Oxidation of aromatic anils by sodium perborate in aqueous acetic Acid Medium

Article abstract of DOI:10.14233/ajchem.2014.15506

The kinetics of oxidation of 9 meta- and 15 para- substituted aromatic anils by sodium perborate were investigated in aqueous acetic acid medium. The reaction was second order with respect to aromatic anil and first order with respect to the sodium perborate. The increase of [H₊] in this oxidation retards the rate of the reaction. The observed rate constant for the substituents were plotted against the Hammett constant, δ and a non-linear concave downward curve was obtained for the anils with substituents in the aniline moiety. The observed break in the log k_obs versus δwas attributed to the transition state whereas the non-linear concave upward curve was observed for the substituents in the benzaldehyde moiety and a non-linear concave upward curve was observed for the substituents in the combination of aniline and benzaldehyde moiety. The electron withdrawing substituents fall on one side of the curve, having a negative ρvalue and the electron releasing substituents fall on the other side, with a positive ρvalue and a suitable mechanism was proposed.

Full text of DOI:10.14233/ajchem.2014.15506

Asian Journal of Chemistry; Vol. 26, No. 3 (2014), 739-744
http://dx.doi.org/10.14233/ajchem.2014.15506
Oxidation of Aromatic Anils by Sodium Perborate in Aqueous Acetic Acid Medium
*
R. VENKATESH and K. KARUNAKARAN
Department of Chemistry, Sona College of Technology (Autonomous), Salem-636 005, India
*Corresponding author: E-mail: karunachemist@gmail.com
Received: 19 March 2013;
Accepted: 27 July 2013;
Published online: 30 January 2014;
AJC-14626
The kinetics of oxidation of 9 meta- and 15 para- substituted aromatic anils by sodium perborate were investigated in aqueous acetic acid
medium. The reaction was second order with respect to aromatic anil and first order with respect to the sodium perborate. The increase of
[H+] in this oxidation retards the rate of the reaction. The observed rate constant for the substituents were plotted against the Hammett
constant, σ and a non-linear concave downward curve was obtained for the anils with substituents in the aniline moiety. The observed
break in the log kobs versus σ was attributed to the transition state whereas the non-linear concave upward curve was observed for the
substituents in the benzaldehyde moiety and a non-linear concave upward curve was observed for the substituents in the combination of
aniline and benzaldehyde moiety. The electron withdrawing substituents fall on one side of the curve, having a negative ρ value and the
electron releasing substituents fall on the other side, with a positive ρ value and a suitable mechanism was proposed.
Keywords: Aromatic anils, Kinetics, Oxidation, Sodium perborate.
purity of the anils was checked by determining their melting
points and FT-IR spectrum. All other chemicals used were of
analytical grade (Merck, India). Acetic acid was purified by
redistillation. All the reagents were prepared just before the
reactions were carried out. A solution of sodium perborate in
acetic acid was prepared fresh and standardized iodometrically.
All the reactions were carried out in a thermostat and the
temperature was controlled to 0.1 ºC. The reactions were
performed under pseudo-first order conditions by maintain
excess of anil over oxidant. The mixture was homogeneous
throughout the course of the reaction. The progress of the
reactions was followed by estimating the unreacted oxidant
iodometrically at regular time intervals. The rate constants (kobs)
were from log (titre) versus time plots. All the rate constants
were averages of two or more determinations.
INTRODUCTION
Sodium perborate (SPB, NaBO3·4H2O), a peroxo salt of
anionic formula B2(O)2(OH)42- , is an inexpensive, innocuous,
industrial chemical, extensively used in detergents as a bleaching
agent1. In aqueous solution it yields hydrogen peroxide and
kinetic studies on sodium perborate oxidation in aqueous and
partly aqueous media confirm the same2-7. In glacial acetic
acid it is a highly effective reagent, the oxidant of choice8-10
and oxidizes anilines to azobenzenes, used in the manufacture
of azo dyes, in good yield11,12. Sodium perborate is a low cost
and widely used industrial chemical. However, its utilization
in organic synthesis has not received much attention. There
have only been a few reported reactions using it as an oxidant,
such as the cleavage of 1,2-diketones13, the oxidation of
alkenylboronic acids14, the oxidations of anilines to azo comp-
Stoichiometry and product analysis: Under kinetic condi-
tions, stoichiometric amounts of the substrate and sodium
perborate were mixed. The reaction mixture ethanol was
extracted. The dark brown extract, when subjected to TLC,
gave two distinct spots. On evaporation of ethanol the products
were found to be benzaldehyde which was confirmed by the
isolation of its 2,4-dinitrophenylhydrazone derivative and
azobenzene identified by its m.p. and UV-visible spectrum.
Formation of the azobenzene was also confirmed by UV-visible
spectra [λmax 429 nm] of the reaction solutions during and after
the completion of the reaction. This was in agreement with
the literature value19. The λmax of azobenzene was confirmed
ounds or nitroarenes and sulfides to sulfoxides or sulfones15,16
.
The epoxidation by use of sodium perborate was also reported,
but it was limited only to quinones17. Herein, we would like to
report the oxidations of aromatic anils using sodium perborate.
EXPERIMENTAL
Anils were prepared18 by refluxing equimolar quantities
of benzaldehyde and aniline (Sigma Aldrich) in ethanol for
ca. 2-3 h. The resulting solution was cooled and poured into
cold water. The precipitated anil was filtered off, washed with
ethanol and dried. It was recrystallized from ethanol. The

740 Venkatesh et al.
Asian J. Chem.
by comparing with that of the authentic sample in aqueous
acetic acid medium.
TABLE-1
EFFECT OF VARYING THE CONCENTRATION
OF ANIL, SPB AND H+ ON THE RATE AT 308 K
102 [Anil]
(mol dm-3)
8.0
103 [SPB]
(mol dm-3)
5.0
[H+]
(mol dm-3)
1.0
HOAc
(%)
50
104 kobs
RESULTS AND DISCUSSION
(s-1)
The reaction followed second order kinetics with respect
to the concentration of anil and is the first order with respect
to the concentration of sodium perborate, respectively.
Under the identical experimental conditions [Anil] >>
[SPB], the pseudo-first order linear plot of log (titre) versus
time is linear up to 80 %, depicted in Fig. 1. The order in
[anil] was two, as revealed by the slope of the plot of log kobs
against log [Anil] (Fig. 2), with correlation coefficient (r =
0.992). The kinetic order in acidity appears to be unity, but
increase in [H+], the rate of the reaction was decreased.
1.21
1.57
2.07
2.46
2.71
1.56
1.84
2.07
1.78
1.56
2.68
2.18
2.07
1.51
1.33
9.0
5.0
1.0
50
10.0
5.0
1.0
50
11.0
5.0
1.0
50
12.0
5.0
1.0
50
10.0
3.0
1.0
50
10.0
4.0
1.0
50
10.0
5.0
1.0
50
10.0
6.0
1.0
50
10.0
7.0
1.0
50
10.0
5.0
0.8
50
10.0
5.0
0.9
50
10.0
5.0
1.0
50
10.0
5.0
1.1
50
1.2
1.1
1.0
0.9
0.8
0.7
0.6
10.0
5.0
1.2
50
Table-2. Temperature dependence studies were conducted for
all the substituted anils (substitution in X andY ring separately)
between 298 and 318 K. The rate constnts for the reaction
systems and various thermodynamic parameters ∆H#, ∆S#, ∆G#
and Ea were found and tabulated (Table-3). When an attempt
is made to fit in the rate data of meta- and para- substituted
anils (substitution in aniline moiety and substitution in benzal-
dehyde moiety separately and substitution in both aniline
moiety and benzaldehyde moiety) into the Hammett equation,
a non-linear concave downward curve was obtained for the
anils with substituents in the aniline moiety, whereas the non-
linear concave upward curve was observed for the substituents
in benzaldehyde moiety and a non-linear concave upward
curve was observed for the substituents in the combination of
aniline and benzaldehyde moiety.
0
500
1000
1500
2000
2500
Time(sec)
Fig. 1. Pseudo-first order plot for sodium perborate oxidation of aromatic
anil at 308 K
TABLE-2
EFFECT OF [Mn(II)], [ACRYLONITRILE] AND
[Na2SO4] ON THE REACTION RATE AT 308 K
Slope = 2.04
-3.55
r = 0.99176
-3.60
[Mn(II)]
(mol dm-3)
[Acrylonitrile]
(mol dm-3)
[Na2SO4]
(mol dm-3)
104 kobs
(s-1)
-3.65
-3.70
-3.75
-3.80
-3.85
-3.90
0
0.02
0.04
0.06
0.08
–
–
–
–
–
2.07
1.31
1.04
1.19
1.17
2.07
1.95
1.63
1.72
1.57
2.07
1.98
1.97
1.73
1.64
–
–
–
–
–
–
0
–
–
0.02
0.04
0.06
0.08
–
–
–
–
–
–
–
–
–
0
-3.95
-1.12 -1.10 -1.08 -1.06 -1.04 -1.02 -1.00 -0.98 -0.96 -0.94 -0.92 -0.90
–
–
0.02
0.04
0.06
0.08
–
–
log [Anil]
–
–
Fig. 2. Plot of log kobs versus log [Anil] at 308 K
–
–
[Anil] = 0.08 mol dm-3; [SPB] = 0.005 mol dm-3 ; [H+] = 1.0 mol dm-3;
HOAc-H2O = 50 % (v/v); temperature = 308 K.
The kinetic results are summarized in Table-1. The ionic
strength of the reaction varies by the addition of Na2SO4 and
its influence on reaction rate was studied and has been found
that it has no significant effect on the reactivity. No polymeri-
zation with acrylonitrile was observed. Addition of Mn(II)
decreases the rate of the reaction and the rate data are listed in
Effect of dielectric constant: The effect of solvent on
the reaction rate was studied by varying the composition of
acetic acid from 30-70 % (Table-4). The oxidations were
carried out by maintaining the concentration of anils, sodium

Vol. 26, No. 3 (2014)
Oxidation of Aromatic Anils by Sodium Perborate in Aqueous Acetic Acid Medium 741
TABLE-3
TEMPERATURE EFFECT AND THERMODYNAMIC PARAMETERS OF OXIDATION OF ANILS BY SPB IN ACETIC ACID
104 kobs (s-1)
∆H#
(kJ
mol-1)
-∆S# (J
K-1
mol-1)
∆G#
(kJ
mol-1)
Ea (kJ
Moiety
S. No. Substituents
mol-1
K-1)
r
SD
25 ºC
30 ºC
35 ºC
40 ºC
45 ºC
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
H
1.41
1.30
0.48
0.59
0.27
1.49
1.23
2.22
0.36
1.41
1.57
1.94
1.52
2.29
1.92
1.37
2.71
2.43
1.41
1.34
1.63
1.46
2.29
1.23
1.34
1.90
1.47
1.72
1.60
0.71
0.93
0.61
1.92
1.59
2.70
0.77
1.72
1.76
2.27
2.28
2.61
2.45
1.78
3.05
2.85
1.72
1.69
2.78
1.77
2.61
1.57
1.63
2.55
1.93
2.07
1.97
1.08
1.29
0.83
2.48
1.75
3.42
1.12
2.07
2.21
3.12
2.74
3.18
2.83
2.06
3.97
3.35
2.07
1.89
3.26
2.91
3.08
2.96
1.95
3.86
3.24
3.41
2.18
2.12
1.81
1.37
2.87
2.21
4.26
2.16
3.41
2.65
4.21
3.41
3.57
3.35
2.65
4.14
5.11
3.41
2.03
4.10
3.56
3.57
3.45
2.27
4.08
3.59
5.11
2.82
2.46
2.04
1.67
3.35
2.98
5.13
2.50
5.11
3.17
5.31
4.55
4.28
3.75
3.29
4.98
5.62
5.11
2.34
4.48
3.87
4.28
3.77
2.86
4.66
4.58
48.49 156.93
26.63 230.03
65.72 107.29
47.12 167.24
67.69 103.95
29.40 219.39
30.35 218.13
30.91 211.33
96.82
97.48
98.77
98.63
99.71
96.97
97.53
95.99
98.85
96.82
97.13
96.18
96.58
95.29
96.52
96.89
95.80
95.80
96.82
97.59
96.26
96.75
96.24
96.92
97.43
96.13
96.56
50.87
29.01
68.10
49.5
0.973
0.993
0.987
0.984
0.977
0.992
0.984
0.999
0.982
0.973
0.990
0.993
0.991
0.944
0.991
0.997
0.979
0.974
0.973
0.977
0.951
0.969
0.997
0.954
0.997
0.957
0.978
0.13
0.04
0.13
0.10
0.17
0.04
0.06
0.01
0.16
0.13
0.04
0.05
0.06
0.06
0.04
0.03
0.05
0.09
0.13
0.04
0.13
0.12
0.02
0.16
0.02
0.12
0.11
m-CH3
p-CH3
p-OC2H5
p-OCH3
p-Cl
Aniline
70.07
31.78
32.73
33.29
77.24
50.87
28.73
41.13
40.71
18.18
25.90
33.67
23.80
35.33
50.87
20.38
37.90
41.63
24.33
47.58
28.77
35.69
45.41
m-Cl
m-NO2
p-NO2
H
74.86
77.88
48.49 156.93
26.35 229.81
38.75 186.49
38.33 189.11
15.80 258.08
23.52 237.02
31.29 213.99
21.42 241.48
32.95 204.06
48.49 156.93
18.00 258.41
35.52 197.20
39.25 186.69
21.95 241.22
45.20 167.92
26.39 230.64
33.31 203.96
43.03 173.80
m-CH3
p-CH3
p-OC2H5
p-OCH3
p-Cl
Benzaldehyde
m-Cl
m-NO2
p-NO2
H
m-CH3
p-CH3
p-OC2H5
p-OCH3
p-Cl
Both in
aniline and
benzaldehyde
m-Cl
m-NO2
p-NO2
[Anil] = 0.1 mol dm-3, [SPB] = 0.005 mol dm-3, [H+] = 1.0 mol dm-3, HOAc-H2O = 50 % (v/v).
TABLE-4
In crystalline state sodium perborate exists as a dimer with
anionic formula B2(O2)2(OH)42-. But in aqueous solution it
affords H2O2. Although perboric acid is reported to exist in
equilibrium with H2O2. Based on the stoichiometry of the
reaction and above experimental results the following reactions
(Scheme-I) are believed to constitute the most possible mecha-
nism of the reaction.
PSEUDO-FIRST ORDER RATE CONSTANTS
FOR THE OXIDATION OF ANILS BY SPB IN
DIFFERENT ACETIC ACID COMPOSITIONS
Acetic acid (%)
Dielectric constant (D)
104 kobs (s-1)
30
40
50
60
70
56.83
49.60
42.37
35.14
27.90
2.84
2.59
2.07
1.80
1.37
K1
+ (O)
H2O
H2O2
[Anil] = 0.1 mol dm-3, [SPB] = 0.005 mol dm-3, [H+] = 1.0 mol dm-3,
k2
temp. = 308 K.
H2O2
[Complex]
C H CH=
6
+
NC6H5
5
slow
perborate and temperature constant. The above observation
clearly reveals that the rate decreases with increase in percen-
tage of acetic acid, i.e., with decreasing dielectric constant
(D) or polarity of the medium. This directs to the conclusion
that there is a charge development in the intermediate state
involving a more polar intermediate than reactants, suggesting
a polar (ionic) mechanism.
+
C6H5 NH +
C6H5CHO
H O
2
[Complex]
2C6H5+NH
+
dimerises
C6H5N
NC6H5 + 2H+
=
Scheme-I: Probable mechanism for the oxidation of aromatic anil by
sodium perborate
The formation of complex is in agreement with the litera-
ture study21,22 and the intermediate formed is experimentally
confirmed at 790 nm. Decomposition of complex is a slow
reaction and is likely to be the rate determining step. Simulta-
neously, at 429 nm, a peak appears and is stable till the end of
the reaction, which is the azobenzene product. Based on the
above experimental observations, a probable mechanism,
shown in Scheme-I is suggested.
Mechanism: The addition of acrylonitrile monomer
showed no variation in the reaction rate and no formation of
turbidity in the oxidation of aromatic anils. This rules out the
presence of free radical mechanism in this oxidation reaction.
So the ionic reaction was preferred in this oxidation study.
Generally, the enhancement of the electrophillic activity of
peroxide will minimize the importance of undesirable free
radical pathways, resulting in a mixture of products20. The in-
vestigation results illustrate a fascinating note that increase of
H+ retards the rate of the reaction.
The above mechanism leads to the following rate law:
d[SPB]
Rate =
= K1k2[Anil][SPB]
dt

742 Venkatesh et al.
Asian J. Chem.
This rate law satisfactory explains all the experimental
results.
In an isoentropic oxidation, the isokinetic temperature lies
at infinite and only enthalpy of activation determines the
reactivity. The isokinetic temperature is zero for an isoenthal-
phic series and the reactivity is determined by the entropy of
activation25.
Effect of substituents on the reaction rate: The kinetics
and oxidation of anil and m-CH3, p-CH3, p-OC2H5, p-OCH3,
p-Cl, m-Cl, m-NO2 , p-NO2 anils were conducted under pseudo-
first order conditions at five different temperatures 298, 303,
308, 313 and 318 K to determine various thermodynamics
parameters. The analysis of the data in the Table-3 indicates
that the oxidation is neither isoenthalphic nor isoentropic but
confirms with isokinetic relationship of compensation law.
Since the reactions are of ion-polar, it is expected that the
entropy of the activated complex for all the anils should be
nearly the same order of magnitude. The effect of substituents
on the rate was studied by varying the substituents H, m-CH3,
p-CH3, p-OC2H5, p-OCH3, p-Cl, m-Cl, m-NO2 and p-NO2 in
one of the rings benzaldehyde (or) aniline.
∆H# = ∆Hº + β∆S#
β is the isokinetic temperature and β was found to be 328 K
which is greater than the experimental temperature.
The values of free energy of activation of the reactions
were found to be more or less similar. This is due to the fact
that at isokinetic temperature the change of substituent has no
influence on the free energy of activation. If the isokinetic
temperature is infinite in isoentropic oxidation reactions, only
the enthalpy of activation finalizes the reactivity and if the
isokinetic temperature is zero in isoenthalpic oxidation reac-
tions, only the entropy of activation finalizes the reactivity26.
The values of the negative and positive ρ values, ρ+ and ρ–,
respectively, at different temperature are given in Table-5. The
unsubstituted anil is the most reactive in this series. Since the
electron releasing substituents also retard the rate of the reac-
tion, the rate-determining step proceeds with the development
of negative charge on the nitrogen atom of anil. It is to be
noted that this kind of transition state has been suggested in
the oxidation of anil by INDC18. Ramalingam and Jayanthi18
have established the order dependence with respect to the
reactants and other kinetic parameters of anils (substitution
only in aniline moiety). The formation of oxalatochromate
species is reported as the intermediate of the reaction since
oxalic acid is used as catalyst.
Deviation from the hammett relationship Hammett
plot for aniline moiety: Hammett made a unique discovery23
about the linear free energy relationships for the side chain.
Application of the Hammett equation with the usual substituent
constant σ to the log kobs data of the meta- and para-substi-
tuted aromatic anils resulted in a non-linear concave down-
ward curve (Fig. 3). Similar types of non-linear Hammett plots
were observed previously in some reaction kinetics24. The
non-linear concave downward curve was obtained for the anils
with substituents in aniline moiety. The electron-releasing
substituents fall on the one side of the curve with a positive
slope and the electron-withdrawing substituents on the other
side with a negative slope. The isokinetic plot and Exner plot
reveal that there is no change in reaction mechanism with
respect to substituents in aniline moiety. para- and meta-Substi-
tuted anils with substituents in aniline moiety in aqueous
acetic acid at 298-318 K confirm the Exner relationship, also
the activation parameters to the isokinetic relationship but not
to any of the linear free energy relationships.
TABLE-5
REACTION CONSTANT FOR THE SPB
OXIDATION OF AROMATIC ANILS
ρ+
ρ–
Substitutents
Temperature (K)
298
303
308
313
318
298
303
308
313
318
298
303
308
313
318
4.14
4.03
4.32
5.95
4.42
1.52
1.53
2.04
1.72
1.14
0.47
0.81
1.83
0.71
0.08
0.34
0.20
0.02
0.10
0.01
2.06
3.10
3.80
1.78
0.18
2.20
2.44
4.42
2.96
0.06
Aniline moiety
3.5
m-NO
2
3.0
2.5
Benzaldehyde
moiety
p-Cl
H
Combination of
aniline and
benzaldehyde
moiety
2.0
m-CH
m-Cl
3
1.5
1.0
0.5
p-OC H
2
5
p-CH
3
Hammett plot for benzaldehyde moiety: The non-linear
concave upward curve was obtained for the anils with substi-
tuents in benzaldehyde moiety (Fig. 4). The electron-releasing
substituents fall on the one side of the curve with a negative
slope and the electron-withdrawing substituents on the other
side with a positive slope. The isokinetic plot and Exner plot
reveals that there is no change in the reaction mechanism with
respect to the substituents in benzaldehyde moiety. The values
of ρ+ and ρ– at five different temperatures are given in Table-5.
The unsubstituted anil is also the less reactive in this series. But
both the electron-releasing and withdrawing substituents
p-NO
2
p-OCH
3
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
σ
Fig. 3. Hammett plot for oxidation of aromatic anils by sodium perborate
(substitution in aniline moiety) at 308 K
Isokinetic temperature is the temperature at which all the
compounds react at same speed since this temperature variation
of substituent has no influence on the free energy of activation.

Vol. 26, No. 3 (2014)
Oxidation of Aromatic Anils by Sodium Perborate in Aqueous Acetic Acid Medium 743
5.5
4.0
m-NO
2
5.0
4.5
3.5
p-NO
2
p-OCH
3
p-CH
4.0
3.5
3.0
2.5
2.0
3
3.0
2.5
2.0
p-Cl
p-OC H
2
5
m-CH
3
m-Cl
H
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
σ
σ
Fig. 6. Hammet plot for the oxidation of anilis by sodium perborate
Fig. 4. Hammett plot for oxidation of aromatic anils by sodium perborate
(substitution in benzaldehyde moiety) at 308 K
temperatures studied. The correlation coefficient (r) of the
dis- persed gram is found to be 0.155 with standard deviation
of 0.947.
Literature study28 also reveals that correlation obtained
with it are poor as the substituents are conjugated with the
reaction centre, where the problem arose with anilinium ions,
where a lone pair of electrons on the NH2 group can be deloca-
lized into substituents such as p-NO2.
accelarate the rate of the reaction considerably dissimilar to
aniline moiety. This shows the opposite kind of behaviour
compared with aniline moiety.
Hammett plot for combination of aniline and benzal-
dehyde moiety: The non-linear concave upward curve was
obtained for the anils with substituents in combination of
aniline and benzaldehyde moiety (Fig. 5) and the electron-
releasing substituents fall on the one side of the curve with a
negative slope and the electron-withdrawing substituents on
the other side with a positive slope like in substituents only in
benzaldehyde moiety. The values of ρ+ and ρ– at five different
temperatures are given in Table-5.
The correlation of oxidation rates of meta- and para-
subnstituted anils are correlated separately with any of the
+
unit parameters (meta: σ, σm ; para: σ, σp+, σp–) was also found
to be ineffective. The oxidation rates measured at 298, 303,
308, 313 and 318 K were analyzed in terms of dual substituent
–
parameter equations (meta: σI, σR; σI, σR ; σI, σR ; F, R; para:
+
–
+
σI, σR ; σI, σR ; σI, σR ; F, R) was also found to be unsuccessful.
The possible reason for the lack of free energy relation-
ship is the isokinetic temperature 328 K which is greater than
the experimental temperature. The same type of results was
also reported by Elango and Buvaneswari29. Operation of
inductive and resonance effect opposing each other is known
but the present study has no parallel30. The less pronounced
substituent can be explained only by the compensation effect.
The influence of the substituents on the reactivity of the
nucleophile is approximately compensated by the influence
of the same substituent on the reactivity of the electrophile31.
In some of the substituted anil exact compensation is doubtful
and the resultant effect is experienced on the oxidation rate.
4.0
m-NO
2
3.5
p-NO
p-CH
2
3
p-OCH
3
3.0
2.5
2.0
1.5
p-Cl
p-OC H
2
5
H
m-Cl
m-CH
3
Conclusion
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
The reaction followed second order kinetics with respect
to the concentration of [anil] and was the first order with respect
to sodium perborate, respectively. Under the experimental
conditions, aromatic anil is oxidized to benzaldehyde and
azobenzene. The addition of H+ retards the rate of the reaction
shows that the protonated anil was less reactive in this oxidation
reaction. The correlations of oxidation rates substituted anils
were correlated separately with any of the unit parameters
which was also found to be ineffective. The oxidation rates
analyzed in terms of dual substituent parameter equations was
also found to be unsuccessful. Correlation of the experimental
σ
Fig. 5. Hammett plot for oxidation of aromatic anils by sodium perborate
(substitution in combination of aniline and benzaldehyde moiety)
at 308 K
Structure-reactivity correlation: Negative entropy of
activation also gives the information that the complex formed
is an associate type with a greater degree of charge in transition
state than initial state27. The rate data obtained is also unsuc-
cessful in proving that the typical Hammet equation log kobs
versus σ is a dispersed gram and a sensitive plot (Fig. 6) at the

744 Venkatesh et al.
Asian J. Chem.
9. A. McKillop and W.R. Sanderson, Tetrahedron, 51, 6145 (1995).
10. J. Muzart, Synthesis, 1325 (1995).
data of solvent reveals that solvent interaction plays a major
role in leading the reactivity. The formation of charged inter-
mediate compound was supported by the high negative values
of entropy of activation and a most probable mechanism has
been proposed for oxidation of anils by sodium perborate. Non-
linear concave downward curve of Hammett plot was obtained
for the anils with substituents in the aniline moiety whereas
the non-linear concave upward curve was obtained for the
substituents in the benzaldehyde moiety and for the substituents
in the combination of aniline and benzaldehyde moiety. With
sodium perborate as oxygen donor, was proved to be a more
effective oxidant in anils.
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