Oxidation of some α-hydroxy acids by tetraethylammonium chlorochromate: A kinetic and mechanistic study

Article abstract of DOI:10.1002/kin.20466

The oxidation of glycolic, lactic, malic, and a few substituted mandelic acids by tetraethylammonium chlorochromate (TEACC) in dimethylsulfoxide leads to the formation of corresponding oxoacids. The reaction is first order each in TEACC and hydroxy acids. Reaction is failed to induce the polymerization of acrylonitrile. The oxidation of α-deuteriomandelic acid shows the presence of a primary kinetic isotope effect (k_H/k_D = 5.63 at 298 K). The reaction does not exhibit the solvent isotope effect. The reaction is catalyzed by the hydrogen ions. The hydrogen ion dependence has the following form: k_obs = a + b[H+ ]. Oxidation of p-methylmandelic acid has been studied in 19 different organic solvents. The solvent effect has been analyzed by using Kamlet's and Swain's multiparametric equations. A mechanism involving a hydride ion transfer via a chromate ester is proposed.

Full text of DOI:10.1002/kin.20466

Oxidation of Some
α-Hydroxy Acids by
Tetraethylammonium
Chlorochromate: A Kinetic
and Mechanistic Study
PREETI SWAMI, D. YAJURVEDI, P. MISHRA, PRADEEP K. SHARMA
Department of Chemistry, J. N. V. University, Jodhpur 342 005, India
Received 20 June 2009; revised 21 September 2009; accepted 21 September 2009
DOI 10.1002/kin.20466
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The oxidation of glycolic, lactic, malic, and a few substituted mandelic acids by
tetraethylammonium chlorochromate (TEACC) in dimethylsulfoxide leads to the formation of
corresponding oxoacids. The reaction is first order each in TEACC and hydroxy acids. Reaction
is failed to induce the polymerization of acrylonitrile. The oxidation of α-deuteriomandelic
acid shows the presence of a primary kinetic isotope effect (kH/kD = 5.63 at 298 K). The re-
action does not exhibit the solvent isotope effect. The reaction is catalyzed by the hydrogen
ions. The hydrogen ion dependence has the following form: kobs = a + b[H+]. Oxidation of
p-methylmandelic acid has been studied in 19 different organic solvents. The solvent effect
has been analyzed by using Kamlet’s and Swain’s multiparametricꢀequations. A mechanism
C
involving a hydride ion transfer via a chromate ester is proposed.
Inc. Int J Chem Kinet 42: 50–55, 2010
2009 Wiley Periodicals,
oxoacids [7], or they may undergo oxidative decar-
boxylation to yield a ketone [8]. There seems to be no
report on the oxidation aspects using TEACC. We have
been interested in the kinetic and mechanistic aspects
of the oxidation by complexed Cr(VI) species, and
several studies on halochromates have already been
reported [9–12]. In continuation of our earlier work,
we now report the kinetics of oxidation of some hy-
droxy acids by TEACC in DMSO as solvent. A suitable
mechanism has also been proposed.
INTRODUCTION
Specific and selective oxidation of organic compounds
under nonaqueous conditions is an important reac-
tion in synthetic organic chemistry. For this, a num-
ber of different chromium(VI) derivatives have been
reported [1–5]. Tetraethylammonium chlorochromate
(TEACC) is one such compound used for the oxidation
of primary aliphatic alcohols [6]. α-Hydroxy acids may
be oxidized either as alcohols, yielding corresponding
Correspondence to: Pradeep K. Sharma; e-mail: drpkvs27@
yahoo.com.
c
ꢀ
2009 Wiley Periodicals, Inc.

OXIDATION OF α-HYDROXY ACIDS BY TETRAETHYLAMMONIUM CHLOROCHROMATE
51
EXPERIMENTAL
Materials
reproducible within 3%. The second-order rate con-
stants, k2, were calculated from the following relation-
ship: k2 = kobs/[hydroxy acid]. All experiments, other
than those for studying the effect of hydrogen ions,
were carried out in the absence of TsOH.
The hydroxy acids were commercial products (Merck,
Ltd., Mumbai, India) of the highest purity available and
were used as such. The preparation and specification of
the substituted mandelic acids have been described ear-
lier [13]. TEACC was prepared by the reported method
[6], and its purity was checked by an iodometric
method. α-Deuteriomandelic acid (PhCD(OH)COOH
or DMA) was prepared by the method of Kemp and
Waters [14]. Its isotopic purity, ascertained by NMR
spectra, was 95 4%. Because of the nonaqueous na-
ture of the solvent, p-toluenesulfonic acid (TsOH) was
used as a source of hydrogen ions. Solvents were puri-
fied by the usual methods.
RESULTS
The rate and other experimental data were obtained for
all the hydroxy acids studied. Since the results were
similar, only representative data are reproduced here.
Stoichiometry
The oxidation of hydroxy acids resulted in the forma-
tion of the corresponding oxoacids. Product analysis
and stoichiometric determinations indicated that the
overall reaction could be written as
Product Analysis
Product analyses were carried out under kinetic con-
ditions, i.e., with an excess of the reductant over
TEACC. In a typical experiment, mandelic acid (7.6 g,
0.05 mol) and TEACC (2.66 g, 0.01 mol) were dis-
solved in 100 mL of DMSO and was allowed to
stand in dark for ≈24 h to ensure the completion
of the reaction. It was then treated with an excess
(250 mL) of a freshly prepared saturated solution
of 2,4-dinitrophenylhydrazine in 2 mol dm−3 HCl
and was kept overnight in a refrigerator. The precipi-
tated 2,4-dinitrophenyl-hydrazone (DNP) was filtered
off, dried, weighed, recrystallized from ethanol, and
weighed again. The product was identical (mp and
mixed mp) to an authentic sample of DNP of phenyl-
glyoxylic acid. Similar experiments with the other hy-
droxy acids yielded the DNP of the corresponding
oxoacids in 79%–88% yields after recrystallization.
The oxidation state of chromium in completely re-
duced reaction mixtures, determined by an iodometric
method, was 3.95 0.10.
ArCH(OH)COOH + O2CrClO−N+Et4
→ ArCOCOOH + H2O + OCrCl O−N+Et4 (1)
TEACC undergoes a two-electron change. This is ac-
cording to the earlier observations with structurally
similar other halochromates. It has already been shown
that both pyridinium fluorochromate (PFC) [15] and
pyridinium chlorochromate (PCC) [16] act as two-
electron oxidants and are reduced to chromium(IV)
species by determining the oxidation state of chromium
by magnetic susceptibility, ESR, and IR studies.
Rate Laws
The reactions are of first order with respect to TEACC.
The individual kinetic runs were strictly first order to
TEACC. Furthermore, the pseudo-first-order rate con-
stant, kobs, does not depend on the initial concentration
of TEACC. The reaction rate increases linearly with
an increase in the concentration of the hydroxy acids
(Table I). Thus, the reaction is first order with respect
to the hydroxy acids also.
Kinetic Measurements
The pseudo-first-order conditions were attained by
keeping a large excess (×15 or greater) of the hydroxy
acid over TEACC. The temperature was kept constant
to 0.1 K. The solvent was DMSO, unless specified
otherwise. The reactions were followed by monitor-
ing the decrease in the concentration of TEACC spec-
trophotometrically at 350 nm for up to 80% of the
reaction. No other reactant or product has any sig-
nificant absorption at this wavelength. The pseudo-
first-order rate constants, kobs, were computed from
the linear least-square plots of log[TEACC] versus
time. Duplicate kinetic runs showed that the rates were
Induced Polymerization of Acrylonitrile. The oxida-
tion of hydroxy acids, by TEACC, in an atmosphere of
nitrogen failed to induce the polymerization of acry-
lonitrile. Furthermore, addition of acrylonitrile had no
effect on the rate (Table I). To further confirm the ab-
sence of free radicals in the reaction pathway, the reac-
tion was carried out in the presence of 0.05 mol dm−3
of 2,6-di-t-butyl-4-methylphenol (butylated hydroxy-
toluene or BHT). It was observed that BHT was recov-
ered unchanged, almost quantitatively.
International Journal of Chemical Kinetics DOI 10.1002/kin

52
SWAMI ET AL.
Table I Rate Constants for the Oxidation of Mandelic
Acid by TEACC at 308 K
DISCUSSION
Reactive Oxidizing Species
103 [TEACC]
[HA]
104 kobs
(s−1
(mol dm−3
)
(mol dm−3
)
)
The observed hydrogen ion dependence suggests that
the reaction follows two mechanistic pathways, one
acid independent and other acid dependent. The acid
catalysis may well be attributed to a protonation of
TEACC to give a stronger oxidant and electrophile.
1.00
1.00
1.00
1.00
1.00
1.00
2.00
4.00
6.00
8.00
1.00
0.10
0.20
0.40
0.60
0.80
1.00
0.40
0.40
0.40
0.40
0.60
3.22
6.21
12.6
18.7
24.3
31.5
13.5
11.7
13.0
12.8
18.9a
+
→
[O2 CrClO−N+ Et4] + H+←− [HO Cr OClO−N+Et4]
(3)
The formation of a protonated Cr(VI) species has ear-
lier been postulated in the reactions of structurally
similar 2,2-bipyridinium chlorochromate [9] and mor-
pholinium chlorochromate [12].
aContained 0.001 mol dm−3 acrylonitrile.
Effect of Acidity. The reaction is catalyzed by hydro-
gen ions. TsOH was used as a source of hydrogen ions.
The hydrogen ion dependence has the following form
(Table II):
Solvent Effect
The rate constants of oxidation, k2, in 18 solvents (CS2
was not considered, as the complete range of solvent
parameters was not available) were correlated in terms
of the linear solvation energy relationship (4) of Kamlet
et al. [17]
kobs = a + b [H+]
(2)
The values of a and b, for p-methylmandelic acid, are
3.05 0.10 × 10−3 s−1 and 5.65 0.17 × 10−3 mol−1
dm3 s−1, respectively (r2 = 0.9964).
log k2 = A0 + pπ∗ + bβ + aα
(4)
In this equation, π∗ represents the solvent polarity, β
the hydrogen bond acceptor basicities, and α is the hy-
drogen bond donor acidity. A0 is the intercept term.
The results of correlation analyses in terms of (4), a bi-
parametric equation involving π∗ and β, and separately
with π∗ and β, are given by
Effect of Temperature. The rates of all the hydroxy
acids were determined at four different temperatures,
and activation parameters were calculated (Table III).
Kinetic Isotope Effect. To ascertain the importance of
the cleavage of the C H bond in the rate-determining
step, the oxidation of DMA was studied. Results
showed the presence of a substantial kinetic isotope
effect (Table III). The value of kH/kD is 5.63 at 298 K.
log k2 = − 4.36 + (1.27 0.16)π∗
+ (0.16 0.14)β − (0.21 0.13)α
(5)
R2 = 0.8567; sd = 0.15; n = 18; ꢀ = 0.41
log k2 = −4.31 + (1.35 0.17)π∗ + (0.09 0.14)β
Effect of Solvents. The rate of oxidation of
p-methylmandelic acid was determined in 19 differ-
ent organic solvents. The choice of the solvents was
limited by the solubility of TEACC and reaction with
primary and secondary alcohols. There was no notice-
able reaction with the solvents chosen. The kinetics
was similar in all the solvents. The values of k2 at
298 K are presented in Table IV.
(6)
R2 = 0.8285; sd = 0.16; n = 18; ꢀ = 0.44
log k2 = −4.33 + (1.38 0.16)π∗
(7)
r2 = 0.8240; sd = 0.16; n = 18; ꢀ = 0.43
log k2 = −2.90 + (0.33 0.30)β
(8)
r2 = 0.0697; sd = 0.36; n = 18; ꢀ = 0.99
Table II Effect of Hydrogen Ion Concentration on the Oxidation of p-Methylmandelic Acid by TEACC
[TEACC] = 0.001 mol dm−3
[HA] = 1.00 mol dm−3
Temperature = 308 K
[H+]
104 kobs(s−1
0.10
36.0
0.20
42.3
0.40
54.0
0.60
62.1
0.80
76.5
1.00
87.3
)
International Journal of Chemical Kinetics DOI 10.1002/kin

OXIDATION OF α-HYDROXY ACIDS BY TETRAETHYLAMMONIUM CHLOROCHROMATE
53
Table III Rate Constants and Activation Parameters for the Oxidation of Hydroxy Acids by TEACC
104 k2 (dm3 mol−1 s−1
)
ꢁH∗
ꢁS∗
ꢁG∗
(kJ mol−1
)
R
288 K
298 K
308 K
318 K
(kJ mol−1
)
(J mol−1 K−1
)
H
p-F
8.19
12.4
5.22
4.32
38.7
33.3
360
1.53
0.30
0.21
3.87
5.62
4.89
1.53
5.82
17.1
23.4
9.90
8.28
70.2
61.2
585
3.17
0.67
0.49
7.11
10.8
9.20
3.04
5.63
31.5
42.3
19.5
16.4
121
60.3
79.2
37.8
31.5
216
45.8 0.7
44.3 0.6
47.8 1.9
48.0 0.8
40.9 0.5
41.4 1.3
34.0 0.6
51.5 0.8
56.9 0.7
58.4 0.5
44.0 0.8
48.2 0.8
45.3 0.7
48.5 0.7
−145
−147
−142
−143
−150
−149
−155
−139
−134
−132
−158
−140
−151
−150
2
2
3
2
2
1
2
3
2
2
3
3
2
2
88.8 0.5
88.0 0.5
90.1 0.7
90.5 0.6
85.3 0.4
85.6 0.2
80.0 0.4
92.9 0.6
96.8 0.5
97.6 0.4
90.9 0.6
89.9 0.7
90.3 0.6
93.1 0.6
p-Cl
p-Br
p-Me
p-Pri
p-OMe
m-Cl
m-NO2
p-NO2
GA
108
936
189
1530
13.0
3.15
2.34
24.3
41.4
32.4
11.5
5.24
6.30
1.44
1.06
13.0
21.2
17.1
5.83
5.40
LA
MLA
DMA
kH/kD
where n is the number of data points and ꢀ is the
Exner’s statistical parameter [18].
with (A + B).
log k2 = (0.38 0.03)A + (1.42 0.02)B − 4.07
Kamlet and coworkers’ [17] triparametric equation
explains ca. 86% of the effect of solvent on the
oxidation. However, by Exner’s criterion the correla-
tion is not even satisfactory (cf. Eq. (5)). The major
contribution is of solvent polarity. It alone accounted
for ca. 82% of the data. Both β and α play relatively
minor roles.
The data on the solvent effect were analyzed in
terms of Swain and coworkers’ equation [19] of cation-
and anion-solvating concept of the solvents also (9).
(10)
R2 = 0.9960; sd = 0.02; n = 19; ꢀ = 0.07
log k2 = 0.18( 0.47)A − 2.85
(11)
(12)
(13)
r2 = 0.0081; sd = 0.38; n = 19; ꢀ = 1.02
log k2 = 1.39( 0.07)B − 4.29
r2 = 0.9587; sd = 0.08; n = 19; ꢀ = 0.21
log k2 = 1.08 0.13(A + B) − 4.20
r2 = 0.7894; sd = 0.18; n = 19; ꢀ = 0.47
log k2 = aA + bB + C
(9)
The rates of oxidation of p-methylmandelic acid in
different solvents showed an excellent correlation in
Swain’s equation (cf. (10)) with the cation-solvating
power playing the major role. In fact, the cation sol-
vation alone accounts for ca. 96% of the data. The
correlation with the anion-solvating power was very
where A and B represent the anion-solvating and
cation-solvating power, respectively. C is the inter-
cept term. (A + B) is postulated to represent the sol-
vent polarity. The rates in different solvents were an-
alyzed in terms of (9), separately with A and B and
Table IV Effect of Solvents on the Oxidation of p-methylmandelic Acid by TEACC at 298 K
Solvents
104 k2 (dm−3 mol−1 s−1
)
Solvents
104 k2 (dm−3 mol−1 s−1
)
Chloroform
1,2-Dichloroethane
Dichloromethane
DMSO
Acetone
DMF
Butanone
Nitrobenzene
Benzene
22.4
30.9
25.7
70.2
27.5
41.7
19.5
30.2
12.0
1.86
Toluene
Acetophenone
THF
t-Butyl alcohol
1,4-Dioxane
1,2-Dimethoxyethane
Carbon disulfide
Acetic acid
9.77
33.9
14.8
10.7
17.4
9.21
5.25
5.22
13.5
Ethyl acetate
Cyclohexane
International Journal of Chemical Kinetics DOI 10.1002/kin

54
SWAMI ET AL.
poor. The solvent polarity, represented by (A + B),
mechanism involving a hydride-ion transfer via either
a chromate ester or a bimolecular reaction.
also accounted for ca. 79% of the data.
Kwart and Nickle [21] have shown that a study of the
dependence of the kinetic isotope effect on tempera-
ture can be gainfully employed to resolve this problem.
The data for protio- and deuteriomandelic acids, fitted
to the familiar expression kH/kD = AH/AD exp(Ea/RT)
[22,23], show a direct correspondence with the proper-
ties of a symmetrical transition state in which the acti-
vation energy difference (Ea) for kH/kD is equal to the
zero-point energy difference for the respective C H
and C D bonds (≈4.5 kJ/mol) and the entropies of
activation of the respective reactions are nearly equal.
Similar phenomena were observed earlier in the oxida-
tion of alcohols by quinolinium bromochromate [24]
and that of hydroxy acids by PFC [25].
Correlation Analysis of Reactivity
The reaction rates of substituted mandelic acids were
correlated with Hammett’s σ values but did not yield
very significant correlation. The rates of oxidation of
substituted mandelic acids correlated well with Brown
and Okamoto’s σ+ values [20]. The reaction constant,
ρ, has a value −2.07, −1.97, −1.89, and −1.80 at
288, 298, 308, and 318 K, respectively. The coefficient
of correlation, r2, ranges from 0.9989 to 0.9999. The
large negative reaction constants and correlation with
σ+ values indicate a carbocationic reaction center in
the transition state.
Bordwell [26] has documented a very cogent ev-
idence against the occurrence of concerted one-step
biomolecular processes by hydrogen transfer, and it
is evident that in the present study also the hydro-
gen transfer does not occur by an acyclic biomolecular
process. It is well established that intrinsically con-
certed sigmatropic reactions, characterized by transfer
of hydrogen in a cyclic transition state, are the only
truly symmetrical processes involving a linear hydro-
gen transfer [27]. Littler [28] has also shown that a
cyclic hydride transfer, in the oxidation of alcohols
by Cr(VI), involves six electrons and, being a Hu¨ckel-
type system, is an allowed process. Thus the overall
mechanism is proposed to involve the formation of a
chromate ester in a fast pre-equilibrium and then a de-
composition of the ester in a subsequent slow step via
a cyclic concerted symmetrical transition state lead-
ing to the product (Schemes 1 and 2). The observed
Mechanisms
The absence of any effect of added acrylonitrile on the
reaction discounts the possibility of a one-electron ox-
idation, leading to the formation of free radicals. The
presence of a substantial kinetic isotope effect in the
oxidation of mandelic acid confirms the cleavage of the
α-C H bond in the rate-determining step. The large
negative reaction constant together with the excellent
correlation with Brown and Okamoto’s σ+ values [20]
points to a highly electron-deficient carbon center in
the transition state. The transition state thus approaches
a carbocation in character. This is supported by the sol-
vent effect also. The greater role played by the cation-
solvating power of the solvents supported the postu-
lation of a carbocationic transition state. Therefore,
the correlation analysis of substituent and solvent ef-
fects on the oxidation of mandelic acid supports the
COOH
O
Ar
H
O
O
O N+Et4
O N+Et4
+
C
Cr
(A)
Ar
C
H
O
Cr
HOOC
OH
Cl
Cl
HO
Slow
#
COOH
C
O
Ar
O
O N+Et4
ArCOCOOH
+
(OH)2CrClO N+Et4
Cr
OH
Cl
H
Scheme 1 Acid-independent path.
International Journal of Chemical Kinetics DOI 10.1002/kin

OXIDATION OF α-HYDROXY ACIDS BY TETRAETHYLAMMONIUM CHLOROCHROMATE
55
COOH
OH
Cr+
O N+Et4
(A) + H+
Ar
C
H
O
Cl
HO
Slow
#
COOH
OH
Ar
C
O
O N+Et4
ArCOCOOH + HOCrClO N+Et4 + H2O
Cr+
Cl
H
OH
Scheme 2 Acid-dependent path.
negative value of entropy of activation also supports
a polar transition state. As the charge separation takes
place, the charged ends become highly solvated. This
results in an immobilization of a large number of sol-
vent molecules, reflected in the loss of entropy [29].
12. Yajurvedi, D.; Purohit, P.; Baghmar, M.; Sharma, P. K.
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We thank the University Grants Commission New Delhi,
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International Journal of Chemical Kinetics DOI 10.1002/kin