Kinetics and mechanism of the oxidation of substituted benzaldehydes by hexamethylenetetramine-bromine

Article abstract of DOI:10.1002/1097-4601(2000)32:10<615::AID-KIN3>3.0.CO;2-6

The oxidation of thirty-six monosubstituted benzaldehydes by hexa-methylenetetramine-bromine (HABR), in aqueous acetic acid solution, leads to the formation of the corresponding benzoic acids. The reaction is first order with respect to HABR. Michaelis-Menten-type kinetics were observed with respect to aldehyde. The reaction failed to induce the polymerization of acrylonitrile. There is no effect of hexamethylenetetramine on the reaction rate. The oxidation of [²H]benzaldehyde (PhCDO) indicated the presence of a substantial kinetic isotope effect. The effect of solvent composition indicated that the reaction rate increases with an increase in the polarity of the solvent. The rates of oxidation of meta- and para-substituted benzaldehydes showed excellent correlations in terms of Charton's triparametric LDR equation, whereas the oxidation of ortho-substituted benzaldehydes correlated well with tetraparametric LDRS equation. The oxidation of para-substituted benzaldehydes is more susceptible to the delocalization effect but the oxidation of ortho- and meta-substituted compounds displayed a greater dependence on the field effect. The positive value of γ suggests the presence of an electron-deficient reaction center in the rate-determining step. The reaction is subjected to steric acceleration when ortho-substituents are present.

Full text of DOI:10.1002/1097-4601(2000)32:10<615::AID-KIN3>3.0.CO;2-6

Kinetics and Mechanism
of the Oxidation
of Substituted
Benzaldehydes by
Hexamethylenetetramine-
Bromine
H. GANGWANI, P. K. SHARMA, K. K. BANERJI
Department of Chemistry, J.N.V. University, Jodhpur, 342 005, India
Received 7 December 1999;accepted 16 May 2000
ABSTRACT: The oxidation of thirty-six monosubstituted benzaldehydes by hexa-methylene-
tetramine-bromine (HABR), in aqueous acetic acid solution, leads to the formation of the
corresponding benzoic acids. The reaction is first order with respect to HABR. Michaelis-
Menten–type kinetics were observed with respect to aldehyde. The reaction failed to induce
the polymerization of acrylonitrile. There is no effect of hexamethylenetetramine on the re-
action rate. The oxidation of [2H]benzaldehyde (PhCDO) indicated the presence of a substan-
tial kinetic isotope effect. The effect of solvent composition indicated that the reaction rate
increases with an increase in the polarity of the solvent. The rates of oxidation of meta- and
para-substituted benzaldehydes showed excellent correlations in terms of Charton’s tripara-
metric LDR equation, whereas the oxidation of ortho-substituted benzaldehydes correlated well
with tetraparametric LDRS equation. The oxidation of para-substituted benzaldehydes is more
susceptible to the delocalization effect but the oxidation of ortho- and meta-substituted com-
pounds displayed a greater dependence on the field effect. The positive value of ␥ suggests
the presence of an electron-deficient reaction center in the rate-determining step. The reaction
is subjected to steric acceleration when ortho-substituents are present. ᭧ 2000 John Wiley & Sons,
Inc. Int J Chem Kinet 32: 615–622, 2000
INTRODUCTION
imes and hydrazones [1,2]. There seems to be only a
few reports on the mechanistic aspects of oxidation
reactions of HABR [3–7]. In continuation of our ear-
lier study, we report here the kinetics of oxidation of
benzaldehyde and thirty-five monosubstituted benz-
aldehydes by HABR in aqueous acetic acid as solvent.
The major objective of this investigation was to study
the structure-reactivity correlation for the substrate un-
dergoing oxidation.
Hexamethylenetetramine-bromine (HABR) has been
reported as a mild and selective synthetic reagent for
the oxidation of alcohol to carbonyl compounds and
for the regeneration of carbonyl compounds from ox-
Correspondence to: K. K. Banerji.
᭧ 2000 John Wiley & Sons, Inc.

616
GANGWANI ET AL.
EXPERIMENTAL
Materials
RESULTS
Oxidation of the aromatic aldehydes by HABR results
in the formation of the corresponding benzoic acids.
Analyses of products and stoichiometric determina-
tions indicate the following overall reaction.
The aldehydes were commercial products. The liquid
aldehydes were purified through their bisulfite addi-
tion compounds and distilling them, under nitrogen,
just before use [8]. The solid aldehydes were recrys-
tallized fromethanol. HABR was prepared and puri-
fied by the reported method [1]. [2H]Benzaldehyde
(PhCDO) was prepared by the reported method [9].
Acetic acid (AcOH) was refluxed with chromic oxide
and acetic anhydride for 6 h and fractionated [10].
2 ArCHO ϩ (CH2)6N4Br4 ϩ 2 H2O !:
2 ArCOOH ϩ (CH2)6N4 ϩ 4 HBrϪ (1)
Rate Laws
The reaction was found to be first order with respect
to the HABR. The individual kinetic runs yielded a
linear plot between log [HABR] and time. Further, the
pseudo-first-order rate constant, kobs, is independent of
initial concentration of HABR. The reaction rate in-
creases with an increase in the concentration of the
aldehyde but not linearly (Table I). The order with
respect to aldehyde is less than one. A plot of 1/kobs
versus 1/[aldehyde] is linear with an intercept on the
rate-ordinate. Thus a Michaelis-Menten–type kinetics
are observed with respect to aldehyde. This leads to
the postulation of the following overall mechanism
[equations (2) and (3)] and the rate-law equation (4):
Product Analysis
The product analysis was carried out under kinetic
conditions. In a typical experiment, freshly distilled
benzaldehyde (5.25 g, 0.05 mol) and HABR (4.59g,
0.01 mol) were made up to 100 cm3 in 1:1 (v/v) acetic
acid-water. The reaction mixture was allowed to stand
for ca. 6 h to ensure completion of the reaction. It was
rendered alkaline with NaOH, filtered, and the filtrate
evaporated to dryness under reduced pressure. The res-
idue was dissolved in a minimum quantity of conc.
HCl and cooled in crushed ice to yield crude acid (2.0
g), which was recrystallized fromhot water to produce
pure benzoic acid (1.76 g, 96%, m.p. 121ЊC).
Aldehyde ϩ HABR 9
9
K: [Complex]
(2)
(3)
k2
Stoichiometry
[Complex] 99: Product
To determine the stoichiometry, HABR (2.30 g, 0.005
mol) and benzaldehyde (0.11 g, 0.001 mol) were made
up to 100 cm3 in 1:1 (v/v) acetic acid-water. The re-
action was allowed to stand for ca. 10 h to ensure the
completion of the reaction. The residual HABR was
determined spectrophotometrically at 394 nm. Several
determinations, with differently substituted benzalde-
hydes, showed that the stoichiometry is 1:1.
Rate ϭ
k2 K[Aldehyde][HABR]/(1 ϩ K[Aldehyde]) (4)
Table I Rate Constants for the Oxidation of
Benzaldehyde by HABR at 298 K
103[HABR]
(mol dm
[PhCHO]
(mol dm
104 kobs
(s )
Ϫ
3
Ϫ
3
Ϫ1
)
)
Kinetic Measurements
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
2.0
4.0
6.0
8.0
1.0
0.10
0.20
0.40
0.60
0.80
1.00
1.50
3.00
0.20
0.20
0.20
0.20
0.40*
3.79
5.63
7.45
8.35
8.89
9.24
9.75
The reactions were carried out under pseudo-first-or-
der conditions by maintaining a large excess of the
aldehyde (% 15 or more) over HABR. The solvent was
a 1:1 (v/v) acetic acid-water mixture (pH ϭ 2.04), un-
less otherwise mentioned. The reactions were carried
out at a constant temperature (Ϯ0.1 K) and followed
up to 80% reaction by monitoring the decrease in ab-
sorption due to HABR at 394 nm. The pseudo-first-
order rate constants, kobs, were computed from the lin-
ear (r Ͼ 0.995) least-squares plots of log [HABR]
versus time. Duplicate kinetic runs showed that the
rate constants were reproducible within Ϯ3%.
10.3
5.72
5.86
5.54
5.60
7.51*
* Contained 0.001 mol dmϪ3 acrylonitrile.

OXIDATION OF SUBSTITUTED BENZALDEHYDES BY HEXAMETHYLENETETRAMINE-BROMINE
617
The dependence of the rate on the concentration of
the aldehyde studied at different temperatures and the
values of K and k2 were calculated fromthe double
reciprocal plots. The thermodynamic parameters of the
formation of the intermediate and the activation pa-
rameters of its decomposition were calculated from the
values of K and k2, respectively, at different temper-
atures (Tables II and III).
Induced Polymerization of Acrylonitrile
The oxidation of benzaldehyde, in an atmosphere of
nitrogen, failed to induce polymerization of acryloni-
trile. Further, the addition of acrylonitrile had no effect
on the reaction rate (Table I).
Kinetic Isotope Effect
To ascertain the importance of the cleavage of the al-
dehydic C-H bond in the rate-determining step, the
oxidation of [2H]benzaldehyde (PhCDO) was studied.
The results (Table III) showed the presence of a sub-
stantial kinetic isotope effect (kH/kD ϭ 5.47 at 298 K).
Effect of Hexamethylenetetramine
The oxidation is not affected by an addition of hexa-
methylenetetramine (HXA) (Table IV).
Table II Formation Constants and Thermodynamic Parameters of the Substituted Benzaldehyde-HABR Complexes
Ϫ
1
K (dm3 mol
)
⌬H
(kJ mol
⌬S
Ϫ
1
Ϫ
Ϫ1
Subst.
288 K
298 K
308 K
318 K
)
(J mol 1 K
)
H
p-Me
p-OMe
p-F
p-Cl
p-NO2
p-CF3
p-COOMe
p-Br
p-NHAc
p-CN
p-SMe
p-NMe2
m-Me
m-OMe
m-Cl
6.15
5.92
5.78
6.05
5.76
6.28
5.98
5.85
5.65
5.81
5.88
5.75
6.03
5.66
5.86
6.02
5.89
5.29
5.45
5.58
6.03
5.56
5.82
5.32
5.87
5.73
5.96
5.93
6.12
5.58
6.10
5.68
6.05
6.13
5.89
6.15
5.70
5.25
5.12
4.96
5.24
4.82
5.46
5.15
5.02
4.72
5.02
5.12
5.00
5.28
4.82
5.02
5.27
5.10
4.50
4.62
4.75
5.28
4.85
5.05
4.85
5.02
4.97
5.12
5.16
5.34
4.75
5.26
4.86
5.24
5.28
5.10
5.32
4.80
4.36
4.25
4.10
4.42
4.05
4.58
4.32
4.26
3.95
4.18
4.32
4.25
4.46
3.97
4.20
4.40
4.32
3.86
3.85
4.02
4.46
4.02
4.23
3.78
4.27
4.02
4.32
4.38
4.55
4.02
4.38
4.05
4.42
4.43
4.32
4.42
3.93
3.44
3.45
3.35
3.52
3.62
3.67
3.53
3.56
3.15
3.33
3.49
3.46
3.48
3.13
3.38
3.52
3.42
3.06
3.08
3.29
3.48
3.26
3.40
3.10
3.40
3.18
3.48
3.47
3.62
3.29
3.51
3.20
3.52
3.64
3.42
3.60
3.15
Ϫ17.1 Ϯ 0.8
Ϫ16.2 Ϯ 0.7
Ϫ16.4 Ϯ 0.6
Ϫ16.5 Ϯ 0.8
Ϫ16.7 Ϯ 0.5
Ϫ16.1 Ϯ 0.9
Ϫ15.8 Ϯ 0.6
Ϫ15.1 Ϯ 0.4
Ϫ16.7 Ϯ 0.7
Ϫ16.5 Ϯ 0.8
Ϫ15.6 Ϯ 0.8
Ϫ15.3 Ϯ 0.7
Ϫ17.0 Ϯ 0.9
Ϫ17.5 Ϯ 0.8
Ϫ16.4 Ϯ 0.7
Ϫ16.0 Ϯ 0.9
Ϫ16.1 Ϯ 0.9
Ϫ16.1 Ϯ 0.8
Ϫ16.9 Ϯ 0.7
Ϫ15.8 Ϯ 0.5
Ϫ16.2 Ϯ 1.0
Ϫ16.1 Ϯ 0.8
Ϫ16.2 Ϯ 0.9
Ϫ16.3 Ϯ 0.6
Ϫ16.2 Ϯ 0.8
Ϫ17.2 Ϯ 0.7
Ϫ16.0 Ϯ 0.7
Ϫ15.9 Ϯ 0.9
Ϫ15.5 Ϯ 0.9
Ϫ15.8 Ϯ 0.5
Ϫ16.5 Ϯ 0.8
Ϫ16.9 Ϯ 0.8
Ϫ16.1 Ϯ 0.9
Ϫ15.7 Ϯ 0.7
Ϫ16.1 Ϯ 0.9
Ϫ16.0 Ϯ 0.7
Ϫ17.5 Ϯ 0.6
Ϫ36 Ϯ 3
Ϫ33 Ϯ 2
Ϫ34 Ϯ 2
Ϫ34 Ϯ 3
Ϫ35 Ϯ 2
Ϫ33 Ϯ 3
Ϫ32 Ϯ 2
Ϫ29 Ϯ 4
Ϫ36 Ϯ 2
Ϫ34 Ϯ 3
Ϫ31 Ϯ 3
Ϫ30 Ϯ 2
Ϫ37 Ϯ 3
Ϫ38 Ϯ 3
Ϫ34 Ϯ 2
Ϫ32 Ϯ 3
Ϫ33 Ϯ 3
Ϫ34 Ϯ 3
Ϫ36 Ϯ 2
Ϫ32 Ϯ 2
Ϫ33 Ϯ 4
Ϫ33 Ϯ 3
Ϫ33 Ϯ 2
Ϫ34 Ϯ 2
Ϫ33 Ϯ 3
Ϫ37 Ϯ 2
Ϫ33 Ϯ 2
Ϫ32 Ϯ 3
Ϫ31 Ϯ 3
Ϫ32 Ϯ 2
Ϫ34 Ϯ 3
Ϫ36 Ϯ 3
Ϫ33 Ϯ 4
Ϫ31 Ϯ 2
Ϫ33 Ϯ 3
Ϫ32 Ϯ 3
Ϫ38 Ϯ 2
m-Br
m-F
m-NO2
m-CO2Me
m-CF3
m-CN
m-SMe
m-NHAc
o-Me
o-OMe
o-NO2
o-COOMe
o-NHAc
o-Cl
o-Br
o-I
o-Cn
o-SMe
o-F
o-CF3
PhCDO

618
GANGWANI ET AL.
Table III Rate Constants and Activation Parameters of the Decomposition of Substituted Benzaldehyde-HABR
Complexes
Ϫ
Ϫ1
104 k2 (dm3 mol 1 s
)
⌬H*
(kJ mol
⌬S*
Ϫ
1
Ϫ
Ϫ1
Subst.
288 K
298 K
308 K
318 K
)
(J mol 1 K
)
H
p-Me
p-OMe
p-F
p-Cl
p-NO2
p-CF3
p-COOMe
p-Br
p-NHAc
p-CN
p-SMe
p-NMe2
m-Me
m-OMe
m-Cl
4.37
9.02
17.4
3.62
2.40
0.18
0.57
0.80
2.30
8.65
0.32
10.5
78.4
8.10
10.0
1.72
1.63
2.30
0.15
0.75
0.55
0.28
6.60
5.82
30.6
31.0
0.43
2.75
41.1
7.81
9.93
16.0
0.85
39.2
5.47
5.45
0.78
5.60
11.0
22.3
42.5
9.39
6.22
0.53
1.55
2.11
6.10
21.4
0.93
25.5
185
19.8
24.0
4.38
4.23
5.71
0.47
2.05
1.50
0.82
15.6
14.0
70.2
73.6
1.20
7.00
92.6
19.3
23.3
36.9
2.33
93.3
13.5
13.3
2.01
5.47
27.6
54.0
98.3
23.7
15.6
1.46
4.12
5.54
15.3
52.8
2.52
62.0
410
47.0
57.1
11.3
10.2
14.3
1.34
5.51
4.06
2.31
38.2
34.2
165
69.1
135
240
61.0
40.3
4.19
10.8
14.3
39.7
127
7.00
150
945
112
135
28.2
25.8
35.6
3.91
14.8
11.0
6.41
90.2
81.3
381
67.5 Ϯ 0.9
56.6 Ϯ 0.9
63.8 Ϯ 0.9
69.0 Ϯ 0.8
68.8 Ϯ 0.9
77.0 Ϯ 0.9
72.1 Ϯ 0.7
70.6 Ϯ 0.9
69.5 Ϯ 0.8
65.7 Ϯ 0.7
75.5 Ϯ 0.8
64.9 Ϯ 0.9
60.4 Ϯ 0.7
64.0 Ϯ 0.7
63.5 Ϯ 0.8
68.5 Ϯ 0.8
67.2 Ϯ 0.9
66.0 Ϯ 0.9
79.9 Ϯ 0.6
73.1 Ϯ 0.9
73.0 Ϯ 0.8
76.8 Ϯ 0.7
64.0 Ϯ 0.9
64.5 Ϯ 0.9
61.5 Ϯ 0.9
61.6 Ϯ 0.5
73.9 Ϯ 0.9
68.6 Ϯ 0.9
59.6 Ϯ 0.9
64.6 Ϯ 0.5
63.5 Ϯ 0.9
62.1 Ϯ 0.9
72.7 Ϯ 0.9
61.1 Ϯ 0.4
66.3 Ϯ 0.8
65.8 Ϯ 0.9
69.2 Ϯ 0.9
Ϫ75 Ϯ 3
Ϫ76 Ϯ 3
Ϫ77 Ϯ 3
Ϫ72 Ϯ 3
Ϫ76 Ϯ 3
Ϫ69 Ϯ 3
Ϫ76 Ϯ 2
Ϫ79 Ϯ 3
Ϫ74 Ϯ 3
Ϫ76 Ϯ 2
Ϫ69 Ϯ 2
Ϫ77 Ϯ 3
Ϫ76 Ϯ 2
Ϫ82 Ϯ 2
Ϫ82 Ϯ 3
Ϫ80 Ϯ 3
Ϫ84 Ϯ 3
Ϫ83 Ϯ 3
Ϫ60 Ϯ 2
Ϫ71 Ϯ 3
Ϫ73 Ϯ 3
Ϫ66 Ϯ 2
Ϫ84 Ϯ 3
Ϫ84 Ϯ 3
Ϫ80 Ϯ 3
Ϫ79 Ϯ 2
Ϫ73 Ϯ 3
Ϫ76 Ϯ 3
Ϫ84 Ϯ 3
Ϫ81 Ϯ 2
Ϫ83 Ϯ 3
Ϫ83 Ϯ 3
Ϫ71 Ϯ 3
Ϫ79 Ϯ 1
Ϫ78 Ϯ 3
Ϫ80 Ϯ 3
Ϫ84 Ϯ 3
m-Br
m-F
m-NO2
m-CO2Me
m-CF3
m-CN
m-SMe
m-NHAc
o-Me
o-OMe
o-NO2
o-COOMe
o-NHAc
o-Cl
o-Br
o-I
o-CN
o-SMe
o-F
172
388
3.21
17.9
210
46.8
56.7
88.2
6.14
215
33.8
32.9
5.18
5.33
8.80
45.3
476
110
133
203
16.6
482
82.2
80.4
13.2
5.23
o-CF3
PhCDO
kH/kD
Effect of Solvent Composition
termine whether the solvent composition is affecting
the formation constant, K, and/or rate constant, k2, the
variation of rate with the concentration of benzalde-
hyde was studied in solutions of different composi-
tions. The results (Table V) showed the rate constants,
The rate of oxidation was determined in solvents con-
taining different amounts of acetic acid and water. It
was observed that the rate increased with an increase
in the amount of water in the solvent mixture. To de-
Table IV Effect of Hexamethylenetetramine (HXA) on the Rate of Oxidation of Benzaldehyde by HABR ([HABR] ϭ
0.001 mol dmϪ3, [benzaldehyde] ϭ 1.0 mol dmϪ3, Temp. ϭ 298 K)
Ϫ
3
103 [HXA] / mol dm
0.0
9.24
0.5
9.33
1.0
9.00
2.0
9.85
3.0
9.63
4.0
9.44
Ϫ
1
104 kobs/s

OXIDATION OF SUBSTITUTED BENZALDEHYDES BY HEXAMETHYLENETETRAMINE-BROMINE
619
Table V Dependence of Rate on the Concentration of
Benzaldehyde in Solvents of Different Compositions
([HABR] ϭ 0.001 mol dmϪ3, Temp. ϭ 298 K)
(CH2)6N4Br4 VJ 2 Br2 ϩ (CH2)6N4
(5)
The probable oxidizing species in a solution of HABR
are, therefore, HABR itself, molecular bromine, and
its acetolysis product. An addition of HXA should re-
sult in the suppression of decomposition of HABR and
thus a decrease in the reaction rate if bromine is the
reactive oxidizing species. Therefore, the lack of an
effect of HXA and strict first-order dependence on
HABR rule out both bromine and its acetolysis prod-
uct as the reactive oxidizing species.
104 kobs/s 1 [at % AcOH (v/v)]
Ϫ
PhCHO
(mol dm
Ϫ
3
)
25
40
50
60
72
0.1
0.2
0.4
0.6
0.8
1.0
18.3
27.5
36.8
41.4
44.2
46.1
7.52
11.1
14.6
16.4
17.4
18.1
3.79
5.63
7.45
8.35
8.88
9.24
1.85
2.77
3.68
4.13
4.41
5.00
0.68
1.01
1.34
1.49
1.60
1.65
A perusal of UV-VIS spectra of HABR (0.001 mol
dmϪ ) and an equivalent amount of bromine (0.002
3
Ϫ
1
K (dm3 mol
)
4.93
55.4
5.42
21.4
5.25
11.0
5.08
5.49
5.35
1.96
3
mol dmϪ ), in acetic acid at Ϸ293 K, showed that the
Ϫ
1
104 k2 (s
)
difference in the spectra of HABR and bromine was
not very striking but their optical densities showed
variations (Fig. 1). HXA had no appreciable absorp-
tion in this range. Further, the spectrumof HABR did
not show any change in the experimental time period
(ca. 2 h). When a solution of HABR in acetic acid was
evaporated to dryness under reduced pressure, HABR
was recovered unchanged. This confirmed that HABR
retained its integrity in acetic acid. Therefore, we pro-
pose that in this reaction, the reactive oxidizing species
is HABR itself.
k2, vary considerably while the formation constant, K,
remained practically independent of solvent compo-
sition.
DISCUSSION
A plot of log k2 at 288 K is linearly related to log k2
at 318 K (r ϭ 0.9993, slope ϭ 0.879 Ϯ 0.005). The
value of the isokinetic temperature is 1269 Ϯ 23 K. A
linear isokinetic relationship is a necessary condition
for the validity of linear free-energy relationships [11].
It also implies that all the aldehydes for which the rates
of oxidation are so correlated are oxidized by the same
mechanism.
Solvent Composition Effect
The increase in the rate constant of decomposition of
the intermediate complex with an increase in the po-
larity of the medium suggests that the transition state
is more polar than the reactants. The solvent effect was
analyzed using Grunwald-Winstein [12] Eq. (6).
In solution, HABR may dissociate to form molec-
ular bromine as Eq. (5).
log k2 ϭ log k0 ϩ mY
(6)
Ϫ
3
Ϫ3
Figure 1 UV-VIS spectra of (A) 0.001 mol dm HABR and (B) 0.002 mol dm bromine; Temperature: 293 K; Solvent:
Acetic Acid.

620
GANGWANI ET AL.
The plot of log k2 versus Y is linear (r ϭ 0.9996)
with m ϭ 0.78 Ϯ 0.01. The value of m suggests that
there is a considerable charge separation in the tran-
sition state of the rate-determining step.
delocalized (resonance) electrical effect parameter
when active site electronic demand is minimal, and ␴
e
represents the sensitivity of the substituent to changes
in electronic demand by the active site. The latter two
substituent parameters are related by Eq. (8).
␴D ϭ ␩␴e ϩ ␴
(8)
Correlation Analysis of Reactivity
d
A perusal of data recorded in Tables II and III results
indicates that the formation constants, K, for the
ArCHO-HABR complexes are not very sensitive to
the nature of the substituent in the aldehyde substrate.
The rate constant for the decomposition of the com-
plex k2, however, showed considerable variation as a
function of substituent in the reductant. Similar obser-
vations have been previously recorded in the oxidation
of benzyl alcohols [13] and mandelic acids [14] by
ceric ammonium nitrate, aliphatic alcohols [4] and
diols [5] by HABR, substituted benzaldehydes by
bis(2,2Ј-bipyridyl) copper(II) permanganate [15] and
aliphatic alcohols by pyridiniumfluorochromate [16]
and pyridinium hydrobromide perbromide [17].
The effect of substituents on reactivity has long
been correlated with the Hammett equation [18] or
with dual substituent-parameter equations [19,20]. In
the late 1980s, Charton [21] introduced a triparametric
LDR equation for the quantitative description of struc-
tural effects on chemical reactivities.
where ␩ represents the electronic demand of the re-
action site and is given by ␩ ϭ R/D, and ␴D represents
the delocalized electrical parameter of the diparame-
tric LD equation.
For ortho-substituted compounds, the LDR equa-
tion has been modified to the LDRS Eq. [21], to ac-
count for the steric effects.
log k2 ϭ L ␴l ϩ D ␴ ϩ R ␴e ϩ S V ϩ h (9)
d
where V is the well known Charton’s steric parameter
based on Van der Waals radii [22].
The rate constants, k2, of the disproportionation of
the intermediate of the oxidation of ortho-, meta-, and
para-substituted benzaldehydes show excellent corre-
lations in terms of the LDR/LDRS equations (Table
VI). All three series of substituted benzaldehydes
meet the requirement of a minimum number of substi-
tuents for analysis by LDR and LDRS equations [20].
We have used the standard deviation (sd), the coeffi-
cient of multiple determination, R2, and Exner’s [23]
parameter, ␺, as the measures of goodness of fit. The
value of ␺ is 0.01 except in one case where it is 0.02.
To test the significance of localized, delocalised,
log k2 ϭ L ␴l ϩ D ␴d ϩ R ␴e ϩ h
(7)
Here, h is the intercept term, ␴ is a localized (field
l
and/or inductive) effect parameter, ␴ is the intrinsic
d
Table VI Temperature Dependence for the Reaction Constants for the Oxidation of Substituted Benzaldehydes by
HABR
T/K
L
D
R
S
␩
R2
h
Para-substituted
Ϫ1.32 Ϯ 0.04
Ϫ1.31 Ϯ 0.02
Ϫ1.24 Ϯ 0.02
Ϫ1.18 Ϯ 0.03
Meta-substituted
Ϫ0.94 Ϯ 0.08
Ϫ0.93 Ϯ 0.02
Ϫ0.95 Ϯ 0.07
Ϫ0.90 Ϯ 0.08
Ortho-substituted
Ϫ1.23 Ϯ 0.06
Ϫ1.13 Ϯ 0.04
Ϫ1.15 Ϯ 0.04
Ϫ1.14 Ϯ 0.05
288
298
308
318
Ϫ1.72 Ϯ 0.01
Ϫ1.63 Ϯ 0.01
Ϫ1.58 Ϯ 0.01
Ϫ1.51 Ϯ 0.01
Ϫ1.86 Ϯ 0.01
Ϫ1.80 Ϯ 0.01
Ϫ1.73 Ϯ 0.01
Ϫ1.68 Ϯ 0.01
—
—
—
—
0.71
0.73
0.72
0.70
0.9998
0.9999
0.9998
0.9998
Ϫ3.36
Ϫ2.96
Ϫ2.56
Ϫ2.16
288
298
308
318
Ϫ1.86 Ϯ 0.02
Ϫ1.76 Ϯ 0.01
Ϫ1.69 Ϯ 0.02
Ϫ1.61 Ϯ 0.02
Ϫ1.57 Ϯ 0.01
Ϫ1.46 Ϯ 0.01
Ϫ1.39 Ϯ 0.01
Ϫ1.30 Ϯ 0.01
—
—
—
—
0.60
0.64
0.68
0.69
0.9996
0.9999
0.9997
0.9996
Ϫ3.35
Ϫ2.96
Ϫ2.56
Ϫ2.17
288
298
308
318
Ϫ1.76 Ϯ 0.01
Ϫ1.68 Ϯ 0.01
Ϫ1.62 Ϯ 0.01
Ϫ1.56 Ϯ 0.01
Ϫ1.66 Ϯ 0.01
Ϫ1.60 Ϯ 0.01
Ϫ1.55 Ϯ 0.01
Ϫ1.46 Ϯ 0.02
1.11 Ϯ 0.01
1.03 Ϯ 0.01
0.99 Ϯ 0.01
0.94 Ϯ 0.01
0.74
0.71
0.74
0.78
0.9997
0.9999
0.9998
0.9998
Ϫ3.37
Ϫ2.96
Ϫ2.56
Ϫ2.16

OXIDATION OF SUBSTITUTED BENZALDEHYDES BY HEXAMETHYLENETETRAMINE-BROMINE
621
and steric effects in the ortho-substituted benzalde-
hydes, multiple linear regression analyses were carried
out with (i) ␴l, ␴d, and ␴e, (ii) ␴d, ␴e, and V, and (iii)
␴l, ␴e, and V. The absence of significant correlations
equations [(10)–(12)] showed that all the four sub-
stituent constants are significant.
(|S| ϫ 100)
PS ϭ
(14)
(|L| ϩ |D| ϩ |S|)
The values of PD and PS are also recorded in Table
VI. The value of PD for the oxidation of para-substi-
tuted benzaldehydes is ca. 52%, whereas the corre-
sponding values for the meta- and ortho-sobstituted
aldehydes are ca. 45 and 47%, respectively. This
shows that the balance of localization and delocaliza-
tion effects is different for differently substituted benz-
aldehydes. The less pronounced resonance effect from
the ortho- position than fromthe para-position may
be due to the twisting away of the aldehydic group
fromthe plane of the benzene ring. The value of the
PS (ca. 24%) shows that the steric effect is significant
in this reaction.
log k2 ϭ (Ϫ1.41 Ϯ 0.36)␴l Ϫ
(1.61 Ϯ 0.29)␴ ϩ (Ϫ3.07 Ϯ 1.65)␴e Ϫ 2.22 (10)
d
R2 ϭ 0.8570, sd ϭ 0.25, n ϭ 12, ⌿ ϭ 0.31
log k2 ϭ (Ϫ1.70 Ϯ 0.42)␴d Ϫ
(1.67 Ϯ 2.58)␴e ϩ (0.73 Ϯ 0.48)v Ϫ 3.06 (11)
R2 ϭ 0.6948, sd ϭ 0.37, n ϭ 12, ⌿ ϭ 0.47
log k2 ϭ (Ϫ1.86 Ϯ 0.66)␴l Ϫ
(0.35 Ϯ 3.15)␴e ϩ (1.12 Ϯ 0.59)v Ϫ 2.26 (12)
R2 ϭ 0.5430, sd ϭ 0.45, n ϭ 12, ⌿ ϭ 0.58
Mechanism
Similarly, in the cases of the oxidation of para- and
meta-substituted benzaldehydes, multiple regression
analyses indicated that both localization and delocal-
ization effects are significant. There is no significant
collinearity between the various substituent constants
for the three series.
The comparison of the L and D values for the sub-
stituted benzaldehydes showed that the oxidation of
para-substituted benzaldehydes is more susceptible to
the delocalization effect than to the localized effect.
However, the oxidation of ortho- and meta-substituted
compounds exhibited a greater dependence on the field
effect. In all cases, the magnitude of the reaction con-
stants decreases with an increase in the temperature,
pointing to a decrease in selectivity with an increase
in temperature.
All three regression coefficients, L, D and R, are
negative, indicating an electron-deficient carbon cen-
ter in the activated complex for the rate-determining
step. The positive value of ␩ adds a negative increment
to ␴d, increasing the electron-donating power of the
substituent and its capacity to stablize a cationic spe-
cies. The positive value of S indicates that the reaction
is subject to steric acceleration by an ortho-substitu-
ent.
The cleavage of the aldehydic C-H bond in the rate-
determining step is confirmed by the presence of a
substantial kinetic isotope effect. The failure to induce
polymerization of acrylonitrile and the lack of effect
of the radical scavenger on the reaction rate point
against the operation of a one-electron oxidation, giv-
ing rise to free radicals. The negative values of the
localization and delocalization electrical effects, that
is, of L, D, and R points to an electron-deficient re-
action center in the rate-determining step. It is further
supported by the positive value of ␩, which indicates
that the substituent is better able to stabilize a cationic
or electron-deficient reactive site. Therefore, a hy-
dride-ion transfer in the rate-determining step is sug-
gested (Scheme I). The observed effect of solvent
composition on the reaction rate also supports a mech-
anisminvolving the transfer of a hydride-ion. The ben-
zoyl cation is reported to have a considerable ketene
character [24] and is thus linear. The linear structure
of acyliumcation has been confirmed by X-ray crys-
tallography also [25]. The change from sp2 to sp re-
sults in steric relief. This relief will be greater in
crowded reductants and is reflected in observed steric
acceleration.
The percent contribution [21] of the delocalized ef-
fect, PD is given by the following Eq. (13):
K
ArCHO ϩ Br…Br…N Ͻ
Ar9C"O…Br…Br…N Ͻ
(|D| ϫ 100)
H
PD ϭ
(13)
(|L| ϩ |D|)
k2
Ar9C"O…Br…Br…N Ͻ
Similarly, the percent contribution of the steric param-
eter [21] to the total effect of the substituent, PS, was
determined by using Eq. (14).
H
ϩ
Ar9C"O ϩ HBr ϩ BrϪ ϩ N Ͻ

622
GANGWANI ET AL.
ϩ
7. Gangwani, H.; Sharma, P. K.; Banerji, K. K. Indian J
Chem, 2000, 39A, 436.
fast
Ar9C"O ϩ H2O
ArCOOH ϩ Hϩ
8. Wiberg, K. B.; Stewart, R. J AmChemSoc 1955, 77,
1786.
9. Wiberg, K. B. J AmChemSoc 1954, 76, 5371.
10. Perrin, D. D.; Armarego, W. L.; Perrin, D. R. Purifica-
tion of Organic Compounds; Pergamon Press: Oxford,
1966.
11. Exner, O. Prog Phys Org Chem1973, 10, 411.
12. Falnberg, A. H.; Winstein, S. J AmChemSoc 1956, 78,
2770.
13. Young, Y. B.; Trahanovsky, W. S. J AmChemSoc
1969, 91, 5060.
Scheme I
The observed negative entropy of activation sup-
ports the above mechanism also. As the charge sepa-
ration takes place, in the transition state of the rate-
determining step, the charged ends become highly
solvated. This results in an immobilization of a large
number of solvent molecules, reflected in the loss of
entropy.
14. Banerji, K. K. J Indian ChemSoc 1975, 52, 573.
15. Mohnot, K.; Sharma, P. K.; Banerji, K. K. J Org Chem
1996, 61, 1310.
16. Banerji, K. K. J ChemSoc, Perkin Trans 2 1988, 1547.
17. Mathur, D.; Sharma, P. K.; Banerji, K. K. J Chem Soc,
Perkin Trans 2 1993, 205.
Thanks are due to the Council of Scientific and Industrial
Research (India) for financial support (Scheme No. 01
(1519)/98/EMR-II).
18. Johnson, C. D. The Hammett Equation; University
Press: Cambridge, 1973; p 78.
BIBLIOGRAPHY
19. Dayal, S. K.; Ehrenson, S.; Taft, R.W. J AmChemSoc
1972, 94, 9113.
20. Swain, C. G.; Unger, S. H.; Rosenquist, N. R.; Swain,
M. S. J AmChemSoc 1983, 105, 492.
21. Charton, M.; Charton, B. Bull Soc ChimFr 1988, 199
and references cited therein.
1. Yavari, I.; Shaabani, G. J ChemRes (S) 1994, 274.
2. Bandgar, B. P.; Admane, S. B.; Jare, S. S. J Chem Res
(S) 1998, 154.
3. Pareek, A.; Kothari, S.; Banerji, K. K. Indian J Chem
1996, 35B, 970.
4. Pareek, A.; Kothari, S.; Banerji, K. K. React Kinet Catal
Lett 1997, 60, 127.
5. Gangwani, H.; Sharma, P. K.; Banerji, K. K. J Chem
Res 1999, (S) 1 ; (M) 854.
6. Bohra, A.; Sharma, P. K.; Banerji, K. K. J Indian Chem
Soc 1998, 75, 784.
22. Charton, M. J Org Chem1975, 40, 407.
23. Exner, O. Collect Czech Chem Commun 1966, 31,
3222.
24. Olah, G. A.; Westermann, P. W. J AmChemSoc 1973,
95, 3706.
25. Boer, F. P. J AmChemSoc 1968, 90, 6706.