Structure-reactivity correlation of anilines in acetic acid

Article abstract of DOI:10.1021/jo0158433

The oxidation of aniline in glacial acetic acid with percarbonate, a dry carrier of hydrogen peroxide, is a second-order reaction conforming to the isokinetic relationship. The hitherto followed method of correlation of the reaction rates in terms of the structure-reactivity relationships is unsatisfactory and erroneous. But the reaction rates of molecular anilines, obtained for the first time, conform to the structure-reactivity relationships.

Full text of DOI:10.1021/jo0158433

1118
J . Org. Chem. 2002, 67, 1118-1124
Str u ctu r e-Rea ctivity Cor r ela tion of An ilin es in Acetic Acid
Chockalingam Karunakaran* and Ramasamy Kamalam
Department of Chemistry, Annamalai University, Annamalainagar 608002, India
karunakaranc@rediffmail.com
Received J une 16, 2001
The oxidation of aniline in glacial acetic acid with percarbonate, a dry carrier of hydrogen peroxide,
is a second-order reaction conforming to the isokinetic relationship. The hitherto followed method
of correlation of the reaction rates in terms of the structure-reactivity relationships is unsatisfactory
and erroneous. But the reaction rates of molecular anilines, obtained for the first time, conform to
the structure-reactivity relationships.
In tr od u ction
(IV) acetate (chloroform-acetic anhydride, F ) -2.4),16
and pyridinium chlorochromate (chlorobenzene-nitroben-
zene, 103[Cl2CHCOOH] ) 3 ( 0.5 mol dm-3, F- ) -3.8)17
were in nonaqueous media. Similar studies in acid
medium include chloramine-T (aqueous acetic acid, 102-
[HClO4] ) 0.62-5.0 mol dm-3),18 N-chlorosuccinimide
(acetic acid, F ) -1.5),19 bromate ion (aqueous acetic acid,
F ) 1.7),20 iodate catalyzed by ruthenium(III) (aqueous
acetic acid, 102[HClO4] ) 1.0 mol dm-3, F ) -2.8),21
periodate (aqueous acetic acid, F+ ) -2.3),22 peroxodis-
ulfate (aqueous acetic acid, F ) 1.9),23 thallium(III)
(aqueous acetic acid, [HClO4] ) 1.0 mol dm-3, F ) -3.0),24
thallium(III) catalyzed by ruthenium(III) (aqueous acetic
acid, [HClO4] ) 1.0 mol dm-3, F ) -0.8),24 and iron(III)-
bipyridyl (aqueous methanol, 102[HClO4] ) 1.2 mol
dm-3, F+ ) -3.1).25 But the present study reveals that
the hitherto followed method of correlation of the reac-
tion rates in acidic solution is erroneous and presents
a modified approach. Sodium percarbonate (Na2CO3‚
1.5H2O2), a dry carrier of hydrogen peroxide,26-29 is an
inexpensive, innocuous, crystalline salt used extensively
in the detergent industry as a bleaching or antiseptic
agent. Its use as an effective reagent in organic synthesis
Studies on the structure-reactivity relationships of
anilines are numerous, and the reaction rates conform
to the usual Hammett equation (reaction constant: F),
its modified form (reaction constant: F-), or the Brown-
Okamoto equation (reaction constant: F+). The oxidation
by chloramine-T (aqueous ethanol, [OH-] ) 0.10 mol
dm-3, F ) -1.0),1 peroxodisulfate (aqueous 2-propanol,
[OH-] ) 0.48 mol dm-3, F ) -1.3),2 aqueous tert-butyl
alcohol, [OH-] ) 0.25 mol dm-3, F ) -1.5),3 carbonate
radical (pH 8.5, F+ ) -1.0),4 borate radical (pH 11.4, F+
) -0.9),5 and hexacyanoferrate(III) (aqueous ethanol,
102[OH-] ) 2.5 mol dm-3, F+ ) -4.1)6 were studied in
basic medium and by N-chloroacetamide (aqueous etha-
nol, F ) -3.3),7 N-bromoacetamide (aqueous methanol,
F ) -0.8),8 periodate (aqueous methanol, F ) -2.4),9
peroxodisulfate (aqueous ethanol, F+ ) -1.4),10 (aqueous
ethanol, pH 7, F+ ) -0.8),11 and peroxomonophosphoric
acid (aqueous acetonitrile, pH 5.4, F ) -1.4, F+ ) -1.3,
F- ) -1.4)12 at neutral pH. The oxidations by hydrogen
peroxide catalyzed by methylrhenium trioxide (methanol,
F ) -1.2),13 peracetic acid (ethanol, F ) -1.9),14 io-
dosobenzene (chloroform, F ) -1.6; manganese(III)salen
complex-catalyzed, methylene chloride, F+ ) -0.8),15 lead-
(15) Larsen, J .; J orgensen, K. A. J . Chem. Soc., Perkin Trans. 2
1992, 1213.
* To whom correspondence should be addressed. Fax: 91-4144-
38145.
(16) Galliani, G.; Rindone, B.; Beltrame, P. L. J . Chem. Soc., Perkin
Trans. 2 1976, 1803.
(1) Radhakrishnamurti, P. S.; Prasada Rao, M. D. Indian J . Chem.
1976, 14B, 790.
(2) Behrman, E. J . J . Am. Chem. Soc. 1967, 89, 2424.
(3) Krishnamurthi, T. K.; Venkatasubramanian, N. Indian J . Chem.
1978, 16A, 28.
(17) Panigrahi, G. P.; Mahapatro, D. D. Int. J . Chem. Kinet. 1982,
14, 977.
(18) Radhakrishnamurti, P. S.; Prasada Rao, M. D. Indian J . Chem.
1976, 14A, 485.
(19) Radhakrishnamurti, P. S.; Sahu, S. N. Indian J . Chem. 1976,
(4) Elango, T. P.; Ramakrishnan, V.; Vancheesan, S.; Kuriacose, J .
Proc. Indian Acad. Sci. Chem. Sci. 1984, 93, 47.
(5) Padmaja, S.; Rajaram, J .; Ramakrishnan, V. Indian J . Chem.
1990, 29A, 422.
(6) Radhakrishnamurti, P. S.; Panda, R. K. Indian J . Chem. 1971,
9, 1247.
(7) Kumar, A.; Bhattacharjee, G. J . Indian Chem. Soc. 1991, 68,
523.
(8) Sharadamani, P. R.; J agannadhan, V. Indian J . Chem. 1990,
29A, 700.
(9) Prasada Rao, M.; Sethuram, B.; Navaneeth Rao, T. Indian J .
Chem. 1979, 17A, 52.
(10) Venkatasubramanian, N.; Sabesan, A. Can. J . Chem. 1969, 47,
3710.
(11) Srinivasan, C.; Perumal, S.; Arumugam, N. Indian J . Chem.
1980, 19A, 160.
(12) Panda, A. K.; Mahapatro, S. N.; Panigrahi, G. P. J . Org. Chem.
1981, 46, 6, 4000.
14B, 898.
(20) Vijayalakshmi; Sundaram, E. V. Indian J . Chem. 1979, 17A,
495.
(21) Prasada Rao, M. D.; Padmanabha, J . Indian J . Chem. 1980,
19A, 984.
(22) Srivastava, S. P.; Bhattacharjee, G.; Malik, P. J . Indian Chem.
Soc. 1990, 67, 347.
(23) Srivastava, S. P.; Gupta, R. C.; Shukla, A. K. Indian J . Chem.
1977, 15A, 605.
(24) Radhakrishnamurti, P. S.; Pati, S. N. Indian J . Chem. 1980,
19A, 980.
(25) Subba Rao, P. V.; Saradamba, G. V.; Ramakrishna, K. Indian
J . Chem. 1985, 24A, 438.
(26) Carrondo, M. A. A. F. de C. T.; Griffith, W. P.; J ones, D. P.;
Skapski, A. C. J . Chem. Soc., Dalton Trans. 1977, 2323.
(27) Adams, J . M.; Pritchard, R. G. Acta Crystallogr. 1977, B33,
3650.
(28) Adams, J . M.; Pritchard, R. G.; Hewat, A. W. Acta Crystallogr.
1979, B35, 1759.
(13) Zhu, Z.; Espenson, J . H. J . Org. Chem. 1995, 60, 1326.
(14) Ibne-Rasa, K. M.; Edwards, J . O. J . Am. Chem. Soc. 1962, 84,
763.
(29) J ones, D. P.; Griffith, W. P. J . Chem. Soc., Dalton Trans. 1980,
2526.
10.1021/jo0158433 CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/29/2002

Structure-Reactivity Correlation of Anilines in Acetic Acid
J . Org. Chem., Vol. 67, No. 4, 2002 1119
composition of acetic acid to 70% by the addition of
methanol, the reaction rate decreases to one-third. The
oxidation is not through radical pathway; addition of
vinyl monomer, acrylonitrile to the reaction solution (0.10
mol L-1) fails to suppress the reaction. Also, the added
vinyl monomer does not polymerize during the course of
the reaction.
Sodium percarbonate is a dry carrier of hydrogen
peroxide.26-29 Chemical tests confirm the presence of
hydrogen peroxide and the absence of peracetic acid in
fresh solutions of percarbonate in glacial acetic acid;
formation of peracetic acid from commercial hydrogen
peroxide and glacial acetic acid requires a catalytic
amount of mineral acid.41 Further, kinetic experiments
with peracetic acid as the oxidant show that the peracetic
acid oxidation under identical conditions is fast, too fast
to follow by titrimetry. The oxidation mechanism is
classical.42 The electrophilic attack of the peroxide oxygen
on the amine nitrogen leading to formation of phenylhy-
droxylamine is slow and rate limiting. Phenylhydroxy-
lamine is further oxidized to nitrosobenzene, which in
turn couples with aniline, present in large excess, to yield
azobenzene; the latter two reactions are fast. Methanol
is a nucleophile, and association of methanol with the
electrophilic peroxide oxygen atom may be a reason for
the suppression of the oxidation with the addition of
methanol.
The structure-reactivity relationships in the oxidation
of anilines in glacial acetic acid were studied with 31
anilines. The activation parameters were calculated from
the measured rates at 45, 55, and 65 °C using the Eyring
relationship (Table 1). The oxidation is neither isoen-
tropic nor isoenthalpic but conforms to the compensation
law also known as isokinetic relationship, a linear
relationship between the enthalpy of activation and the
entropy of activation (Figure 2: correlation coefficient, r
) 0.98, standard error, sd ) 3.0). The maximum possible
errors in activation enthalpy (δ) and activation entropy
are 3.6 kJ mol-1 and 11.6 J K-1 mol-1, respectively. The
error criterion is satisfied in the present study, that is,
∆∆Hq (74.4 kJ mol-1) . 2δ (7.2 kJ mol-1), and hence the
correlation between ∆Hq and ∆Sq is significant. However,
this relationship is to be viewed with skepticism, as these
parameters are mutually interdependent.43,44 But the
existence of the Exner relationship, the linear double
logarithmic relationship between the rates at two differ-
ent temperatures, confirms the isokinetic relationship
and reveals that all the anilines, the reaction rates of
which were measured, are oxidized through a common
mechanism (Figure 3: r ) 0.99, sd ) 0.09, number of
data points, n ) 29, slope ) 1.21).43,44 The isokinetic
temperature, obtained from the Exner plot, is -25 °C,
which is far from the experimental temperature, justify-
ing the structure-reactivity correlation. However, it is
possible that anomalous behavior of one or two substit-
uents of very high or very low reactivity (points at the
extremes of the Exner plot, and here it is o-phenylene-
diamine) may lead to error in slope and hence in the
isokinetic temperature. Inclusion of o-phenylenediamine
in the Exner plot involves extrapolated values; this
F igu r e 1. Rate dependence on [aniline].
is beginning to emerge,30-40 and this is the first report
on the kinetics of the oxidation; the reaction was carried
out in glacial acetic acid, a solvent suitable for the
oxidation.
Resu lts a n d Discu ssion
The kinetics of the oxidation, studied in glacial acetic
acid under the condition [aniline] . [oxidant], were
followed by iodometric estimation of the unreacted oxi-
dizing agent. The reaction is sluggish at room tempera-
ture but is smooth at 45-65 °C; the decomposition of the
oxidant is large above 65 °C. The reaction is first order
with respect to the oxidant. The reaction rates were
measured from 15 to 75% consumption of the oxidant,
and the plot of log [oxidant] versus reaction times is
linear. The pseudo-first-order rate constant (k′), obtained
from the least-squares slope of the pseudo-first-order plot,
remains constant at different initial concentrations of the
oxidant (102[oxidant]0 ) 0.40-1.80 mol L-1, [aniline]0 )
0.50 mol L-1; 104k′ ) 2.3 s-1 at 60 °C). The reaction rate
is reproducible to (4%. The oxidation is also first order
with respect to aniline. The reaction rate increases with
the concentration of aniline, and the plot k′ versus
[aniline] is a straight line passing through the origin
(Figure 1). Glacial acetic acid is a suitable solvent for
percarbonate oxidation. At 65 °C, on decreasing the
(30) McKillop, A.; Sanderson, W. J . Chem. Soc., Perkin Trans. 1
2000, 471.
(31) Ait-Mohand, S.; Lunak, S.; Muzart, J . Recueil 1997, 130, 1655.
(32) Maignien, S.; Ait-Mohand, S.; Muzart, J . Synlett 1996, 439.
(33) McKillop, A.; Sanderson, W. R. Tetrahedron 1995, 51, 6145.
(34) Muzart, J . Synthesis 1995, 1325.
(35) Muzart, J .; Ait-Mohand, S. Tetrahedron Lett. 1995, 36, 5735.
(36) Muzart, J .; Ait-Mohand, S. New J . Chem. 1995, 19, 207.
(37) Ait-Mohand, S.; Levina, A.; Lunak, S.; Muzart, J . Inorg. Chim.
Acta 1995, 238, 183.
(38) Ait-Mohand, K.; Levina, A.; Muzart, J . Synth. Commun. 1995,
25, 2051.
(41) Vogel’s Textbook of Practical Organic Chemistry Including
Qualitative Organic Analysis; Furniss, B. S., Hannaford, A. J ., Rogers,
V., Smith, P. W. G., Tatchell, A. R., Eds.; Longman: London, 1978.
(42) Ogata, Y.; Shimizu, H. Bull. Chem. Soc. J pn. 1979, 52, 635.
(43) Exner, O. Collect. Czech. Chem. Commun. 1964, 29, 1094.
(44) Exner, O. Nature 1964, 201, 488.
(39) Ait-Mohand, S.; Muzart, J . Synth. Commun. 1995, 25, 2373.
(40) Muzart, J .; N’Ait Ajjou, A.; Ait-Mohand, S. Tetrahedron Lett.
1994, 35, 1989.

1120 J . Org. Chem., Vol. 67, No. 4, 2002
Karunakaran and Kamalam
Ta ble 1. P seu d o-F ir st-Or d er Ra te Con sta n ts (k′), Activa tion P a r a m eter s, a n d Sp ecific Oxid a tion Ra tes of Molecu la r
An ilin es (k) in Gla cia l Acetic Acid a
104k′ (s-1
)
103k (dm3 mol-1 s-1
)
entry
substituent
45 °C
0.795
1.01
1.60
55 °C
65 °C
∆Hq (kJ mol-1
)
-∆Sq (J K-1 mol-1
)
45 °C
55 °C
65 °C
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
H
1.58
2.69
4.17
1.27
0.596
2.33
1.65
0.801
1.64
2.15
1.87
3.11
5.30
8.66
2.23
1.24
6.06
3.52
1.59
3.52
6.10
4.63
3.59
3.82
0.746
1.58
1.38
4.38
0.921
4.74
4.42
5.41
2.19
0.472
1.99
1.20
0.854
58.2
71.5
72.9
56.5
54.0
80.5
70.9
56.9
69.4
87.4
78.3
69.0
71.3
52.9
74.0
66.3
77.9
52.6
62.6
64.9
71.1
86.6
65.0
68.1
56.4
56.7
127
141
96.8
88.9
149
161
69.7
102
151
107
49.1
78.2
108
52.1
168
101
124
80.1
168
125
118
98.0
60.6
135
116
154
156
-114
73.9
116
4.60
7.41
5.04
0.474
0.108
0.447
13.9
0.180
11.7
25.5
18.5
1.23
1.08
0.0472
0.0856
0.185
4.17
0.0650
4.63
9.14
19.7
13.1
1.01
0.185
1.11
33.7
0.345
27.3
67.6
45.8
2.70
2.41
0.0887
0.168
0.429
9.89
0.125
9.80
12.3
11.6
0.210
0.0587
0.234
0.191
0.127
2940
112
87.3
18.0
38.9
27.3
1.78
0.384
2.88
71.9
0.685
58.7
192
113
6.10
m-CH3
m-OH
m-Cl
0.594
0.348
0.942
0.679
0.419
0.702
0.811
0.755
0.721
0.774
0.217b
0.283
0.295
m-NO2
m-COOH
m,p-(CH3)2
m,p-Cl2
p-CH3
p-OCH3
p-OC2H5
p-Cl
p-Br
p-NO2
p-COOH
p-COOC2H5
p-NHCOCH3 0.721
1.59
1.72
5.34
0.408
0.555
0.686
1.71
0.514
2.33
0.162
0.478
0.863
25.3
0.224
19.9
22.0
23.7
0.749
0.0962
0.531
0.355
0.238
o,m-Cl2
o-CH3
o-OCH3
o-OC2H5
o-Cl
0.267
1.10
0.976
1.04
0.295
0.129
0.408
0.320
0.226
104c
2.46
2.65
4.86
4.56
0.613
0.288
0.877
0.647
0.456
457c
11.2
0.101
0.0263
0.109
0.0947
0.0631
670
47.3
35.5
o-NO2
o-COOH
o-COOCH3
o-COCH3
o-NH2
-NHCH3
-N(CH3)2
o-OH
1920c
26.9
8.04
12400
269
157
4.74
1.82
fast
very fast
74.9
63.8
4.47
o-SH
a
b
[oxidant]0 ) 0.010 mol dm-3; [aniline]0 ) 0.50 mol dm-3
.
Extrapolated using Arrhenius plot: 0.576 at 60 °C. c Extrapolated using
Arrhenius plot: 1.42, 10.7, and 43.1 at 20, 30, and 40 °C, respectively.
F igu r e 2. Isokinetic plot.
F igu r e 3. Exner plot.
substrate is highly reactive and may largely define the
plot. However, even if o-phenylenediamine is omitted, the
correlation is satisfactory (r ) 0.97, sd ) 0.09, n ) 28,
slope ) 1.12; figure not given); the isokinetic temperature
drops to -58 °C. The spread of the k′ values increases
with temperature and is in accordance with the fact that
the isokinetic temperature is lower than the experimental
temperature. Current views do not attach much signifi-
cance to the isokinetic temperature, though linear cor-
relation is usually a necessary condition for the validity
of the Hammett equation.
The rates of oxidation of para- and meta-substituted
anilines in acetic acid do not conform to the usual
Hammett equation at any of the temperatures studied

Structure-Reactivity Correlation of Anilines in Acetic Acid
J . Org. Chem., Vol. 67, No. 4, 2002 1121
steric effects. In the present study, the activation enthal-
pies and activation entropies of all the para-, meta-, and
ortho-substituted anilines conform to the isokinetic re-
lationship. So are the rates at 65 and 45 °C to the Exner
relationship. This indicates that the mechanism of oxida-
tion of the ortho-substituted anilines is the same as that
of para- and meta-substituted anilines and justifies the
analysis of the reaction rates of the ortho-substituted
anilines in terms of the structure-reactivity relation-
ships. The rates of oxidation of ortho-substituted ani-
lines also fail to conform to Charton’s LDS equations;
the carboxy-, nitro-, and methoxycarbonyl groups indi-
vidually take either planar or orthogonal orientations
requiring appropriate steric substituent constants, the
acetyl substituent was excluded due to nonavailability
of the steric substituent constant. The values of the
steric substituent constants (υ) used were those of
Charton.50
Anilines in basic and neutral media are present as free
bases but in acid medium exist in dual forms, the free
bases and the conjugate acids. And, the ratio of the
concentrations of the free base to the conjugate acid
([XC6H4NH2]/[XC6H4NH3+]) depends on the pKa of the
aniline and the acidity of the medium. The reported
oxidations of anilines (vide supra) were carried out under
pseudo-first-order conditions ([anilines] . [oxidant]), and
the concentration of the oxidants at different reaction
times were determined by titrimetry or spectrophotom-
etry. The pseudo-first-order rate constants (k′) were
obtained from the least-squares slopes of log [oxidant]
versus time plots, and the second-order rate constants
are k′/[aniline]T where [aniline]T is the total concentration
of aniline. Since the pKa varies from 5.36 (p-OCH3) to
-0.28 (o-NO2) and molecular anilines are the reactive
species (nucleophile), the reported k′/[aniline]T values are
not the rate constants of the oxidant-molecular aniline
reactions. And, the analysis of k′/[aniline]T in terms of
the Hammett, Brown-Okamoto, DSP, and LDS equa-
tions is erroneous. Now, for the first time, the specific
reaction rates of molecular anilines with the oxidant (k)
have been obtained and correlated in terms of the
structure-reactivity relationships. In the reactions of
anilines in acid medium, as the free bases are the nu-
cleophiles, the specific reaction rates of anilines are to
be obtained using the concentrations of the free bases
but not the total concentrations of anilines. The concen-
trations of the free bases may be deduced from the acid
strength of the medium and the pKa values of the anilines
and acetic acid.51-55 Although the pKa values correspond
to aqueous solutions, detailed examination reveals that
they may be used to obtain the concentrations of the free
bases in glacial acetic acid. Coupling the ionization
equilibrium of acetic acid with that of anilinium ion
results in the elimination of [H2O] and [H3O+]. The ratio
of the ionization constant of acetic acid to that of
F igu r e 4. Hammett plot with the rate constants obtained by
the hitherto reported method at 45 °C.
(e.g., Figure 4). In anilines, the reaction site may conju-
gate with the para substituent, but correlation of the
oxidation rates of para- and meta-substituted anilines
separately with the usual Hammett substituent con-
stants or the Brown-Okamoto substituent constants or
the modified Hammett substituent constants (para: σp,
σp+, σp-, meta: σm, σm+) was also unsuccessful. The
reaction rates at all the temperatures measured were
analyzed in terms of the dual substituent parameter
(DSP) equations (para: σI, σR; σI, σR+; σI, σR-; F, R; meta:
σI, σR; σI, σR+; σI, σR-; F, R) but with failure. The σ values
of H, m-CH3, m-OH, m-Cl, m-NO2, m-COOH, p-CH3,
+
p-OCH3, p-Cl, p-Br, p-NO2, p-COOH, σp of CH3, OCH3,
-
+
Cl, Br, σp of NO2, COOH, and σI, σR, σR of OH used
were those compiled by Shorter;45 for the meta substit-
+
uents (m-CH3, m-Cl, m-NO2, m-COOH), σm values of
Brown were employed.46 Taft’s σI, σR, σR+, and σR
-
values47 of H, CH3, OCH3, Cl, Br, NO2, COCH3, COOC2H5,
and NHCOCH3 were used in the DSP equations. The
values of σ of p-OC2H5, p-COOC2H5, p-NHCOCH3, σp+ of
H, OC2H5, NO2, COOH, COOC2H5, NHCOCH3, σp- of H,
CH3, OCH3, OC2H5, Cl, Br, COOC2H5, NHCOCH3, σR- of
OH, and σI, σR, σR+, σR- of OC2H5, COOH, and COOCH3
employed were those compiled by Hansch.48 The F and
R values used were those of Swain.49 The Hammett
equation and its different modified forms are applicable
to para- and meta-substituted benzene derivatives but
not to the ortho compounds; besides the well-known
inductive and resonance effects associated with the para
and meta substituents, the ortho substituents cause
(50) Aslam, M. H.; Burdon, A. G.; Chapman, N. B.; Shorter, J .;
Charton, M. J . Chem. Soc., Perkin Trans. 2 1981, 500.
(51) Dean, J . A. Handbook of Organic Chemistry; McGraw-Hill: New
York, 1987; p 8-1.
(45) Shorter, J . Correlation Analysis in Organic Chemistry: an
Introduction to Linear Free-Energy Relationships; Clarendon, Oxford,
1973.
(46) Brown, H. C.; Okamoto, Y. J . Am. Chem. Soc. 1958, 80, 4979.
(47) Dayal, S. K.; Ehrenson, S.; Taft, R. W. J . Am. Chem. Soc. 1972,
94, 9113.
(52) Buckingham, J ., Donaghy, S. M., Cadogan, J . I. G., Raphael,
R. A., Rees, C. W., Eds. Dictionary of Organic Compounds; Chapman
and Hall: New York, 1982.
(53) Lyman, W. J .; Reehl, W. F.; Rosenblalt, D. H. Handbook of
Chemical Property Estimation Methods; American Chemical Society:
Washington, DC, 1990; p 6-1.
(48) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165.
(49) Swain, G. C.; Unger, S. H.; Rosenquist, N. R.; Swain, M. S. J .
Am. Chem. Soc. 1983, 105, 492.
(54) Takayasu, K. Chem. Pharm. Bull. 1978, 26, 59.
(55) Perrin, D. D. Dissociation Constants of Organic Bases in
Aqueous Solution; Butterworth: London, 1965.

1122 J . Org. Chem., Vol. 67, No. 4, 2002
Karunakaran and Kamalam
acid could be brought into the line using the σp- value, it
is illogical. The large deviation of the p-carboxy substitu-
ent is probably due to complication of zwitterion forma-
tion not allowed for in the calculation of the concentration
of the molecular form of the aniline. The meta acid does
not deviate significantly, and a possible explanation is
that the zwitterion formation is insignificant.
The specific reaction rates of ortho-substituted anilines
were analyzed in terms of Charton’s LDS equations. The
oxidation of o-mercaptoaniline is very fast. The UV-vis
spectra of the reaction solution during and after the
completion of the reaction show no formation of azo
compound; the reaction is not the oxidation of amino
group but that of thiol. The oxidation of o-aminophenol
is also too fast; o-aminophenol enters into intramolecular
hydrogen bonding. o-Phenylenediamine in glacial acetic
+
acid may exist in three forms, viz., o-+NH3C6H4NH3
,
o-NH2C6H4NH3+, and o-NH2C6H4NH2. Calculation of the
individual concentrations, using the pKa values, reveals
that o-phenylenediamine exists predominantly as o-
NH2C6H4NH3+; the other two forms exist in trace amounts.
The protonated amino group (NH3+-) is highly electron
withdrawing (σp ) 0.60), and the free energy of activation
for the oxidation of the conjugated acid is likely to be
large; hence, the free base is the probable reactive
species; the reported k values correspond to this species.
The oxidation of molecular o-phenylenediamine encoun-
ters two identical reaction sites; one acts as the substitu-
ent and the other is the reaction center. This requires
that only half of the rate to be considered for the
correlation analysis. Because of these complications and
also that the reported reaction rates are only the ex-
trapolated values, o-phenylenediamine was omitted from
the regression. Correlation analysis of the specific rates
of the ortho-substituted molecular anilines in terms of
the LDS equations gave fairly satisfactory results. The
electronic parameters used were the σI values of Taft,47
along with σR, σR+, or σR-; for ethoxy, carboxy, and
methoxycarbonyl substituents, the values employed were
those compiled by Hansch.48 Alternatively, the field F
and resonance R parameters of Swain49 were employed.
The steric parameters used were the υ values of Char-
ton.50 Each data set involved eight points (methyl,
methoxy, ethoxy, chloro, nitro, carboxy, methoxycarbonyl,
and hydrogen), acetyl being excluded due to the un-
availability of the steric parameter. Table 2 reveals
F igu r e 5. Hammett plot with the rate constants obtained by
the modified method at 45 °C.
anilinium ion yields the equilibrium constant of the
+
equilibrium PhNH2 + HOAc h PhNH3 + OAc-. In the
absence of water, self-ionization of acetic acid is un-
likely, and hence, [PhNH3+] ) [OAc-]. Also, [PhNH3+] +
[PhNH2] ) [PhNH2]T. Solving the quadratic expression
on [PhNH2] yields [PhNH2]. Examination of the pKa and
pKb values of acetic acid and ammonia in water reveals
that the variation of these values with temperature is
small; the pKa of acetic acid varies between 4.76 and 4.79
in the temperature range 0-50 °C, and the pKb of
ammonia decreases from 4.78 to 4.72 on raising the
temperature from 15 to 50 °C.53,56 As the pKa values of
anilines at the experimental temperatures are not avail-
able, as an approximation, the pKa values at 25 °C51-55
were used in the calculation. The specific reaction rates
of molecular anilines in glacial acetic acid (k) thus
obtained conform to the usual Hammett equation at all
the temperatures studied; Figure 5 is the Hammett plot
at 45 °C (r ) 0.98, sd ) 0.20, number of data points, n )
17, F ) -2.7 ( 0.1 at 45-65 °C). The Hammett plots
reveal some noteworthy features. The plots involve the
ordinary benzoic acid-based σ values throughout and
include p-nitro, p-ethoxycarbonyl, and p-carboxy substit-
uents. In anilines, these substituents enter into cross-
conjugation with the reaction center and withdraw
electrons strongly through resonance, necessitating the
use of σp-. But the plots, e.g., Figure 5, show that the
p-nitro and p-ethoxycarbonyl substituents fit the Ham-
-
that σR is less satisfactory in explaining the variation
of the rate with the substituent. This is in accordance
with the literature of the oxidation of anilines (vide
supra); most of the satisfactory correlations involve
either σ or σp+. Better results were found when it was
assumed that the carboxy, nitro, and methoxycarbonyl
groups were in the orthogonal orientation, and the results
+
obtained by using σI and σR were fairly better. One of
the most satisfactory correlation equations was as fol-
lows:
-
mett line with the usual Hammett σ values; use of σp
-log k ) 2.28((0.15) + 2.90((0.28)σI +
1.15((0.12)σR+ + 1.18((0.35)υ
values lead to deviations. This is not uncommon. Exami-
nation of the literature of the oxidation of anilines
provides many satisfactory correlations using benzoic
+
acid-based σ values or the Brown-Okamoto σp values
45 °C, n ) 8, 100R2 ) 98.6, sd ) 0.156
-
rather than σp (vide supra). Although p-aminobenzoic
The regression coefficients yield the percentages of
inductive (σI), resonance (σR+), and steric (υ) terms as 55,
22, and 23, respectively, at 45 °C.
(56) Weast, R. C., Selby, S. M., Hodgman, C. D., Eds. Handbook of
Chemistry and Physics, 45th ed.; The Chemical Rubber Co.: Clevelan,
OH, 1964; p D-79.

Structure-Reactivity Correlation of Anilines in Acetic Acid
J . Org. Chem., Vol. 67, No. 4, 2002 1123
Ta ble 2. Cor r ela tion An a lysis of th e Ra tes of
Or th o-Su bstitu ted An ilin esa
45 °C
100R2
55 °C
100R2
65 °C
100R2
explanatory
variables
sd
sd
sd
σI, σR
94.0
94.6
94.1
96.5
96.4
94.0
94.9
95.2
97.0
0.286
0.303
0.316
0.243
0.249
0.318
0.295
0.287
0.227
93.5
93.9
93.6
95.8
95.8
93.6
94.5
94.5
96.4
0.303
0.327
0.336
0.270
0.271
0.335
0.310
0.311
0.253
98.0
97.1
96.7
98.1
98.5
96.2
96.5
96.6
98.0
0.230
0.224
0.237
0.178
0.160
0.255
0.245
0.240
0.186
σI, σR, υb
σI, σR, υc
σI, σR, υd
σI, σR, υe
σI, σR, υf
σI, σR, υg
σI, σR, υh
σI, σR, υi
-
σI, σR
90.9
91.2
91.2
92.6
93.5
91.5
91.5
91.3
93.7
0.352
0.388
0.387
0.354
0.333
0.381
0.379
0.385
0.327
89.8
89.9
90.1
91.4
92.4
90.6
90.5
90.0
92.5
0.379
0.421
0.418
0.389
0.366
0.406
0.408
0.418
0.364
94.0
94.2
94.8
95.0
96.5
94.8
94.2
94.0
95.7
0.287
0.314
0.299
0.292
0.246
0.298
0.315
0.321
0.271
σI, σR-, υb
σI, σR-, υc
σI, σR-, υd
σI, σR-, υe
σI, σR-, υf
σI, σR-, υg
σI, σR-, υh
σI, σR-, υi
+
σI, σR
94.6
96.0
95.0
97.9
97.6
94.6
95.0
96.3
98.6
0.271
0.263
0.291
0.188
0.204
0.303
0.293
0.251
0.153
94.3
95.5
94.7
97.5
97.3
94.3
94.8
95.9
98.3
0.282
0.283
0.306
0.209
0.217
0.316
0.302
0.269
0.172
96.3
98.3
97.5
99.2
99.4
96.3
96.3
97.2
99.0
0.225
0.172
0.207
0.118
0.103
0.251
0.250
0.218
0.130
σI, σR+, υb
σI, σR+, υc
σI, σR+, υd
σI, σR+, υe
σI, σR+, υf
σI, σR+, υg
σI, σR+, υh
σI, σR+, υi
F igu r e 6. Exner plot with the rate constants obtained by the
modified method (o-phenylenediamine excluded).
they are not so as the combination is not in any simple
form; hence, they lack any physical significance.
F, R
93.9
94.7
94.0
96.8
95.9
94.0
95.0
95.9
97.4
0.287
0.301
0.321
0.233
0.264
0.321
0.292
0.265
0.213
93.6
94.2
93.7
96.3
95.6
93.6
94.8
95.3
97.0
0.299
0.320
0.334
0.254
0.277
0.334
0.301
0.286
0.231
96.5
97.6
96.3
98.8
98.5
96.5
96.9
97.5
98.6
0.220
0.205
0.234
0.140
0.159
0.245
0.229
0.207
0.155
F, R, υb
F, R, υc
F, R, υd
F, R, υe
F, R, υf
F, R, υg
F, R, υh
F, R, υi
Con clu sion s
The hitherto followed correlation of the reaction rates
of anilines in acid medium is erroneous, found to be
unsatisfactory in the reaction in glacial acetic acid. But
the specific reaction rates of molecular anilines, obtained
for the first time, conform to the structure-reactivity
relationships.
a
n ) 8 (o-NH2 and o-COCH3 excluded); 100R2: the percentage
b
of variation explained. COOH: planar, NO2: planar, COOCH3:
planar. cCOOH: orthogonal, NO2: planar, COOCH3: planar.
Exp er im en ta l Section
COOH: planar, NO2: orthogonal, COOCH3: planar. e COOH:
d
orthogonal, NO2: orthogonal, COOCH3: planar. f COOH: planar,
Gen er al P r ocedu r e. Sodium percarbonate, Na2CO3‚1.5H2O2
(Fluka), was used as received. The anilines (AR or LR) were
redistilled or recrystallized before use; the experimentally
determined physical constants confirm the identity of the
anilines employed. Percarbonate was dissolved in glacial acetic
acid, standardized iodometrically, and used afresh. The kinet-
ics of the oxidation, at fixed temperature, was followed by
mixing required volumes of the oxidant and aniline in glacial
acetic acid and estimating the unreacted oxidant at different
reaction times iodometrically. Percarbonate (0.01 mol) was
added to aniline (0.015 mol) in acetic acid at 60 °C and diluted
with water after 4 h, and the separated solid was filtered,
recrystallized from petroleum ether (40-60), and identified as
azobenzene by its mp, mixed mp, and IR spectrum. Formation
of azobenzenes in the reaction of all the anilines listed, except
o-mercaptoaniline, N-methylaniline, and N,N-dimethylaniline,
was confirmed by the UV-visible spectra of the reaction
solutions, during and after the completion of the reaction. For
the experiments with peracetic acid, solution of peracetic acid
in glacial acetic acid was prepared by standard method,41
standardized iodometrically, and used afresh.
P h ysica l Con sta n ts a n d Solven ts of Recr ysta lliza tion
of An ilin es. Aniline, bp 183 (184) °C; m-methylaniline, bp 201
(203) °C; m-aminophenol, mp (water) 126 (124-6) °C; m-
chloroaniline, bp 229 (230) °C; m-nitroaniline, mp (water) 114
(114) °C; m-aminobenzoic acid, mp (water) 177 (178) °C; m,p-
dimethylaniline, mp (petroleum ether 40-60) 50 (51) °C; m,p-
dichloroaniline, mp (ligroin) 71 (72) °C; p-methylaniline, mp
(water) 42 (42) °C; p-methoxyaniline, mp (petroleum ether 40-
60) 57 (57) °C; p-ethoxyaniline, bp 254 (253) °C; p-chloro-
g
NO2: planar, COOCH3: orthogonal. COOH: orthogonal, NO2:
planar, COOCH3: orthogonal. COOH: planar, NO2: orthogonal,
COOCH3: orthogonal. COOH: orthogonal, NO2: orthogonal,
COOCH3: orthogonal.
h
i
Figures 4 and 5 clearly show that the values of k′,
although obtained under identical conditions, are inca-
pable of yielding a meaningful Hammett plot, and this
is because they involve a highly variable concentration
of free aniline. Similarly, an Exner plot involving k′, even
though reasonably linear, is not meaningful to afford the
isokinetic temperature. Hence, as a rough approximation,
an Exner plot with the tabulated k values at 45 and 65
°C was made (Figure 6); it is satisfactorily linear yielding
an isokinetic temperature of -133 °C (r ) 0.997, sd )
0.087, n ) 28, slope ) 1.05). o-Phenylenediamine was
omitted, as its inclusion involves extrapolated values and
also complication of the reactive species (i.e., either
o-NH2C6H4NH3+ or o-NH2C6H4NH2 or both). The statisti-
cal parameters of the fit remain unaltered even on
inclusion of o-phenylenediamine (r ) 0.996, sd ) 0.115,
n ) 29, slope ) 1.08, figure not given), and the isokinetic
temperature raises to -89 °C. Although it appears that
the reported thermodynamic properties correspond to the
combined process of ionization of anilinium ions and the
reaction of molecular anilines with hydrogen peroxide

1124 J . Org. Chem., Vol. 67, No. 4, 2002
Karunakaran and Kamalam
aniline, mp (petroleum ether 40-60) 70 (70-1) °C; p-bromo-
aniline, mp (aqueous ethanol) 65 (66) °C; p-nitroaniline, mp
(water) 147 (148) °C; p-aminobenzoic acid, mp (water) 188
(188) °C; ethyl p-aminobenzoate, mp (ethanol) 90 (92) °C;
p-aminoacetanilide, mp (water) 163 (162-3) °C; o, m-dichlo-
roaniline, bp 250 (252) °C; o-methylaniline, bp 198 (200) °C;
o-methoxyaniline, bp 223 (225) °C; o-ethoxyaniline, bp 228
(230) °C; o-chloroaniline, bp 207 (209) °C; o-nitroaniline, mp
(water) 71 (72) °C; o-aminobenzoic acid, mp (water) 145 (144-
8) °C; methyl o-aminobenzoate, bp 256 (256) °C; o-amino-
acetophenone, bp 249 (250-2) °C; o-phenylenediamine, mp
(water) 102 (104) °C; N-methylaniline, bp 194 (196) °C; N,N-
dimethylaniline, bp 191 (193) °C; o-aminophenol, mp (water)
173 (174) °C; o-aminothiophenol, bp 232 (234) °C; given in
parentheses are the literature values.
Su p p or tin g In for m a tion Ava ila ble: The method of
calculation of the concentration of free aniline base and the
pKa values used. This material is available free of charge via
the Internet at http://pubs.acs.org.
J O0158433