Kinetics and Mechanism of the Oxidation of Some α-Hydroxy Acids by Benzyltrimethylammonium Dichloroiodate

Article abstract of DOI:10.1246/bcsj.76.2335

The oxidation of lactic acid, mandelic acid, and ten monosubstituted mandelic acids by benzyltrimethylammonium dichloroiodate (BTMACI) in glacial acetic acid leads to the formation of the corresponding oxo acid. The reaction is first order with respect to hydroxy acid, BTMACI, and zinc chloride. An addition of benzyltrimethylammonium chloride enhances the rate slightly. [PhCH2Me3N](1+)[IZn2Cl6](1-) is postulated to be the reactive oxidizing species. The oxidation of α-deuteriomandelic acid exhibited the presence of a substantial kinetic isotope effect (k_H/k_D = 5.97 at 298 K). The rate of oxidation of the substituted mandelic acids showed an excellent correlation with Brown's ?⁺ values. The reaction constants are negative. A mechanism involving the transfer of hydride ion to the oxidant is postulated.

Full text of DOI:10.1246/bcsj.76.2335

Ó 2003 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 76, 2335–2339 (2003) 2335
Kinetics and Mechanism of the Oxidation of Some ꢀ-Hydroxy Acids
by Benzyltrimethylammonium Dichloroiodate
ꢀ
Archana Goyal and Seema Kothari
Department of Chemistry, J N V University, Jodhpur 342005, India
Received January 14, 2003; E-mail: seema@goraexports.com
The oxidation of lactic acid, mandelic acid, and ten monosubstituted mandelic acids by benzyltrimethylammonium
dichloroiodate (BTMACI) in glacial acetic acid leads to the formation of the corresponding oxo acid. The reaction is first
order with respect to hydroxy acid, BTMACI, and zinc chloride. An addition of benzyltrimethylammonium chloride en-
þ
ꢁ
hances the rate slightly. [PhCH2Me3N] [IZn2Cl6] is postulated to be the reactive oxidizing species. The oxidation of ꢀ-
deuteriomandelic acid exhibited the presence of a substantial kinetic isotope effect (kH=kD ¼ 5:97 at 298 K). The rate of
þ
oxidation of the substituted mandelic acids showed an excellent correlation with Brown’s ꢁ values. The reaction con-
stants are negative. A mechanism involving the transfer of hydride ion to the oxidant is postulated.
acid is 0.0005 mol dm 3. The addition of zinc chloride (0.002
ꢁ
Benzyltrimethylammonium polyhalides are widely used as
These
1–3
ꢁ3
halogenating agents in synthetic organic chemistry.
mol dm ) increased the solubility of BTMACI and a saturated so-
lution of BTMACI, under these conditions, has strength of 0.0016
mol dm .
Product Analysis. The formation of oxo acids as the product
of the oxidation was confirmed by their characteristic positive spot
compounds are more suitable than molecular halogens because
of their solid nature, ease of handling, stability, selectivity, and
excellent product yields. However, not many reports are avail-
able on their use as oxidizing agents in synthetic organic
ꢁ3
14
4–6
test, given by a completely reduced reaction mixture. The quan-
titative estimation of the oxo acids was carried out under kinetic
conditions. In a typical experiment, mandelic acid (0.1 mol) and
BTMACI (0.01 mol) were dissolved in 100 mL of glacial acetic
acid in the presence of 0.03 moles of zinc chloride. The reaction
mixture was allowed to stand in the dark for ca. 6 h to ensure com-
pletion of the reaction. Most of the solvent was removed by distil-
lation under reduced pressure. The solution was then treated with
an excess (250 mL) of a saturated solution of 2,4-dinitrophenyl-
chemistry.
We are interested in the kinetic and mechanistic
studies of the newer oxidizing agents and have reported, per-
haps for the first time, the use of benzyltrimethylammonium di-
chloroiodate (BTMACI) in the oxidation of organic com-
7,8
pounds. A perusal of the literature showed that the kinetics
and mechanism of the oxidation of hydroxy acids by BTMACI
has not been investigated so far. Hydroxy acids may be oxi-
9
dized either as alcohols yielding corresponding oxo acids, or
10
may undergo decarboxylation to yield an aldehyde. In the
present article the kinetics of oxidation of lactic acid (LA),
mandelic acid (MA), and ten substituted mandelic acids by
BTMACI in glacial acetic acid in the presence of zinc chloride
have been described. Attempts have been made to correlate the
rate and structure in this reaction. Mechanistic aspects are dis-
cussed.
ꢁ3
hydrazine in 2 mol dm HCl and kept overnight in a refrigerator.
The precipitated 2,4-dinitrophenylhydrazone (DNP) was filtered
off, dried, weighed, recrystallized from ethanol, and weighed
again. The yields of DNP before and after recrystallization were
2.20 g (90%) and 1.98 g (81%) respectively. The identity of the
product was established by comparing the mp of the DNP deriva-
15
tive with the literature value. The DNP was found to be identical
ꢃ ꢃ
(
mp = 190 C and mixed mp = 193 C) with the DNP of phenyl-
Experimental
ꢃ
15
glyoxylic acid (mp = 196–7 C). In similar experiments with
other hydroxy acids, the yield of the corresponding oxo acids
was in the range of 77–87% after recrystallization.
Stoichiometry. To determine the stoichiometry, BTMACI
(0.05 mol) and MA (0.01 mol) were mixed with 100 mL of glacial
acetic acid in the presence zinc chloride (0.15 mol). The reaction
was allowed to stand for ca. 10 h to ensure the completion of the
reaction. The residual BTMACI was determined spectrophotomet-
rically at 364 nm. Several determinations, with different concen-
trations of MA, showed that the stoichiometry is 1:1.
Spectral Studies. UV–vis spectra of 0.0005 mol dm of
BTMACI alone and in the presence of 0.002, 0.003 and 0.006
mol dm of ZnCl2 were obtained using an HP-diode array spec-
trophotometer (Model 8452A), at 298 ꢂ 3 K. The solvent and
blank were glacial acetic acid. The scanning speed was 600
1
Materials. BTMACI was prepared by the reported method,
and its purity was checked iodometrically. LA, MA, and nitroman-
delic acids were commercial products of the highest purity availa-
ble and were used as such. Halogeno-, alkyl-, and p-methoxyman-
delic acids were prepared from the corresponding benzaldehydes
11
by cyanohydrine formation followed by acid hydrolysis. Solu-
tions were standardized by alkalimetry. ꢀ-Deuteriomandelic acid
12
(
DMA) was prepared by the method of Kemp and Waters. The
isotopic purity, as ascertained by its NMR spectrum, was 94 ꢂ 5%.
Glacial acetic acid was purified by refluxing it with CrO3 and ace-
ꢁ3
13
tic anhydride for 6 h followed by distillation.
ꢁ3
BTMACI is only slightly soluble in acetic acid at room
temperature. However, the addition of zinc chloride renders it
readily soluble in acetic acid. We found that in the absence of zinc
chloride, the strength of a saturated solution of BTMACI in acetic
ꢁ
1
nm s
.
Published on the web December 15, 2003; DOI 10.1246/bcsj.76.2335

2
336 Bull. Chem. Soc. Jpn., 76, No. 12 (2003)
Oxidation of Acids by Polyhalogen Compound
Kinetic Measurements. The reactions were studied under
Table 1. Rate Constants for the Oxidation of Mandelic Acid
by BTMACI at 318 K
pseudo-first-order conditions by keeping an excess (ꢄ20 or great-
er) of the hydroxy acid over BTMACI. The solvent was glacial
acetic acid. The reactions were carried out at constant temperature
3
4
[MA]
/mol dm
10 [BTMACI]
ꢁ3
10 kobs
ꢁ3
ꢁ1
ꢂ0:1 K) in the presence of zinc chloride (0.003 mol dmꢁ3 unless
/mol dm
/s
(
stated otherwise) and were followed by monitoring the decrease in
0.2
0.4
0.6
1.0
2.0
3.0
2.0
2.0
2.0
1.0
1.0
1.0
1.0
1.0
1.0
0.2
0.4
0.6
0.8
1.0
1.0
0.45
0.93
1.30
2.25
4.61
6.69
4.68
4.55
4.60
4.71
[
BTMACI] spectrophotometrically at 364 nm for up to 80% of the
reaction. Beer’s law is valid for BTMACI within the concentration
range used in our experiments. Pseudo-first-order rate constants,
kobs, were evaluated from linear plots (r >0:9990) of log-
[
BTMACI] against time. Duplicate kinetic runs showed that the
rates were reproducible to within ꢂ4%. The third order rate con-
stant, k3, was calculated by the following relation: k3 ¼ kobs/
[
hydroxy acid] [ZnCl2].
2
2
2
.0
.0
.0
Results
The rate and other experimental data were obtained for all
aÞ
bÞ
4.64
4.57
ꢁ
3
the hydroxy acids. Since the results are similar, only represen-
tative data are reported here.
a) and b) contained 0.002 and 0.005 mol dm acrylonitrile re-
spectively.
The oxidation of the hydroxy acids by BTMACI results in
the formation of the corresponding oxo acids. Analyses of
products and stoichiometric determinations indicate the follow-
ing overall reaction.
RCH(OH)COOH þ [PhCH2Me3N]ICl2
ð1Þ
!
RCOCOOH þ PhCH2Me3NCl þ HCl þ HI
Rate Laws. The reactions are first order with respect to
BTMACI, as evidenced by a linear plot of log[BTMACI] vs
time (Fig. 1). The pseudo-first-order rate constants do not de-
pend on the initial concentration of BTMACI (Table 1). This
further confirms the first-order dependence of rate on the oxi-
dant concentration. The reaction rate increases linearly with
an increase in the concentration of the hydroxy acid (Table 1).
2
A plot of log kobs vs log[hydroxy acid] is linear (r >0:9990)
with a slope = 0:99 ꢂ 0:01, indicating that the order with re-
spect to the hydroxy acid is also one (Fig. 2).
Induced Polymerization of Acrylonitrile. The oxidation
of hydroxy acids, under a nitrogen atmosphere, failed to induce
polymerization of acrylonitrile. Further, the addition of acry-
Fig. 2. A log–log plot for the oxidation of mandelic acid
ꢁ3
ꢁ3
(
0.2–3.0 mol dm ) by BTMACI (0.001 mol dm ) at
18 K.
3
lonitrile had no effect on the rate (Table 1).
Effect of Temperature. The rates of oxidation of hydroxy
acids were determined at different temperatures between 298 K
and 328 K and the activation parameters were calculated
(Table 2).
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. The results showed (Table 2)
that the reaction exhibited a substantial kinetic isotope effect
(
kH=kD ¼ 5:97 at 298 K). The rates for the deuteriated mandel-
ic acid were corrected for the amount of the protio mandelic
acid present.
Effect of Zinc Chloride. With an increase in the concentra-
tion of zinc chloride, the reaction rate increases linearly
2
(
0
Table 3). A plot of kobs versus [ZnCl2] is linear (r ¼
:9990) and passes through the origin.
Effect of Benzyltrimethylammonium Chloride. An addi-
ꢁ3
Fig. 1. Oxidation of mandelic acid (3.0 mol dm ) by
ꢁ
tion of benzyltrimethylammonium chloride (BTMAC) enhanc-
es the reaction rate slightly (Table 4).
BTMACI (0.001 mol dm 3) at 318 K.

A. Goyal et al.
Bull. Chem. Soc. Jpn., 76, No. 12 (2003) 2337
aÞ
Table 2. Rate Constants and Activation Parameters of the Oxidation of Substituted Mandelic Acids by BTMACI
4
6
ꢁ2 ꢁ1
ꢀ
ꢀ
K
ꢀ
ꢀG
kJ mol
Subst.
10 k3/dm mol
s
ꢀH
ꢀS
ꢁ1
ꢁ1 ꢁ1
ꢁ1
2
98 K
308 K
318 K
328 K
kJ mol
J mol
H
p-F
185
277
101
82.0
995
839
379
531
221
187
1700
1520
17000
54.6
12.2
8.50
52.7
120
65.0
750
1080
438
381
3070
2770
27100
120
31.7
19.5
116
160
2280
979
55:6 ꢂ 1:1
54:5 ꢂ 1:6
58:4 ꢂ 1:3
60:3 ꢂ 1:0
46:7 ꢂ 2:1
48:8 ꢂ 1:8
33:4 ꢂ 3:3
66:6 ꢂ 0:8
74:8 ꢂ 1:3
78:1 ꢂ 2:2
67:4 ꢂ 0:9
55:7 ꢂ 0:4
57:3 ꢂ 1:2
ꢁ92 ꢂ 4
ꢁ93 ꢂ 5
ꢁ88 ꢂ 4
ꢁ83 ꢂ 3
ꢁ108 ꢂ 7
ꢁ103 ꢂ 6
ꢁ132 ꢂ 9
ꢁ73 ꢂ 2
ꢁ58 ꢂ 4
ꢁ52 ꢂ 7
ꢁ71 ꢂ 3
ꢁ101 ꢂ 1
ꢁ102 ꢂ 4
83:0 ꢂ 0:9
82:0 ꢂ 1:2
84:5 ꢂ 1:0
84:9 ꢂ 0:8
78:9 ꢂ 1:6
79:3 ꢂ 1:4
72:6 ꢂ 2:6
88:3 ꢂ 0:6
92:0 ꢂ 1:1
93:4 ꢂ 1:7
88:4 ꢂ 0:7
85:8 ꢂ 0:3
87:4 ꢂ 0:9
p-Cl
p-Br
p-Me
p-Pri
p-OMe
m-Cl
m-NO2
p-NO2
m-Br
LA
851
6190
5670
49000
278
83.5
52.0
271
13000
21.2
4.80
2.52
20.5
57.8
31.0
253
131
492
286
DMA
kH=kD
5.97
5.83
5.73
5.59
ꢁ
3
ꢁ3
a) [Oxidant] = 0.001 mol dm ; [hydroxy acid] = 1.0 or 5.0 mol dm depending on the reactivity of the hydroxy acid.
Table 3. Effect of Zinc Chloride on the Rate of Oxidation of Mandelic Acid by BTMACI
3
ꢁ3
1
1
0 [ZnCl2]/mol dm
3.0
2.25
4.0
3.10
6.0
4.61
8.0
6.35
10.0
7.65
15.0
11.6
20.0
16.0
04 kobs/mol dm
ꢁ3
MA] = 1.0 mol dm 3; [BTMACI] = 0.001 mol dm ; Temperature = 318 K.
ꢁ
ꢁ3
[
Table 4. Effect of Benzyltrimethylammonium Chloride on the Rate of Oxidation of Mandelic Acid by BTMACI
3
ꢁ3
1
1
0 [BTMAC]/mol dm
ꢁ3
0.0
2.25
0.4
2.68
0.6
2.90
1.0
3.25
2.0
3.50
3.0
3.85
4.0
4.66
04 kobs/mol dm
[
MA] = 1.0 mol dm 3; [BTMACI] = 0.001 mol dm ; Temperature = 318 K.
ꢁ ꢁ3
However, there is an initial sharp decrease in the absorbance
followed by a regular but gradual decrease in the absorbance
of BTMACI on further addition of ZnCl2. This clearly shows
that a strong complex [A] is formed initially which undergoes
further complexation to give complex [B] whose concentration
increases with an increase in the concentration of ZnCl2.
Discussion
7
From our data on the solubility of BTMACI in the absence
and presence of ZnCl2, the value of the equilibrium constant,
ꢁ1
3
K1, comes to be ca. 2400 mol dm . This indicates that even
at the lowest concentration of ZnCl2 used, almost whole of
BTMACI will be in the form of complex [A] (Eq. 2). The linear
increase in the rate with an increase in the concentration of
ZnCl2 points to a further complexation to give complex [B]
ꢁ
(Eq. 3). The role of ZnCl2 is to co-ordinate with ICl2 . Inter-
halogen compounds are known to form complexes with Lewis
acids like zinc chloride.
16
ꢁ
3
Fig. 3. UV–vis spectra of [A] 0.0005 mol dm BTMACI,
K1
þ
ꢁ
þ
ꢁ
ꢁ ꢁ3
3
[PhCH2Me3N] ICl2 þ ZnCl2 ꢀ [PhCH2Me3N] [IZnCl4]
[A]
[
[
[
B] 0.0005 mol dm BTMACI + 0.002 mol dm ZnCl2,
C] 0.0005 mol dm BTMACI + 0.003 mol dm ZnCl2,
ꢁ
ꢁ
3
ꢁ3
ꢁ3
3
D] 0.0005 mol dm BTMACI + 0.006 mol dm ZnCl2.
Solvent: Glacial acetic acid; Temperature: 298 ꢂ 3 K.
ð2Þ
K2
þ
B]
ꢁ
[
A] þ ZnCl2 ꢀ [PhCH2Me3N] [IZn2Cl6]
ð3Þ
Spectral Studies. A comparison of the UV–vis spectra
Fig. 3) of BTMACI alone and in the presence of different con-
[
(
centrations of ZnCl2 showed that the nature of the spectra is not
much different in the presence or absence of zinc chloride.
In the complexes [A] and [B], the formal oxidation state of io-
dine is þ1. Despite the lack of evidence for the existence of

2
338 Bull. Chem. Soc. Jpn., 76, No. 12 (2003)
Oxidation of Acids by Polyhalogen Compound
þ
discrete I ions, its stable complexes with donors have been
16,17
known for a long time.
species in the sulfuric acid medium has been reported
The formation of positive iodine
18
recently.
Acetic acid is a relatively poor ionizing solvent,
and the formation of ion-pairs in it is a distinct possibility.
Therefore, it is probable that complexes [A] and [B] exist as
ion-pairs in the solvent.
The observed dependence on the concentration of zinc chlo-
ride indicates that the equilibrium between [A] and [B] is rapid,
that the equilibrium constant, K2, is small that the reaction is
not complete even at high concentrations of ZnCl2, and that on-
ly complex [B] is reactive. The small rate-enhancing effect of
BTMAC suggests that iodine monochloride (Eq. 4) is not in-
volved in the oxidation process.
K3
þ
ꢁ
þ
ꢁ
[
PhCH2Me3N] ICl2 ꢀ [PhCH2Me3N] Cl þ ICl ð4Þ
Therefore, [B] is the only reactive oxidizing species in the
oxidation of hydroxy acids. The formation of the complex is
also supported by spectral studies. The existence of the anion
Fig. 4. Brown plot at 298 K.
2ꢁ
[
Zn2Cl6] in tertahydrofuran has been confirmed by X-ray
19
2ꢁ
crystallography.
known.
The linear correlation between log k3 of twelve hydroxy
acids at 298
0
Various metallic salts of [Zn2Cl6] are
Here, k0 is rate constant for MA.
Mechanism. A hydrogen abstraction mechanism leading to
the formation of free radicals may be discounted in view of the
20
2
24
failure to induce the polymerization of acrylonitrile, and the
large value of the reaction constants. In most hydrogen abstrac-
K
and 328
K
(r ¼ 0:9998; slope
=
:805 ꢂ 0:003) shows that an isokinetic relationship exists in
21
the oxidation of hydroxy acids by BTMACI. The value of
the isokinetic temperature is 562 ꢂ 7 K. An isokinetic relation-
ship is a necessary condition for the validity of linear free en-
ergy relationships. It also implies that all the hydroxy acids
with this correlation are oxidized by a similar mechanism.
Correlation Analysis of Reactivity. The rate constants of
the oxidation of MA and ten monosubstituted mandelic acids
tion reactions, the reaction constants, ꢂ, have small magni-
tude.
22
The observed kinetic isotope effect suggested that the ꢀ-C–
H bond is cleaved in the rate determining step. The value of the
kinetic isotope effect (kH=kD ¼ 5:97 at 298 K) is of the order of
the values obtained in the oxidation of MA by pyridinium hy-
21
25
drobromide perbromide (kH=kD ¼ 5:07 at 323 K) and pyridi-
22
26
were correlated in terms of the Hammett equation, but they
failed to yield a significant correlation with Hammett’s ꢁval-
ues, Eq. 5. The main deviating points correspond to para-sub-
stituents capable of electron-donation by resonance viz. meth-
oxy, methyl, isopropyl, fluoro, etc. Their rates are higher than
those required by their Hammett’s ꢁvalues.
nium bromochromate (kH=kD ¼ 6:35 at 298 K). Both these
oxidations suggest the cleavage of a ꢀ-C–H bond in the rate de-
termining step. This supports the proposition of the cleavage of
the ꢀ-C–H bond in the rate-determining step of the oxidation of
hydroxy acids by BTMACI.
The correlation analysis of the substituent effect indicated
the presence of a highly electron-deficient reaction centre in
the rate-determining step. The fact that an excellent correlation
log k3 ¼ ꢁ3:03 ꢂ 0:26ꢁꢁ 1:36;
ð5Þ
þ
is obtained with Brown’s ꢁ values suggests that there is a res-
2
r ¼ 0:9372; sd ¼ 0:29; n ¼ 11;
¼ 0:26;
onance interaction in the transition state between a developing
positive centre at the reaction site and the substituent. Thus, the
transition state approaches a carbocation in character. Below,
the transfer of a hydride-ion from the acid to the oxidant is in-
dicated (Scheme 1).
We were unable to study the effect of polarity, which might
have given supportive evidence to our mechanism, because of
the decomposition of the oxidant in water and its insolubility in
Temperature ¼ 298 K:
Here, n is the number of data points.
The rate constants were, therefore, correlated in terms of
23
þ
Brown’s ꢁ values (Eq. 6).
An excellent correlation
Fig. 4) with large reaction constants was obtained (Table 5).
(
þ
þ
log k3 ¼ ꢂ ꢁ þ log k0:
ð6Þ
Table 5. Temperature Dependence of the Reaction Constant for the Oxidation of Substituted Mandelic Acids by
aÞ
BTMACI
þ
r2
bÞ
h
Temp/K
ꢂ
sd
298
308
318
328
ꢁ2:36 ꢂ 0:01
ꢁ2:13 ꢂ 0:02
ꢁ2:01 ꢂ 0:01
ꢁ1:90 ꢂ 0:01
0.9999
0.9995
0.9998
0.9998
0.004
0.023
0.010
0.008
0.010
0.023
0.015
0.015
ꢁ1:73
ꢁ1:42
ꢁ1:13
ꢁ0:79
a) No. of data points = 11. b) h = intercept.

A. Goyal et al.
Bull. Chem. Soc. Jpn., 76, No. 12 (2003) 2339
Scheme 1.
any other suitable solvent. The proposed mechanism is, how-
ever, supported by the observed negative entropy of activation
for the conversion of the reactant molecules into the activated
complex. As the process involves a large charge separation
in the transition state which results in an increase in the total
number of charges, the two ends become highly solvated. This
results in an immobilization of a large number of solvent mole-
cules, reflected in the loss of entropy.
11 A. I. Vogel, ‘‘A Text Book of Practical Organic Chemis-
try,’’ Longmans Green, London (1956), p. 774.
12 T. J. Kemp and W. A. Waters, J. Chem. Soc., 1964, 1194.
1
3
K. J. P. Orton and A. Bradfield, J. Chem. Soc., 1924, 960;
927, 983.
F. Feigl, ‘‘Spot Tests in Organic Analysis,’’ Elsevier Pub-
lishing Co., Amsterdam (1966), p. 485.
‘‘Dictionary of Organic Compounds,’’ ed by J. R. A.
1
1
4
1
5
Pollock and R. Stevans, Eyre and Spottiswoode Publishers Limit-
ed, London (1964), Vol. 1, p. 359.
Thanks are due to University Grants Commission (India) for
financial support.
1
6
A. J. Downs and C. J. Adams, ‘‘Comprehensive Inorganic
Chemistry,’’ ed by J. C. Bailer, H. J. Emeleus, R. N. Yholm, and
A. F. Tritman-Dickenson, Pergamon Press, Oxford (1973), Vol.
b, p. 1344.
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