Lack of linear free energy relationship: Tungsten(VI) catalyzed perborate oxidation of anilines

Article abstract of DOI:10.1002/(SICI)1097-4601(1999)31:8<571::AID-KIN6>3.0.CO;2-4

Operation of linear free energy relationships in tungsten(VI) catalyzed perborate oxidation was studied with 29 para-, meta- and ortho-substituted anilines. The activation parameters were calculated from k*( = rate/[substrate]²) at 35, 40, 45, 50 and 55 °C using the Erying relationship by the method of least squares. The oxidation is not isoentropic; in an isoentropic series only enthalpy of activation determines the reactivity and the isokinetic temperature is at infinity. At the isokinetic temperature all the compounds of the reaction series react at equal rate, the variation of substituent at this temperature has no influence on the free energy of activation.

Full text of DOI:10.1002/(SICI)1097-4601(1999)31:8<571::AID-KIN6>3.0.CO;2-4

Lack of Linear Free Energy
Relationship: Tungsten(VI)
Catalyzed Perborate
Oxidation of Anilines
C. KARUNAKARAN, P. N. PALANISAMY
Department of Chemistry, Annamalai University, Annamalainagar 608 002, India
Received 20 July 1998; accepted 26 March 1999
ABSTRACT: The rates of tungsten(VI) catalyzed perborate oxidation of 29 para-, meta- and
ortho-substituted anilines in aqueous acetic acid at 35–55ЊC conform to the Exner relation-
ship, also the activation parameters to the isokinetic relationship but not to any of the linear
free energy relationships. The results are rationalized. ᭧ 1999 John Wiley & Sons, Inc. Int J Chem
Kinet 31: 571–575, 1999
INTRODUCTION
to the substrate resulting in the formation of aniline
oxide or phenylhydroxylamine which reacts with an-
other molecule of aniline in a fast step to give the
product or (b) rate-limiting abstraction of hydride ion
from molecular aniline by the oxidant, followed by the
Sodium perborate, NaBO .4H O, is a cheap, eco-
3
2
friendly, large-scale industrial chemical, primarily
used in detergents as a bleaching agent. It is a mild
oxidant and in glacial acetic acid effectively oxidizes
aniline, present in excess, to azobenzene [1,2]. In
aqueous acetic acid the oxidation is sluggish but tung-
sten(VI) catalyzes the same. The catalyzed oxidation
is zero, first and second order in the oxidant, catalyst,
and substrate, respectively [3]. Hydrogen peroxide is
the reactive species of perborate [4]. Rapid generation
of dioxoperoxotungsten(VI) followed by its stabiliza-
tion by aniline, present in swamping excess as (ani-
line)dioxoperoxotungsten(VI), which acts as the oxi-
dizing agent and transfers oxygen to molecular aniline
in a rate-limiting step is the probable mechanism of
oxidation [3]. This article reports absence of well-
defined substituents effect; the oxidation rates of 29
para-, meta- and ortho-substituted anilines do not con-
form to any of the linear free energy relationships.
Generally, the oxidation of aniline involves either
ϩ
fast reaction of PhNH with aniline to yield the prod-
uct. Kinetics of oxidation of anilines by a variety of
oxidants have been reported and they conform to the
Hammett equation (reaction constant: ␳) or the mod-
Ϫ
ified Hammett equation (reaction constant: ␳ ) or the
ϩ
Brown-Okamoto equation (reaction constant: ␳ ).
The oxidation by chloramine-T (aqueous ethanol,
Ϫ
Ϫ3
[OH ] ϭ 0.10 mol dm , ␳ ϭ Ϫ1.0) [5], peroxodi-
Ϫ
Ϫ3
sulfate (aqueous 2-propanol, [OH ] ϭ 0.48 mol dm ,
␳ ϭ Ϫ1.3) [6], (aqueous t-butanol, [OH ] ϭ 0.25
mol dm , ␳ ϭ Ϫ1.48) [7], carbonate radical (pH 8.5,
␳ ϭ Ϫ1.0) [8], borate radical (pH 11.4, ␳ ϭ Ϫ0.87)
Ϫ
Ϫ3
ϩ
ϩ
2
[9] and hexacyanoferrate(III) (aqueous ethanol, 10
Ϫ
Ϫ
3
ϩ
[OH ] ϭ 2.5 mol dm , ␳ ϭ Ϫ4.1) [10] were stud-
ied in basic medium and that by N-bromoacetamide
(aqueous methanol, ␳ ϭ Ϫ0.79) [11], periodate
(aqueous methanol, ␳ ϭ Ϫ2.44) [12], peroxodisulfate
ϩ
(a) rate-determining oxygen transfer from the oxidant
(aqueous ethanol, ␳ ϭ Ϫ1.41) [13], peroxomono-
phosphoric acid (aqueous acetonitrile, pH 5.4, ␳ ϭ
ϩ
Ϫ
Ϫ1.37, ␳ ϭ Ϫ1.31, ␳ ϭ Ϫ1.38) [14], peracetic
acid (ethanol, ␳ ϭ Ϫ1.86) [15] and lead(IV) acetate
Correspondence to: C. Karunakaran
᭧
1999 John Wiley & Sons, Inc.
CCC 0538-8066/99/080571-05

5
72
KARUNAKARAN AND PALANISAMY
Ϫ3
acid, [HClO ] ϭ 1.0 mol dm , ␳ ϭ Ϫ3.0) [23], thal-
4
lium(III) catalyzed by ruthenium(III) (aqueous acetic
Ϫ3
acid, [HClO ] ϭ 1.0 mol dm , ␳ ϭ Ϫ0.76) [23],
4
iron(III)-bipyridyl complex (aqueous methanol,
2
Ϫ3
ϩ
1
0 [HClO ] ϭ 1.2 mol dm , ␳ ϭ Ϫ3.10) [24] and
4
pyridinium chlorochromate (chlorobenzene-nitroben-
3
Ϫ3
zene, 10 [Cl CHCOOH] ϭ 3 Ϯ 0.5 mol dm ,
␳
2
Ϫ
ϭ Ϫ3.75) [25].
EXPERIMENTAL
Sodium perborate was used as received. Anilines were
redistilled or recrystallized before use. All chemicals
were of analytical grade. Aqueous solution of perbo-
rate was prepared afresh and standardized iodometri-
cally. Kinetic studies were made under pseudo-first-
order conditions with catalytic concentration of
tungsten(VI) and large excess of anilines in aqueous
acetic acid [3]. Sodium tungstate was used as the cat-
alyst. The progress of the oxidation was followed up
to about 80% completion by iodometric estimation of
the unconsumed oxidant at different reaction time and
the temperature was controlled to Ϯ0.1ЊC. The oxi-
dation rates were found from the least squares slopes
of the linear plots of [oxidant] versus time. The oxi-
dation is sluggish below 35ЊC and above 55ЊC evap-
oration of solvent (water) occurs; the kinetic study was
restricted to 35–55ЊC. In all the cases, formation of
the corresponding substituted azobenzene was con-
firmed by UV-visible spectra of the reaction solutions
during and after completion of the oxidation.
Figure 1 The Isokinetic Plot. The numbers correspond to
substituents as given in Table I.
(chloroform-acetic anhydride, ␳ ϭ Ϫ2.4) [16] at neu-
tral pH. Similar studies in acidic medium include chlo-
2
ramine-T (aqueous acetic acid, 10 [HClO ] ϭ 0.62
4
Ϫ3
Ϫ5.0 mol dm ) [17], N-chlorosuccinimide (acetic
acid, ␳ ϭ Ϫ1.5) [18], bromate ion (aqueous acetic
acid, ␳ ϭ 1.74) [19], iodate catalyzed by ruthe-
nium(III) (aqueous acetic acid, 10 [HClO ] ϭ 1.0 mol
dm , ␳ ϭ Ϫ2.8) [20], periodate (aqueous acetic acid,
2
4
Ϫ3
ϩ
␳
ϭ Ϫ2.3) [21], peroxodisulfate (aqueous acetic
acid, ␳ ϭ 1.88) [22], thallium(III) (aqueous acetic
RESULTS AND DISCUSSION
Operation of linear free energy relationships in tung-
sten(VI) catalyzed perborate oxidation was studied
with 29 para-, meta- and ortho-substituted anilines.
The activation parameters were calculated from
2
k*(ϭ rate/[substrate] ) at 35, 40, 45, 50, and 55ЊC us-
ing the Eyring relationship by the method of least
squares (Table I). The oxidation is not isoentropic; in
an isoentropic series only enthalpy of activation de-
termines the reactivity and the isokinetic temperature
(␤) is at infinity [26]. At the isokinetic temperature all
the compounds of the reaction series react at equal
rate; the variation of substituent at this temperature has
no influence on the free energy of activation. Trans-
gressing the isokinetic temperature results in the re-
versal of the order of reactivities. The title reaction is
neither isoenthalpic. Only activation entropy deter-
mines the reactivity in an isoenthalpic series, the iso-
7
ϩ log k*(35°C)
Figure 2 The Exner Plot. The numbers are as in Table I.

LACK OF LINEAR FREE ENERGY RELATIONSHIP
573
kinetic temperature is zero. On the other hand, com-
pensation law, also known as isokinetic relationship,
exists. It is the linear relationship between enthalpy of
activation and entropy of activation (Fig. 1., correla-
tion coefficient, r ϭ 0.957; standard deviation, sd ϭ
be viewed with scepticism as these parameters are mu-
tually interdependent [27,28]. But existence of Exner
relationship, the linear double logarithmic relationship
between rates at two different temperatures, confirms
the isokinetic relationship and reveals that all the an-
ilines are oxidized through a common mechanism
(Fig. 2., r ϭ 0.976, sd ϭ 0.122, n ϭ 28). The isoki-
netic temperature is Ϫ1197 K. There is no general
theory which excludes negative values of isokinetic
temperature [26,29,30]. Also, negative value of iso-
kinetic temperature is not unknown [26]; the isokinetic
temperature of peroxodisulfate oxidation of ortho-
3
.36; number of data sets, n ϭ 28; o-phenylenedi-
amine is excluded). The maximum possible errors in
activation enthalpy (␦) and activation entropy are
Ϫ1
Ϫ1
Ϫ1
3
.4 kJ mol and 11.2 J K mol , respectively. The
error criterion is satisfied in the present study, that is,
#
#
⌬
⌬H ϾϾ 2␦ and hence the correlation between ⌬H
#
and ⌬S is significant. However, this relationship is to
Table I Tungsten(IV) Catalyzed Perborate Oxidation of Para-, Meta-, and Ortho-Substituted Anilinesx
05 k* dmϪ molϪ
3
1
s
Ϫ1
1
⌬
H#
Ϫ⌬S
#
Ϫ1
JK mol 1
Ϫ
1
Ϫ
No Substituent
35ЊC
40ЊC
45ЊC
50ЊC
55ЊC
6.24
7.12
4.48
5.72
kJ mol
1
2
3
4
5
6
7
8
9
. H
. p-CH3
. p-OCH3
. p-OC H
. p-Cl
. p-Br
. p-COOH
. p-NHCOCH
. p-NO2
1.04
0.80
0.66
0.81
2.7
3.6
4.67
1.7
3.55
1.04
1.11
1.28
3.0
1.84
2.44
14.7
3.62
2.4
1.97
1.38
1.93
1.42
1.03
1.8
1.3
1.17
1.1
1.24
3.9
2.42
1.99
1.5
1.84
5.64
7.4
11.4
5.6
16.3
2.32
2.25
3.5
3.85
2.87
2.94
3.16
9.44
11.8
14.6
8.96
18.9
3.4
3.18
5.16
8.1
6.24
7.84
70.7
85.6
78.1
78.8
69.4
61.5
70.3
89.2
84.1
65.7
56.6
73.8
56.8
61.2
60.4
81.4
50.4
52.2
53.6
64.0
61.1
78.4
59.7
73.2
69.6
71.3
57.4
89.2
95.3
65.8
91.2
87.5
108
131
99.6
47.0
56.3
128
156
98.9
147
137
37.5
54.9
168
165
161
130
136
83.3
147
97.5
107
121
107
59.9
25.9
2
5
14.8
5.04
8.3
15.9
31.0
14.4
28.0
5.44
4.68
8.1
13.9
8.24
10.5
110
12.4
8.2
7.04
6.8
9.44
10.8
4.4
11.5
14.6
1.53
1610
2.88
a
2.72
6.92
1.58
1.66
2.17
5.6
2.78
3.46
21.3
4.43
2.6
2.71
2.13
3.26
2.51
1.58
3.36
3.69
0.38
3
b
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
0. p-COOC H5
2
1. p-COCH3
2. m-CH3
3. m-OCH3
4. m-Cl
6.92
4.4
5. m-Br
6. m-COOH
7. m-NO2
5.52
38.6
5.25
3.83
4.0
3.29
5.68
4.56
2.31
5.84
5.8
b
57.5
a
8.83
5.92
6.12
4.56
6.0
5.2
3.45
7.55
8. m-COCH3
9. m, p-Cl2
0. o-CH3
1. o-OCH3
2. o-OC H
2
5
3. o-Cl
4. o-Br
a
5. o-COOH
2.78
0.29
364
0.34
19.5e
10.4
1.1
820
6. o-NO
7. o-OH
0.64
564
1.46
274g
2
c
442
0.60
86.8f
8. o-COCH
9. o-NH2
1.89
3
d
102
x
0[Aniline] ϭ 5.0 mol dmϪ3,
a
b
c
10[Aniline] ϭ 2.5 mol dmϪ3.
d
10 [Aniline] ϭ 5.0 mol dmϪ3,
2
1
0
0
0
2
10[Aniline] ϭ 1.0 mol dmϪ3.
10 [Perborate] ϭ 5.0 mol dmϪ3,
3
1
1
0 [Perborate] ϭ 1.0 mol dmϪ3,
0
0
0
03[W(VI)] ϭ 1.0 mol dmϪ3,
10 [Aniline] ϭ 6.0 mol dmϪ3.
2
4
10 [W(VI)] ϭ 5.0 mol dmϪ3.
0
Rate
Aniline]2
e
f
g
Medium: 50% (v/v) aq. HOAc.
k* ϭ
25ЊC. 35ЊC. 45ЊC.
[
0

5
74
KARUNAKARAN AND PALANISAMY
stituted anilines also fail to conform to any of the tri-
parametric equations; one of the parameters being the
steric susceptibility constant, ␷.
Operation of inductive and resonance effects op-
posing each other is known but the present study has
no parallels [32]. The less pronounced substituents ef-
fect on the oxidation rate may be explained by the
‘compensation effect’. The oxidation involves two an-
iline molecules in the rate-determining step [3]; the
catalyst-aniline complex is likely to be the electrophile
and the free aniline is the nucleophile. The influence
of the substituent on the reactivity of the nucleophile
is approximately compensated by the influence of the
same substituent on the reactivity of the electrophile.
As exact compensation is unlikely, especially in some
of the substituted anilines, the resultant effect is ex-
perienced on the oxidation rate.
Figure 3 The Hammett Plot. The numbers are as in
Table I.
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