Correlation analysis of reactivity in the oxidation of substituted benzaldehydes by pyridinium hydrobromide perbromide

Article abstract of DOI:10.1002/poc.413

The oxidation of benzaldehyde and 35 monosubstituted benzaldehydes by pyridinium hydrobromide perbromide (PHPB) in aqueous acetic acid leads to the formation of the corresponding benzoic acids. The reaction is first order with respect to each of the benzaldehydes and PHPB. Addition of pyridinium bromide has no effect on the rate of oxidation. 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 increase in the polarity of the solvent. The rates of oxidation of meta- and para-substituted benzaldehydes were correlated in terms of Charton's triparametric LDR equation whereas those of ortho-substituted benzaldehydes were correlated with a tetraparametric LDRS equation. The oxidations of para- and ortho-substituted benzaldehydes are more susceptible to the delocalization effect while the oxidation of meta-substituted compounds displays a greater dependence on the fi eld effect. The positive value of η suggests the presence of an electron-deficient reaction centre in the rate-determining step. The reaction is subjected to steric hindrance by the ortho substituents.

Full text of DOI:10.1002/poc.413

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2001; 14: 650–656
DOI:10.1002/poc.413
Correlation analysis of reactivity in the oxidation of
substituted benzaldehydes by pyridinium hydrobromide
perbromide
Meenakshi Aneja, Seema Kothari and Kalyan K. Banerji*
Department of Chemistry, J. N. V. University, Jodhpur 342 005, India
Received 3 August 2000; revised 2 March 2001; accepted 5 April 2001
ABSTRACT: The oxidation of benzaldehyde and 35 monosubstituted benzaldehydes by pyridinium hydrobromide
perbromide (PHPB) in aqueous acetic acid leads to the formation of the corresponding benzoic acids. The reaction is
first order with respect to each of the benzaldehydes and PHPB. Addition of pyridinium bromide has no effect on the
rate of oxidation. 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 increase in the polarity of the
solvent. The rates of oxidation of meta- and para-substituted benzaldehydes were correlated in terms of Charton’s
triparametric LDR equation whereas those of ortho-substituted benzaldehydes were correlated with a tetraparametric
LDRS equation. The oxidations of para- and ortho-substituted benzaldehydes are more susceptible to the
delocalization effect while the oxidation of meta-substituted compounds displays a greater dependence on the field
effect. The positive value of Z suggests the presence of an electron-deficient reaction centre in the rate-determining
step. The reaction is subjected to steric hindrance by the ortho substituents. Copyright  2001 John Wiley & Sons,
Ltd.
KEYWORDS: Kinetics; mechanism; oxidation; correlation analysis; polyhalide; aromatic aldehydes
INTRODUCTION
Analyses of products and stoichiometric determinations
indicate the following overall reaction:
Pyridinium hydrobromide perbromide (PHPB) is one of
the quaternary polyhalides, which have been used as
effective halogenating and oxidizing agents in synthetic
organic chemistry.1–3 The polyhalides are more suitable
than molecular halogens because of their solid nature,
ease of handling, stability, selectivity and excellent
product yield. We have been interested in the kinetic
and mechanistic aspects of oxidation by polyhalide
compounds and many reports have emanated from our
laboratory,4–8 including that on the oxidation of aliphatic
aldehydes by PHPB.9 In continuation of our earlier
studies, we report here the kinetics of oxidation of
benzaldehyde and 35 monosubstituted benzaldehydes by
PHPB in aqueous acetic acid as solvent. The major
objective of this investigation was to study the structure–
reactivity correlation for the substrate undergoing oxida-
tion.
‡
À
ArCHO ‡ PyH Br3 ‡ H2O
! ArCOOH ‡ PyH BrÀ ‡ 2HBr
ꢀ1†
‡
Rate laws
The reactions were found to be first order with respect to
PHPB. The individual kinetic runs were strictly first order
in PHPB. Further, the pseudo-first-order rate constants,
kobs, do not depend on the initial concentration of PHPB.
The reaction rate increases linearly with increase in the
concentration of aldehydes (Table 1).
Induced polymerization of acrylonitrile
The oxidation of benzaldehyde, in an atmosphere of
nitrogen, failed to induce polymerization of acrylonitrile.
Further, the addition of acrylonitrile had no effect on the
reaction rate (Table 1).
RESULTS
Oxidation of the aromatic aldehydes by PHPB results in
the formation of the corresponding benzoic acids.
*Correspondence to: K. K. Banerji, Department of Chemistry, J. N. V.
University, Jodhpur 342 005, India.
Effect of substituent
Contract/grant sponsor: Council of Scientific and Industrial Research
(India).
The rates of the oxidation of benzaldehyde and 35
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 650–656

REACTIVITY IN OXIDATION OF SUBSTITUTED BENZALDEHYDES
651
Table 1. Rate constants for the oxidation of benzaldehyde by
PHPB at 318 K
monosubstituted benzaldehydes were determined at
different temperatures and the activation parameters
were calculated (Table 2).
103 [PHPB]
[PhCHO]
(mol dmÀ3
)
(mol dmÀ3
)
104 kobs(sÀ1
)
1.0
1.0
1.0
1.0
1.0
1.0
2.0
4.0
6.0
8.0
1.0
0.05
0.10
0.20
0.30
0.50
1.00
0.20
0.20
0.20
0.20
0.30a
3.65
7.19
15.0
22.5
35.8
72.1
14.5
15.3
15.5
14.8
22.0
Kinetic isotope effect
To ascertain the importance of the cleavage of the
aldehydic C—H bond in the rate-determining step, the
oxidation of [2H]benzaldehyde (PhCDO) was studied.
The results (Table 2) showed the presence of a substantial
kinetic isotope effect (kH/kD = 5.75 at 298 K).
Effect of solvent composition
a
Contained 0.005 mol dmÀ3 acrylonitrile.
The rate of oxidation was determined in solvents
containing different amounts of acetic acid and water.
Table 2. Rate constants and activation parameters of the oxidation of substituted benzaldehydes by PHPB
104 k2 (dm3 molÀ1 sÀ1
)
DH*
DS*
DG*
Substituent
288 K
298 K
308 K
318 K
72.1
(kJ molÀ1
)
(J molÀ1 KÀ1
)
(kJ molÀ1
)
H
3.81
10.5
52.1
6.51
4.12
4.14
0.58
0.91
0.26
20.5
0.43
30.6
10.2
25.0
110
27.0
64.1
72.0 Æ 1.0
66.8 Æ 1.4
61.5 Æ 2.3
68.4 Æ 1.8
71.4 Æ 1.6
71.3 Æ 1.6
80.7 Æ 0.9
78.5 Æ 0.9
83.9 Æ 0.9
65.8 Æ 2.6
81.0 Æ 0.8
63.9 Æ 2.6
53.4 Æ 3.6
70.6 Æ 1.5
69.6 Æ 1.2
77.0 Æ 0.7
77.2 Æ 0.6
76.9 Æ 0.8
86.3 Æ 0.4
78.9 Æ 0.8
80.3 Æ 0.4
83.5 Æ 0.9
71.6 Æ 0.7
71.1 Æ 1.2
75.4 Æ 0.8
68.4 Æ 0.8
89.7 Æ 0.8
85.9 Æ 1.1
76.9 Æ 0.9
81.7 Æ 0.7
83.1 Æ 1.3
83.8 Æ 0.2
87.2 Æ 1.2
74.1 Æ 0.5
75.2 Æ 0.7
92.4 Æ 1.8
74.2 Æ 1.4
À61 Æ 3
À71 Æ 5
À76 Æ 7
À69 Æ 6
À62 Æ 5
À63 Æ 5
À46 Æ 3
À50 Æ 3
À42 Æ 3
À69 Æ 5
À48 Æ 3
À72 Æ 6
À84 Æ 8
À62 Æ 5
À67 Æ 4
À54 Æ 2
À54 Æ 2
À54 Æ 3
À37 Æ 2
À50 Æ 3
À48 Æ 1
À42 Æ 3
À61 Æ 2
À65 Æ 4
À55 Æ 3
À68 Æ 3
À31 Æ 3
À37 Æ 4
À52 Æ 3
À44 Æ 2
À42 Æ 4
À41 Æ 1
À37 Æ 4
À58 Æ 2
À57 Æ 2
À28 Æ 6
À68 Æ 5
90.0 Æ 0.8
87.7 Æ 1.1
84.0 Æ 1.8
88.9 Æ 1.4
89.9 Æ 1.3
89.9 Æ 1.2
94.4 Æ 0.7
93.3 Æ 0.7
96.2 Æ 0.7
86.2 Æ 2.1
95.1 Æ 0.6
85.3 Æ 2.0
78.4 Æ 2.9
88.8 Æ 1.2
89.4 Æ 1.0
93.1 Æ 0.6
93.0 Æ 0.5
93.0 Æ 0.6
97.2 Æ 0.4
93.6 Æ 0.6
94.6 Æ 0.3
96.0 Æ 0.7
89.7 Æ 0.5
90.2 Æ 0.9
91.6 Æ 0.7
88.4 Æ 0.7
98.8 Æ 0.6
96.8 Æ 0.9
92.3 Æ 0.7
94.7 Æ 0.6
95.4 Æ 4.0
95.8 Æ 0.2
98.0 Æ 1.0
91.1 Æ 0.4
92.0 Æ 0.5
101 Æ 1.4
94.3 Æ 1.1
p-Me
160
p-OMe
p-F
p-Cl
p-Br
p-CF3
p-CO2Me
p-NO2
p-NHAc
p-CN
p-SMe
p-NMe2
m-Me
m-OMe
m-Cl
260
650
105
76.2
76.0
15.4
22.0
7.70
311
11.4
420
15.3
10.5
10.5
1.75
2.65
0.81
47.5
1.30
65.6
1000
16.4
13.1
3.00
3.10
3.14
0.57
2.40
1.64
0.90
11.8
9.38
5.46
19.7
0.31
0.70
4.17
1.55
1.27
1.00
0.43
6.70
4.62
0.16
1.77
5.75
41.0
27.8
28.0
5.27
7.85
2.67
110
4.09
160
2240
42.0
33.3
8.55
595
5300
115
88.0
23.7
24.2
24.5
6.38
5.10
1.03
1.04
1.06
0.17
0.81
0.53
0.29
4.30
3.57
1.92
7.60
0.081
0.20
1.40
0.50
0.36
0.30
0.12
2.37
1.60
0.038
0.65
5.91
m-Br
m-F
8.67
8.70
1.89
7.05
4.95
2.93
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
5.58
20.0
13.8
8.47
30.7
80.3
65.3
41.5
24.4
15.1
49.2
125
3.06
6.55
32.5
13.8
10.9
9.00
4.25
48.5
34.5
1.68
13.5
5.60
1.04
2.04
11.4
4.73
3.50
3.10
1.28
18.4
12.6
0.48
4.77
5.66
o-CF3
PhCDO
kH/kD
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 650–656

652
M. ANEJA, S. KOTHARI AND K. K. BANERJI
Table 3. Effect of solvent composition on the oxidationÀo3f
more polar than the reactants. The solvent effect was
analysed using the Grunwald–Winstein equation:11
benzaldehyde by PHPB with [PHPB] = 0.001 mol dm
[benzaldehyde] = 1.0 mol dmÀ3 and temperature = 318 K
,
log k2 ˆ log k0 ‡ mY
ꢀ4†
AcOH (%)
20
30
50
60
70
80
104 kobs (sÀ1
)
410
205
72.1 27.7 19.8 10.0
The plot of log k2 versus Y is linear (r2 = 0.9980) with
m = 0.81 Æ 0.02. The value of m suggests that there is a
considerable charge separation in the transition state of
the reaction.
It was observed that the rate increased with increase in
the amount of water in the solvent mixture (Table 3).
Correlation analysis of reactivity
Effect of pyridinium bromide
The reaction rates of the meta- and para-substituted
compounds were correlated in terms of the Hammett
equation12 but the correlation was not satisfactory [Eqn.
(5)]. We used the standard deviation (sd), the coefficient
of determination (r2/R2) and Exner’s13 parameter, , as
the measures of goodness of fit:
The rates of oxidation were not affected by an addition of
pyridinium bromide (Table 4).
DISCUSSION
A plot of logk2 at 288 K is linearly related to logk2 at
318 K (r2 = 0.9994, slope = 0.830 Æ 0.003). The value of
the isokinetic temperature is 646 Æ 15 K. A linear
isokinetic relationship is a necessary condition for the
validity of linear free energy relationships.10 It also
implies that all the aldehydes for which the rates of
oxidation are so correlated are oxidized by the same
mechanism.10
log k2 ˆ À2:22 Æ 0:13ꢀ À 2:61
r2 ˆ 0:9245; sd ˆ 0:22; n ˆ 24; ˆ 0:20
ꢀ5†
The data showed wide scatter. The rate constants of many
para-substituted compounds, capable of electron dona-
tion by resonance, are higher than those expected from
their Hammett s values. This indicates that in the
transition state of the reaction, there is an electron-
deficient centre, which is stabilized by cross-conjugation
with the electron-donating substituent at the para
position. Hence the rate constants of para-substituted
In solutions, PHPB may undergo the following
reactions.
‡
À
‡
PyH Br3 „ Br2 ‡ PyH BrÀ
ꢀ2†
ꢀ3†
compounds were correlated with Brown’s s values,14
but the correlation was not good:
‡
‡
À
À
‡
PyH Br3 „ Br3 ‡ PyH
The probable oxidizing species in a solution of PHPB
are, therefore, PHPB itself, tribromide ion and molecular
bromine. However, a strict first-order dependence on
PHPB and the absence of any effect of pyridinium
bromide rule out both bromine and tribromide ion as the
reactive oxidizing species. Hence PHPB itself is the
reactive oxidizing species in this reaction. Similar results
were obtained in the oxidation of aliphatic aldehydes.9
‡
log k2 ˆ À1:23 Æ 0:07ꢀ À 2:94
ꢀ6†
r2 ˆ 0:9624; sd ˆ 0:18; n ˆ 13; ˆ 0:14
In view of the failure of the correlation analyses in terms
of Hammett and Brown’s equations, the rates were
correlated in terms of the Yukawa–Tsuno15 equation.
Although the results are better, the correlation is still not
very good:
Solvent composition effect
‡
log k2 ˆ À1:38 Æ 0:24‰ꢀ0 ‡ 0:80ꢀꢀ À ꢀ0†Š À 2:87 ꢀ7†
r2 ˆ 0:9646; sd ˆ 0:19; n ˆ 12; ˆ 0:15
The increase in the rate of oxidation with increase in the
polarity of the medium suggests that the transition state is
The data for the p-NHAc compound were not included in
this correlation as the s0 value is not available.15
Since the correlations with single substituent par-
ameter equations and the Yukawa–Tsuno equation are
not very good, the rates were correlated in terms of
Charton’s16 LDR equation:
Table 4. Effect of pyridinium bromide on the rate of
oxidation of benzaldehyde by PHPB wiÀth3 [PHPB] =
0.001 mol dmÀ3, [benzaldehyde] = 1.0 mol dm and tem-
perature = 318 K
[PyHBr] (mol dmÀ3
104 kobs (sÀ1
)
0.0 0.01 0.02 0.04 0.08 0.12
72.1 73.5 71.5 72.0 71.2 74.0
)
log k2 ˆ Lꢀl ‡ Dꢀd ‡ Rꢀe ‡ h
ꢀ8†
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 650–656

REACTIVITY IN OXIDATION OF SUBSTITUTED BENZALDEHYDES
653
Table 5. Temperature dependence of the reaction constants for the oxidation of substituted benzaldehydes by PHPB
Temperature
Substitution
(K)
L
D
R
S
Z
R2
sd
PD
PS
Para
288
298
308
318
288
298
308
318
288
298
308
318
À1.46 Æ 0.02 À2.45 Æ 0.01 À3.42 Æ 0.06
À1.38 Æ 0.02 À2.24 Æ 0.02 À3.13 Æ 0.07
À1.27 Æ 0.03 À2.10 Æ 0.02 À3.08 Æ 0.08
À1.23 Æ 0.03 À2.04 Æ 0.02 À3.06 Æ 0.08
À1.89 Æ 0.01 À1.09 Æ 0.01 À1.49 Æ 0.05
À1.76 Æ 0.02 À1.02 Æ 0.02 À1.39 Æ 0.09
À1.63 Æ 0.02 À0.92 Æ 0.02 À1.34 Æ 0.09
À1.58 Æ 0.02 À0.88 Æ 0.02 À1.25 Æ 0.09
—
—
—
—
—
—
—
—
1.40 0.9998 0.013 0.11 62.7
1.40 0.9997 0.016 0.014 61.9
1.47 0.9995 0.019 0.018 62.3
1.50 0.9996 0.019 0.016 62.4
1.37 0.9998 0.010 0.012 36.6
1.36 0.9994 0.014 0.020 36.7
1.46 0.9990 0.013 0.022 36.1
1.42 0.9992 0.015 0.023 35.8
—
—
—
—
—
—
—
—
Meta
Ortho
À1.60 Æ 0.01 À1.97 Æ 0.01 À2.74 Æ 0.06 À1.31 Æ 0.01 1.39 0.9999 0.010 0.009 55.2 26.8
À1.46 Æ 0.01 À1.79 Æ 0.01 À2.51 Æ 0.06 À1.17 Æ 0.01 1.40 0.9998 0.010 0.015 55.1 26.5
À1.37 Æ 0.02 À1.71 Æ 0.02 À2.51 Æ 0.11 À1.14 Æ 0.02 1.47 0.9995 0.020 0.012 55.5 27.0
À1.32 Æ 0.01 À1.60 Æ 0.01 À2.23 Æ 0.07 À1.05 Æ 0.01 1.39 0.9998 0.010 0.015 54.8 26.4
where h is the intercept term, sl is a localized (field and/or
inductive) effect parameter, sd is the intrinsic delocalized
(resonance) electrical effect parameter when the active
site electronic demand is minimal and se represents the
sensitivity of the substituent to changes in electronic
demand by the active site. The last two substituent
parameters are related by the equation
positive value of Z adds a negative increment to sd,
increasing the electron-donating power of the substituent
and its capacity to stabilize a cationic species. The
negative value of S indicates that the reaction is subject to
steric hindrance by an ortho substituent.
To test the significance of localized, delocalized and
steric effects in the ortho-substituted benzaldehydes,
multiple linear regression analyses were carried out with
(i) sl, sd and se, (ii) sl, se and ꢂ, (iii) sd, se and ꢂ and (iv)
sl, sd and ꢂ. However, the correlations were not
significant, showing that all the four substituent constants
are significant:
ꢀD ˆ ꢁꢀe ‡ ꢀd
ꢀ9†
where Z represents the electronic demand of the reaction
site and is given by ꢁ = R/D, and sD represents the
delocalized electrical parameter of the diparametric LD
equation.
For ortho-substituted compounds, it is necessary to
account for the possibility of steric effects and Charton,
therefore, modified the LDR equation to generate the
LDRS equation:16
log k2 ˆ À1:70 Æ 0:43ꢀl À 1:72 Æ 0:34ꢀd
À 0:24 Æ 1:95ꢀe À 3:39
ꢀ11†
ꢀ12†
ꢀ13†
ꢀ14†
R2 ˆ 0:8359; sd ˆ 0:30; n ˆ 13; ˆ 0:33;
temperature 298 K
log k2 ˆ Lꢀl ‡ Dꢀd ‡ Rꢀe ‡ Sꢂ ‡ h
ꢀ10†
log k2 ˆ À1:73 Æ 0:76ꢀl À 1:58 Æ 3:65ꢀe
À 1:04 Æ 0:68ꢂ À 2:64
where ꢂ is the well-known Charton’s steric parameter
based on Van der Waals radii.17
R2 ˆ 0:4970; sd ˆ 0:52; n ˆ 13; ˆ 0:61;
The rates of oxidation of ortho-, meta- and para-
substituted benzaldehydes show excellent correlations in
terms of the LDR/LDRS equations (Table 5). All three
series of substituted benzaldehydes meet the requirement
of a minimum number of substituents for analysis by the
LDR and LDRS equations.16 The comparison of the L
and D values for the substituted benzaldehydes showed
that the oxidation of para- and ortho-substituted
benzaldehydes is more susceptible to the delocalization
effect than to the localized effect. However, the oxidation
of meta-substituted compounds exhibited a greater
dependence on the field effect. In all cases, the magnitude
of the reaction constants decreases with increase in
temperature, pointing to a decrease in selectivity with an
increase in temperature.
temperature 298 K
log k2 ˆ À1:92 Æ 0:38ꢀd À 2:98 Æ 2:32ꢀe
À 1:41 Æ 0:43ꢂ À 3:44
R2 ˆ 0:7980; sd ˆ 0:33; n ˆ 13; ˆ 0:37;
temperature 298 K
log k2 ˆ À1:49 Æ 0:17ꢀl À 1:76 Æ 0:14ꢀd
À 1:01 Æ 1:15ꢂ À 2:95
R2 ˆ 0:9737; sd ˆ 0:12; n ˆ 13; ˆ 0:13;
temperature 298 K
All three regression coefficients, L, D and R, are
negative, indicating an electron-deficient carbon centre in
the activated complex for the rate-determining step. The
Similarly, in the oxidation of para- and meta-
substituted benzaldehydes, multiple regression analyses
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 650–656

654
M. ANEJA, S. KOTHARI AND K. K. BANERJI
indicated that both localization and delocalization effects
are significant. There is no significant collinearity
between the various substituent constants for the three
series.
The percentage contribution16 of the delocalized
effect, PD, is given by the equation
Scheme 1
ꢀjDj  100†
PD ˆ
ꢀ15†
The correlation of vinyl cation formation, in terms of the
Yukawa–Tsuno equation,15 is reported to yield ꢃ ꢁ À4
and r ꢁ 1.1. In the present reaction the value of r is
À1.38 and that of r is 0.80 [cf. Eqn. (7)]. The significantly
low r value and resonance demand may be attributed to a
stronger electron donation from the carbonyl oxygen than
ꢀjLj ‡ jDj†
Similarly, the percentage contribution of the steric
parameter16 to the total effect of the substituent, PS,
was determined by using the equation
ꢀjSj  100†
from
=CH2. It is of interest to compare here the results
PS ˆ
ꢀ16†
obtained in earlier studies using Charton’s LDR/LDRS
equations. Charton16 reported that in the solvolysis of 4-
substituted cumyl chlorides, the magnitudes of L, D and
R are much larger (À5.02, À7.37 and À9.73, respec-
tively, at 298 K) than those obtained in the present study.
This may well be due to the dispersal of the positive
charge, on the carbon, by the adjacent carbonyl oxygen,
in addition to that by the phenyl group. However, the
value of Z, the electronic demand of the reaction site, is
comparable in the two reactions (1.3 and 1.4). We have
applied these equations to many oxidation reactions.18–22
The values of the reaction constants are given in Table
6. These reactions involve the formation of a cationic
species in the rate-determining step either by a hydride-
ion transfer from the reductant to the oxidant or by an
addition of halogen to the sulfide. The magnitudes of R
and Z in the oxidation of aromatic aldehydes by
benzyltrimethylammonium chlorobromate (BTMACB)
are lower than those observed in the present reaction.
This shows that in the oxidation by BTMACB,18 the
transition state is more reactant-like rather than product-
like. In the rest of the reactions, the polar reaction
constants have comparable values. The positive steric
constant in the oxidation of alcohols22 implies a steric
acceleration, whereas in other reactions a steric hindrance
by the ortho substituents is indicated. The above
comparison supports the proposed mechanism.
The abstraction of a hydride ion from an aldehydic C—
H bond has been proposed for several oxidizing species.
Formation of an acylium cation has been suggested in the
oxidation of benzaldehyde23 and acetaldehyde.24 Simi-
larly oxidation of aromatic aldehydes by BTMACB18 is
also proposed to involve a hydride-ion transfer in the
rate-determining step.
A mechanism involving the transfer of a hydride ion is
supported by the observed effect of solvent composition.
The observed negative value of the entropy of activation
also supports the proposed mechanism. As PHPB and the
aldehyde come together in the transition state to form a
single activated complex, their freedom to move
separately is curtailed. This is reflected in a loss of
entropy.
ꢀjLj ‡ jDj ‡ jSj†
ThevaluesofPDandPSaregiveninTable5.Thevalueof
PD for the oxidation of para-substituted benzaldehydes is
ca62%whereasthecorrespondingvaluesforthemeta-and
ortho-substituted aldehydes are ca 36 and 55%, respec-
tively. This shows that the balance of localization and
delocalizationeffectsisdifferentfordifferentlysubstituted
benzaldehydes.Thelesspronouncedresonanceeffectfrom
theorthopositionthanfromtheparapositionmaybedueto
the twisting away of the aldehydic group from the plane of
thebenzene ring. The magnitude ofthe PS value showsthat
the steric effect is significant in this reaction.
Mechanism
The cleavage of the aldehydic C—H bond in the rate-
determining step is confirmed by the presence of a
substantial kinetic isotope effect. A one-electron oxida-
tion, giving rise to free radicals, is unlikely in view of the
failure to induce polymerization of acrylonitrile and the
zero effect of the radical scavenger on the reaction rate.
The negative values of the localization and delocalization
electrical effects, i.e. of L, D and R, point to an electron-
deficient reaction centre in the transition state of the rate-
determining step. This is further supported by the positive
value of Z, which indicates that the substituent is better
able to stabilize a cationic or electron-deficient reactive
site. Therefore, a hydride-ion transfer in the rate-
determining step is suggested (Scheme 1). The large
negative values of L, D and R indicate that the electron
demand of the reaction on the substituents is very high.
This, coupled with large deuterium isotope effect, points
to a considerable carbocationic character in the transition
state. Hence the rate-determining step can be visualized
as a hydride-ion transfer involving a late product-like
transition state. The structure of the transition state
should therefore be close to a linear acylium cation,
‡
Ar À C O. The energy profile of this reaction should
resemble that of solvolytic formation of a vinyl cation.
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 650–656

REACTIVITY IN OXIDATION OF SUBSTITUTED BENZALDEHYDES
655
Table 6. Reaction constants of the oxidation reactions in terms of Charton's LDR/LDRS equations at 298 K
Substitution
Oxidant/reductanta
L
D
R
Z
S
Ref.
Para
BTMACB/ArCHO
PHPB/ArSMe
BTMAB/ArSMe
HABR/ArSMe
BTMACI/ArCH2OH
PHPB/ArCHO
BTMACB/ArCHO
PHPB/ArSMe
BTMAB/ArSMe
HABR/ArSMe
BTMACI/ArCH2OH
PHPB/ArCHO
BTMACB/ArCHO
PHPB/ArSMe
BTMAB/ArSMe
HABR/ArSMe
À1.59
À1.43
À1.40
À1.41
À1.59
À1.38
À1.64
À1.72
À1.68
À1.72
À1.89
À1.76
À1.61
À1.46
À1.42
À1.47
À1.87
À1.46
À1.75
À2.11
À2.09
À2.09
À2.15
À2.24
À1.08
À0.99
À1.01
À1.05
À1.04
À1.04
À1.53
À1.66
À1.72
À1.71
À1.69
À1.79
À1.31
À2.89
À2.85
À3.01
À3.10
À3.13
À0.72
À0.95
À1.03
À1.29
À1.46
À1.39
À1.10
À2.25
À2.10
À2.67
À2.53
À2.51
0.75
1.37
1.36
1.44
1.44
1.40
0.67
0.96
1.02
1.23
1.40
1.36
0.79
1.36
1.22
1.56
1.50
1.40
—
—
18
19
20
21
22
—
—
—
—
This work
Meta
—
18
—
19
—
20
—
21
—
22
—
This work
Ortho
À1.03
À1.13
À1.15
À1.14
1.23
À1.17
18
19
20
21
22
BTMACI/ArCH2OH
PHPB/ArCHO
This work
a
BTMACB = benzyltrimethylammonium chlorobromate; BTMAB = benzyltrimethylammonium tribromide; HABR = hexamethy-
lenetetramine-bromine; BTMACI = benzyltrimethylammonium dichloroiodate.
Although the contribution of the steric term is
significant, its interpretation is not straightforward. The
structure of the transition state where the aryl ring is
perpendicular to the elongated C—H bond should lead to
a lesser degree of steric interaction between the ortho
substituent and the reaction centre. Perhaps the observed
steric hindrance by the ortho substituent is due to the
hindrance to the approach of the oxidizing species by the
ortho substituent.
degree of charge separation is much greater in the
oxidation of the aromatic aldehydes than it is in the
oxidation of aliphatic aldehydes. This is also consistent
with the proposal that the oxidation of aromatic
aldehydes involves a direct hydride-ion transfer whereas
that of aliphatic aldehydes proceeds via an intermediate
complex.9
It is of interest to compare here the results of the
oxidation of aliphatic aldehydes9 by PHPB with those of
the aromatic aldehydes. Aliphatic aldehydes exhibited
Michaelis–Menten kinetics with respect to the reductant
whereas the aromatic aldehydes exhibited a first-order
dependence on the reductant. This difference may well be
due to the fact that the aliphatic aldehydes are extensively
hydrated in aqueous or mixed aqueous solvents25 to yield
a gem-diol. Aromatic aldehydes are not known to
undergo hydration to any significant extent.25 The gem-
diols are likely to behave more like alcohols and the
oxidation of alcohols by PHPB is known to exhibit
Michaelis–Menten kinetics.26 The magnitude9 of the
kinetic isotope effect is much less in the oxidation of
acetaldehyde (kH/kD = 3.23 at 298 K) than that obtained
in the present reaction. This is consistent with a non-
linear transition state implied in the rate-determining
disproportionation of an intermediate complex. The
higher magnitude of the kinetic isotope effect in the
oxidation of benzaldehyde indicates a bimolecular
reaction via a linear transition state. Both the reactions
showed a linear relationship in terms of the Grunwald–
Winstein equation. However, the value of m is relatively
small (0.47) in the oxidation of acetaldehyde compared
with that in benzaldehyde (0.81). This indicates that the
EXPERIMENTAL
Materials. The aldehydes were commercial products. The
liquid aldehydes were purified through their hydrogen-
sulfite addition compounds and distilling them, under
nitrogen, just before use.27 The solid aldehydes were
recrystallized from ethanol. PHPB was prepared by the
reported method.1 Its purity was checked by an
iodometric method. [2H]Benzaldehyde (PhCDO) was
prepared by a reported method.28 Acetic acid was
refluxed with chromic oxide and acetic anhydride for
6 h and fractionated.
Product analysis. The product analysis was carried out
under kinetic conditions. In a typical experiment, freshly
distilled benzaldehyde (5.30 g, 0.05 mol) and PHPB
(3.20 g, 0.01 mol) were diluted to 100 ml in 1:1 (v/v)
acetic acid–water. The reaction mixture was allowed to
stand for ca 10 h to ensure completion of the reaction. It
was rendered alkaline with NaOH, filtered and the filtrate
evaporated to dryness under reduced pressure. The
residue was dissolved in the minimum quantity of
concentrated HCl and cooled in crushed ice to yield
crude acid (1.13 g), which was recrystallized from hot
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 650–656

656
M. ANEJA, S. KOTHARI AND K. K. BANERJI
2. Djerassi C, Schloz CR. J. Am. Chem. Soc. 1948; 70: 417; Fieser
LF. J. Chem. Educ. 1954; 31: 291.
3. Perelman M, Farkas E, Fornefeld EJ, Kraay RJ, Rapala EJ. J. Am.
Chem. Soc. 1960; 82: 2402.
water to produce pure benzoic acid (1.08 g, 87%, m.p.
120°C).
Stoichiometry. To determine the stoichiometry,
PHPB (1.60 g, 0.005 mol) and benzaldehyde (0.106 g,
0.001 mol) were diluted to 100 ml in 1:1 (v/v) acetic
acid–water. The reaction mixture was allowed to stand
for ca 10 h to ensure the completion of the reaction. The
residual PHPB was determined spectrophotometrically at
358 nm. Several determinations, with differently sub-
stituted benzaldehydes, showed that the stoichiometry is
1:1.
4. Goel S, Kothari S, Banerji KK. J. Chem. Res. (S) 1996; 230: (M)
1318; J. Chem. Res. (S) 510: (M) 2901.
5. Suri D, Kothari S, Banerji KK. J. Chem. Res. (S) 1996; 228: (M)
1301; J. Chem. Res. (S) 1995; 274: (M) 1734.
6. Anjana, Kothari S, Banerji KK. J. Chem. Res. (S) 1999; 476: (M)
2118.
7. Rao PSC, Goel S, Kothari S, Banerji KK. Indian J. Chem. 1998;
37B: 1129.
8. Goswami G, Kothari S, Banerji KK. J. Chem. Res. (S) 1999; 176:
(M) 813.
9. Devi J, Kothari S, Banerji KK. J. Chem. Res. (S) 1993; 400: (M)
2680.
Kineticmeasurements. The reactions were carried out
under pseudo-first-order conditions by maintaining a
large excess of the aldehyde (Â15 or more) over PHPB.
The solvent was 1:1 (v/v) acetic acid–water, unless
mentioned otherwise. The reactions were carried out at a
constant temperature (Æ0.1 K) and were followed up to
80% reaction by monitoring the decrease in [PHPB] at
358 nm. The pseudo-first-order rate constants, kobs, were
computed from the linear (r2 > 0.995) least-squares plots
of log[PHPB] versus time. Duplicate kinetic runs showed
that the rate constants were reproducible to within Æ4%.
10. Exner O. Collect. Czech. Chem. Commun. 1964; 29: 1094.
11. Fainberg AH, Winstein S. J. Am. Chem. Soc. 1956; 78: 2770.
12. Johnson CD. The Hammett equation. Cambridge: Cambridge
University Press, 1973; 78.
13. Exner O. Collect. Czech. Chem. Commun. 1966; 31: 3222.
14. Brown HC, Okamoto Y. J. Am. Chem. Soc. 1958; 80: 4979.
15. Tsuno Y, Fujio M. Adv. Phys. Org. Chem. 1999; 32: 267.
16. Charton M. Prog. Phys. Org. Chem. 1986; 18: 287.
17. Charton M. J. Org. Chem. 1975; 40: 407.
18. Raju VS, Sharma PK, Banerji KK. J. Org. Chem. 2000; 65: 3322.
19. Vyas VK, Jalani N, Kothari S, Banerji KK. J. Chem. Res. (S) 1996;
370: (M) 2201.
20. Goel S, Varshney S, Kothari S, Banerji KK. J. Chem. Res. (S)
1996; 510: (M) 2901.
21. Choudhary K, Suri D, Kothari S, Banerji KK. J. Phys. Org. Chem.
2000; 13: 283.
Acknowledgements
22. Rao PSC, Suri D, Kothari S, Banerji KK. J. Chem. Res. (S) 1998;
510: (M) 2251.
23. Pearlmutter-Hayman P, Weissmann Y. J. Am. Chem. Soc. 1962;
Thanks are due to the Council of Scientific and Industrial
Research (India) for financial support.
84: 2323.
24. Kaplan L. J. Am. Chem. Soc. 1958; 80: 2639.
25. Bell RP. Prog. Phys. Org. Chem. 1964; 1: 10.
26. Mathur D, Sharma PK. Banerji KK. J. Chem. Soc., Perkin Trans. 2
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Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 650–656