
0 [AN]
5
10 k (s )
-1
30-70
72.0
63.3
56.0
45.5
38.5
0.0138
0.0158
0.0178
0.0219
0.0259
7.96
8.10
10.80
12.20
13.50
16.40
18.16
20.68
22.90
27.96
1
mol dm )
1
-3
40-60
50-50
60-40
70-30
(
GA
9.00
9.10
8.90
9.12
9.08
LA
MA
9.00
11.44
13.80
0.0
1.0
2.0
3.0
4.0
5.0
13.50
13.48
13.44
13.36
13.40
22.90
22.96
22.94
22.88
22.80
22.86
19.52
34.78
-3
2
-3
3
-3
+
0 [HA] = 2 mol dm ; 10 [PFC] = 1mol dm ; 10[H ] = 1 mol dm
1
The dielectric constant or permittivity (ε) is a dimension-
9.16
3
13.52
2
-3
-3
+
10 [HA] = 2.0 mol dm ; 10 [PFC] = 1.0mol dm ; 10 [H ] = 1.0 mol
dm ; Solvent composition = 50 % AcOH – 50 % H O (v/v)
less constant that indicates how easy a material can be polarized
by imposition of an electric field on an insulating material.
The constant is the ratio between the actual material ability to
carry an alternating current to the ability of vacuum to carry
the current. The dielectric constant can be expressed as:
-
3
2
TABLE-3
EFFECT OF IONIC STRENGTH ON THE OXIDATION
OF HYDROXY ACIDS BY PFC AT 303 K
ε = ε
where; ε = the dielectric constant; ε
of the material; ε = vacuum permittivity.
s
/ε
0
5 –1
0 k (s )
1
2
0 [NaClO ]
1
1
s
= the static permittivity
4
GA
LA
MA
0
0
1
2
3
4
5
.0
.0
.0
.0
.0
.0
9.00
9.00
8.90
9.12
9.08
9.16
13.50
13.36
13.38
13.44
13.56
13.32
22.90
22.88
22.82
22.96
22.92
22.94
The effect from solvent composition on the reaction rate
was studied by varying the concentration of acetic acid from
30 to 70 %. The pseudo-first-order rate constants were
estimated for the oxidation of hydroxyl acids, with pyridinium
fluorochromate in the presence of perchloric acid at a constant
ionic strength. The reaction rate is increases markedly with
the increase in the proportion of acetic acid in the medium
2
0 [HA] = 2.0 mol dm ; 10 [PFC] = 1.0 mol dm ; 10[H ] = 1.0 mol
–3
3
–3
+
1
dm ; Solvent composition = 50 % AcOH – 50 % H O (v/v)
–3
2
(
Table-5). When the acid content increases in the medium, the
Effect of acidity: The reaction is catalyzed by hydrogen
ions (Table-1). The acid-catalysis may well be attributed to a
protonation of pyridinium fluorochromate to give a stronger
oxidant and electrophile.
acidity of the medium is increased whereas the dielectric
constant of the medium is decreased. These two effects cause
the rate of the oxidation to increase markedly. The enhance-
ment of the reaction rate with an increase in the amount of
acetic acid generally may be attributed to two factors, viz., (i)
+
CrFO-PyH + H
+
+
O
2
(OH)OCrFO-PyH
(2)
+
the increase in acidity occurring at constant [H ] and (ii) the
The formation of a protonated Cr(VI) species has earlier
been postulated in the reactions of structurally similar
decrease in the dielectric constant with an increase in the acetic
acid content.
The plot of log k versus 1/D (dielectric constant) is linear
20
21
pyridinium chlorochromate and pyridinium fluorochromate .
Kinetic isotope effect: To ascertain the importance of
the cleavage of the α-C-H bond in the rate-determining step,
oxidation of α-deuterio mandelic acid (DMA) was studied.
Results showed the presence of a substantial primary kinetic
isotope effect (Table-4).
1
with positive slope suggesting the presence of either dipole-
dipole or ion-dipole type of interaction between the oxidant
22,23
and the substrate (Fig. 2). Plot of log k versus (D-1)/(2D+1)
1
is a curvature indicating the absence of dipole-dipole interaction
in the rate determining step. Positive slope of log k versus
1
1/D plot indicates that the reaction involves a cation-dipole
type of interaction in the rate determining step.
Effect of solvent polarity on reaction rate:The oxidation
of α-hydroxy acid has been studied in the binary mixture of
acetic acid and water as the solvent medium. For the oxidation
of all hydroxy acids, the reaction rate increased remarkably
with the increase in the proportion of acetic acid in the solvent
medium. These results are presented in Table-5.
2
4
Amis holds the view that in an ion-dipole reaction
involving a positive ionic reactant, the rate would decrease
with increasing dielectric constant of the medium and if the
reactant were to be a negatively charged ion, the rate would

924 Ahmed et al.
Asian J. Chem.
TABLE-6
ACTIVATION PARAMETERS AND SECOND ORDER RATE CONSTANTS FOR THE
OXIDATION OF HYDROXY ACIDS BY PFC IN AQUEOUS ACETIC ACID MEDIUM
3
0 k (dm mol s )
3
-1 -1
#
#
#
-1
∆G (kJ mol )
1
Ea
(kJ mol )
∆H
(kJ mol )
−∆S
(JK mol)
2
Substrate
-1
-1
-1
298 K
303 K
308 K
313 K
(at 303 K)
GA
LA
3.40
5.40
4.50
6.75
6.00
8.72
8.40
11.60
19.23
46.52
38.30
41.93
43.84
36.95
39.44
144.92
164.46
151.72
87.87
86.78
85.41
MA
8.60
11.45
15.23
2
0 [HA] = 2.0 mol dm ; 10 [PFC] = 1.0 mol dm ; 10 [H ] = 1.0 mol dm ; Solvent composition = 50 % AcOH – 50 % H O (v/v)
-3
3
-3
+
-3
1
2
1
1
1
1
1
1
1
1
1
1
1
0
0
0
0
.50
.45
.40
.35
.30
.25
.20
.15
.10
.05
.00
.95
.90
.85
.80
_
O PyH
+
_
O PyH
+
O
O
+
+
Cr
+
H
Cr
O
F
HO
F
PFC
COOH
COOH
OH
_
O PyH
+
O
+
_
+
O
PyH
K
+
+
Cr
R - C - OH
Cr
R - C
O
F
HO
F
H
H
OH
slow
R = H (GA); CH3 (LA); C6H5 (MA);
k
COOH
O
COOH
_
+
O
PyH
0
.012
0.014
0.016
0.018
0.020
0.022
0.024
0.026
OH
fast
_
+
O
PyH
1
/ D
R - CO +
Cr
R -
+
H O
2
C
H
O
Fig. 2. Plot of 1/D against log kobs showing effect of solvent polarity
+
Oxo acid
F
Cr
Cr (IV)
O
F
H
increase with the increasing dielectric constant. In this case,
there is a possibility of a positive ionic reactant, as the rate
decreases with the increasing dielectric constant of the
Scheme-I:Mechanism of oxidation of hydroxy acids by pyridinium
fluorochromate
24
medium . Due to the polar nature of the solvent, transition
state is stabilized, i.e., the polar solvent molecules surround
the transition state and result in less disproportion.
log k (at T
The linear relationship in Exner plots
303 K) and 4 + log k (308 K) observed in the present study
2
) = a + b log k (at T
1
)
(3)
2
6,27
at 4 + log k
2
(
2
Thermodynamic parameters: The kinetics of oxidation
of hydroxy acids was studied at four different temperatures
viz., 298, 303, 308 and 313 K. The second order rate constants
imply the validity of the isokinetic relationship. Isokinetic tempe-
rature obtained is 418 ±12 K. The linear isokinetic correlation
implies that glycolic acid, lactic acid and mandelic acid are
oxidized by the same mechanism and the changes in the rate
are governed by the changes in both the enthalpy and entropy
were calculated (Table-6). The Arrhenius plot of log k
/T is found to be linear. The enthalpy of activation, entropy
of activation and free energy of activation were calculated from
at 298, 303, 308 and 313 K using the Eyring relationship
2
versus
1
28
of activation .
Mechanism of oxidation: From the product analysis, 2,4-
k
2
by the method of least square and presented in Table-6. The
entropy of activation is negative for hydroxyl acids. The negative
entropy of activation in conjunction with other experimental
data supports the mechanism outlined in Scheme-I.
Isokinetic relationship: The reaction is neither isoenthalpic
nor isoentropic but complies with the compensation law also
known as the isokinetic relationship.
dinitro phenyl hydrazone was confirmed. Hence, it shows that
under the experimental conditions employed in the present
study, α-hydroxyl acids were oxidized to the corresponding
oxo acids. Based on the above kinetic observations the
following mechanism is proposed for the reaction. Absence
of any effect of added acrylonitrile on the reaction discounts
the possibility of a one-electron oxidation, leading to the
formation of free radicals. The presence of a substantial kinetic
isotope effect in the oxidation of DMA confirms the cleavage
of the α-C-H bond in the rate-determining step. Therefore, a
hydride-ion transfer in the rate determining step is suggested
(Scheme-I).
#
H = ∆Hº + β∆S
#
∆
(3)
The isokinetic temperature β is the temperature at which
all the compounds of the series react equally fast. Also, at the
isokinetic temperature, the variation of substituent has no
25
influence on the free energy of activation. Exner suggested a
method of testing the validity of isokinetic relationship. The
isokinetic relationship is tested by plotting the logarithms of
Conclusion
rate constants at two different temperatures (T
each other according to eqn. 4.
2
> T
1
) against
The kinetics of oxidation of α-hydroxy acids has been
investigated in aqueous acetic acid medium in the presence of

Vol. 25, No. 2 (2013)
Oxidation of Some α-Hydroxy Acids by Pyridinium Fluorochromate 925
8
.
.
M.N. Bhattacharjee, M.K.Chaudhuri, H.S. Dasgupta, N. Roy and D.K.
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chromate is first order each with respect to the hydroxy acids,
pyridinium fluorochromate and hydrogen ion. The oxidation
is catalyzed by mineral acid. The lowering of dielectric
constant of reaction medium increases the reaction rate signi-
ficantly. The ionic strength of the reaction medium does not
affect the rate of the oxidation. The reaction does not shows
the polymerization, which indicates the absence of free radical
intermediate in the oxidation. The order of reactivity is glycolic
acid < lactic acid < mandelic acid. The reaction rate is higher
in lactic acid than in glycolic acid due to the inductive effect.
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9
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