Correlation analysis of reactivity in the oxidation of anilines by nicotinium dichromate in nonaqueous media

Article abstract of DOI:10.1002/kin.20199

The kinetics of oxidation of 15 para- and meta-substituted anilines by nicotinium dichromate (NDC) in different organic solvent media in the presence of p-toluenesulfonic acid (TsOH) has been investigated. The rate of the reaction is zero order with respect to substrate, first order in NDC, and is found to increase with increase in [TsOH]. The various thermodynamic parameters for the oxidation have been reported and discussed along with the validity of the isokinetic relationship. The specific rate of oxidizing species-anilines reaction (k₂) correlates with Hammett's substituent constants affording negative reaction constants. The effect of paraand meta-substituents on the oxidation rates confirms to Swain et al.'s substituent constants F and R. both with negative reaction constants. The rate data failed to correlate with macroscopic solvent parameters such as ε_r, and E _T^N while showing satisfactory correlation with Kamlet-Taft's solvatochromic parameters (α. β. and π*).

Full text of DOI:10.1002/kin.20199

Correlation Analysis of
Reactivity in the Oxidation
of Anilines by Nicotinium
Dichromate in Nonaqueous
Media
D. S. BHUVANESHWARI, K. P. ELANGO
Department of Chemistry, Gandhigram Rural Institute (Deemed University), Gandhigram 624 302, India
Received 10 November 2005; revised 18 March 2006; accepted 29 March 2006
DOI 10.1002/kin.20199
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The kinetics of oxidation of 15 para- and meta-substituted anilines by nicotinium
dichromate (NDC) in different organic solvent media in the presence of p-toluenesulfonic acid
(TsOH) has been investigated. The rate of the reaction is zero order with respect to substrate,
first order in NDC, and is found to increase with increase in [TsOH]. The various thermodynamic
parameters for the oxidation have been reported and discussed along with the validity of the
isokinetic relationship. The specific rate of oxidizing species-anilines reaction (k2) correlates
with Hammett’s substituent constants affording negative reaction constants. The effect of para-
and meta-substituents on the oxidation rates confirms to Swain et al.’s substituent constants F
and R, both with negative reaction constants. The rate data failed to correlate with macroscopic
solvent parameters such as εr and EN wh∗ile showing satisfactory correlation with Kamlet–Taft’s
T
solvatochromic parameters (α, β, and π ). ꢀ 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38:
C
657–665, 2006
tablished that the technique of correlation analysis
may well be used to separate, quantify, and rationalize
such solvent–solvent–solute interactions on reactivity
[1–5].
INTRODUCTION
It has been shown that reactivity is influenced by
the preferential solvation of the reactants and/or
the transition state through nonspecific and specific
solvent–solvent–solute interactions. The kinetics of
oxidation of organic compounds in nonaqueous and
aquo-organic solvent media have revealed the im-
portant role of nonspecific and specific solvent ef-
fects on the reactivity. Furthermore, it has been es-
Further, one of the important tools in deciding the
mechanism of reactions is the study of substituent
effects and thermodynamic parameters. The Hammett
equation and its modified forms [6], all known as linear
free energy relationships (LFER), have been found to
be useful for correlating reaction rates and equilibrium
constants for side chain reactions for meta- and para-
substituted derivatives. The isokinetic relationship is
also an important tool for deciding the nature of a
mechanism. Keeping this in view, a systematic study of
Correspondence to: K. P. Elango; e-mail: drkpelango@rediff
mail.com.
c
ꢀ
2006 Wiley Periodicals, Inc.

658
BHUVANESHWARI AND ELANGO
various LFERs and the isokinetic relationship has been
made to establish the role of solvent and substituents
on reactivity and to decide the nature of the mechanism
being followed in the nicotinium dichromate (NDC, a
mild and selective oxidant [7]) oxidation of some meta-
and para-substituted anilines.
Data Analysis
Correlation analyses were carried out using Microcal
origin (version 6) computer software. The goodness of
the fit was discussed using the correlation coefficient,
standard deviation (SD), and Exner’s statistical
parameter (ψ). The percentage contribution (Px)
of a parameter to the total effect on reactivity was
computed as reported earlier [8].
This article focuses on the study of kinetics and
mechanism of oxidation of substituted anilines by
NDCinnonaqueousmedia. Anilines(aromaticamines)
are the most widespread and principal contaminants
of industrial wastewaters. These comprise an impor-
tant class of environmental contaminants: they are the
building blocks for many textile dyes, agrochemicals,
and other classes of synthetic chemicals. The reac-
tion pathways of aromatic amines in natural systems
are dominated by redox reactions with soil and sedi-
ment constituents. Better understanding of the mech-
anism of oxidation of such compounds/contaminants
to harmless products is an important goal for basic re-
search and industrial applications; hence, the present
study.
Stoichiometry
The stoichiometry of the reaction was determined by
carrying out several sets of experiments with vary-
ing amounts of [NDC] largely in excess over [sub-
strate]. The estimation of unconsumed NDC showed
that 3 mol of the substrate react with 2 mol of
NDC.
Product Analysis
The oxidation product was analyzed using preparative
TLC on silica gel, which yielded azobenzene mp 66◦C
(lit. 68◦C), UV (EtOH) λmax 320 nm.
EXPERIMENTAL
Materials
RESULTS AND DISCUSSION
All the chemicals and solvents used were of analyt-
ical grade. The solvents 1,4-dioxane (Diox), acetic
acid (AcOH), tert-butanol (t-BuOH), ethyl methyl ke-
tone (EMK), acetone (MeCOMe), nitrobenzene (NB),
dimethyl formamide (DMF), and acetonitrile (MeCN)
are of analytical grade and were purified by con-
ventional methods. The anilines used were with sub-
stituents H, p-Me, p-OMe, p-COMe, p-NHCOMe,
p-NO2, p-Cl, p-Br, p-F, m-Me, m-COOH, m-NO2,
m-Et, m-OMe, and m-COMe. The solid anilines
were used as such, and the liquid anilines were
used after vacuum distillation. Nicotinium dichro-
mate was prepared by the reported method [7],
and its purity was checked by the iodometric
method.
The kinetic studies were carried out under pseudo-first-
order conditions with the [substrate] ꢁ [NDC]. The
first-order dependence of the reaction on NDC is ob-
vious from the linearity of the plots of log [NDC]
versus time. Further, the pseudo-first-order rate con-
stants, kobs, do not depend on the initial concentra-
tion of NDC (Table I). The oxidation is zero order
in the substrate, in both the presence and absence
of acid (Table I). The oxidation of anilines by NDC
in DMF is remarkably slow, but is catalyzed in the
presence of p-toluene sulfonic acid, and the reaction
proceeds at a comfortable rate. The kobs values in-
crease with increase in the initial concentration of
TsOH, and a plot of log kobs versus log [TsOH] is
linear (figure is not shown, r = 0.997, SD = 0.021,
ψ = 0.10). The reaction did not promote polymeriza-
tion of acrylonitrile indicating the absence of free
radicals. However, addition of Mn(II) (0.003 M), in
the form of MnSO4, retards the rate of the oxida-
tion process (Table I) indicating two electron oxida-
tion. This indicates the involvement of Cr(IV) inter-
mediate in the oxidation of anilines by Cr(VI) reagent.
Mn(II) ion reduces Cr(IV) formed to Cr(III). In the ab-
sence of Mn(II) ion, formed Cr(IV) reduces Cr(VI)
to Cr(V) and the oxidation of aniline by Cr(V) is
fast [9].
Kinetic Measurements
The reactions were carried out under pseudo-first-order
conditions by keeping an excess of substrate over NDC.
The progress of the reactions was followed by esti-
mating the unreacted oxidant iodometrically at 26, 34,
42, and 49 ( 0.1)◦C. The rate constants were deter-
mined by the least-squares method, from the linear
plots (r > 0.96) of log [NDC] versus time. Replicate
runs showed that the rate constants were reproducible
to within 3%.

CORRELATION ANALYSIS OF REACTIVITY IN THE OXIDATION OF ANILINES
659
Table I Pseudo-First-Order Rate Constants for the
Oxidation of Aniline by NDC at 299 K in DMF Medium
magnitude. However, because of the difference in the
polarity of different anilines, the extent of solvation
should be different and hence the experimental value
of ꢀS# may be different for different anilines, as ob-
served by us in the present case [11].
The activation energies and the entropies and en-
thalpies of activation were also calculated for the ox-
idation of aniline in the eight organic solvent media
(Table III). Negative entropy of activation indicates a
greater degree of ordering in the transition state than
in the initial state, due to an increase in solvation dur-
ing the activation process. The existence of a linear
relationship (Fig. 2, r = 0.977, SD = 0.076, ψ = 0.26)
between log kobs at 307 K and log kobs at 299 K in-
dicates that a single mechanism is operating in all the
solvent systems studied.
× 102[aniline] × 103[NDC] × 102[H+] × 105 kobs
(M)
(M)
(M)
(s−1
)
2.0
3.0
4.0
5.0
2.0
3.0
4.0
5.0
2.0
3.0
4.0
5.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
3.0
4.0
5.0
2.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.5
2.0
2.5
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
4.0
5.0
6.0
7.0
–
12.1
12.3
12.2
11.9
7.3a
7.1a
6.8a
7.0a
23.8b
23.5b
24.0b
23.9b
12.1
11.8
12.2
12.0
8.1
Structure-Reactivity Correlation
The effect of substituents on the oxidation rate was
studied with 15 para- and meta-substituted anilines in
eight organic solvent media (Table IV). The results in
Table IV reveal that the rate constants vary with sub-
strate in a particular solvent, though the rate of the
reaction is independent of [substrate]. This may be due
to the fact that because of the difference in polarity
of different anilines, the extent of solvation should be
different and hence the experimental values of rate con-
stantsmaybedifferentfordifferentanilinesasobserved
by us in the present study. The rate data failed to con-
form to either the usual Hammett equation or modified
ones; plots of log kobs versus substituent constants are
nonlinear (not shown). The dual substituent parameter
(DSP) equations [12,13] also failed to correlate the rate
data with the substituent.
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 ra-
tio of the concentrations of the free bases to the con-
jugate acid ([XC6H4NH2]/[XC6H4NH+3 ]) depends on
the pKa of the aniline and the acidity of the medium.
Thereportedoxidationsofanilines, inthepresentstudy,
were carried out under pseudo-first-order conditions
([anilines] ꢁ [NDC]), and the concentration of NDC
at different reaction times was determined by titrime-
try. The pseudo-first-order rate constants (kobs) were
obtained from the least-squares slopes of log [NDC]
versus time plots, and the second-order rate constants
are kꢂ = kobs/[aniline]T, where [aniline]T is the total
concentration of aniline. Since the pKa varies from
5.35 (p-OMe) to 1.01 (p-NO2) and molecular ani-
lines are the reactive species (nucleophile), the reported
kobs/[aniline]T values are not the rate constants of the
oxidant-molecular aniline reactions. And the analysis
12.1
19.5
25.7
0.63
0.59
0.60
0.58
10.4c
–
–
–
5.0
a
p-Me substituted aniline.
b in MeCN.
c In the presence of 0.003 M Mn(II).
Activation Parameters
The activation parameters were calculated from kobs at
299, 307, 315, and 322 K using the van’t Hoff plot by
themethodofleastsquaresandarecollectedinTable II.
The operation of isokinetic relationship is tested by
plotting the logarithms of rate constants at two temper-
atures (T2 > T1) against each other according to Eq. (1)
as suggested by Exner [10].
log k (at T2) = a + b log k(at T1)
(1)
In the present study, linear plots imply the validity of
isokinetic relationship. A representative plot is shown
in Fig. 1 (in DMF medium, r = 0.984, SD = 0.036,
ψ = 0.19, isokinetic temperature = 435 21 K). The
operation of isokinetic relationship reveals that all the
substituted anilines examined are oxidized through a
common mechanism.
Since the reactions are of ion-dipolar type, it is ex-
pected that the entropy of the activated complex for all
these anilines should be nearly of the same order of

660
BHUVANESHWARI AND ELANGO
Table II Effect of Temperature on the Rate of Oxidation of Substituted Anilines by NDC in Dimethylformamide and
Activation Parameters for the Oxidation
Rate Constants × 105 kobs (s−1
)
Substituents
in Aniline
Ea
ꢀH#
(kJ mol−1
−ꢀS#
ꢀG#
(kJ mol−1
)
299 K 307 K 315 K
322 K
(kJ mol−1 K−1
)
)
(J K−1 mol−1
)
H
p-Me
12.1
7.36
16.8
21.9
15.7
12.3
14.9
12.4
12.5
17.2
15.9
15.5
27.9
12.1
15.9
19.7
25.8
18.5
29.8
21.9
21.5
28.0
19.6
20.4
20.2
21.5
18.5
37.7
19.8
21.7
22.2
31.6
22.0
34.8
30.8
25.2
35.2
22.2
22.6
23.2
25.3
19.8
50.9
21.2
27.0
27.6
40.6
16
41
32
48
54
48
38
19
35
26
60
55
37
21
32
14
39
30
46
52
46
36
17
31
24
57
53
35
19
30
273
188
220
170
150
171
201
261
216
240
128
149
205
254
218
95.3
95.6
95.6
96.9
96.5
97.1
96.5
95.4
96.0
96.1
95.8
97.5
96.1
95.1
94.8
p-OMe
p-COMe
p-NHCOMe
p-NO2
p-Cl
p-Br
p-F
m-Me
m-COOH
m-NO2
m-Et
m-OMe
m-COMe
9.60
4.44
5.06
3.78
5.75
11.3
7.36
7.35
5.18
2.76
6.89
12.3
12.2
[Substrate] = 2 × 10−2 M; [NDC] = 1 × 10−3 M; [H+] = 5 × 10−2 M.
of kobs and kobs/[aniline]T in terms of the Hammett and
modified Hammett equations is erroneous.
in Table V. p-NHCOMe, p-F, and m-Et are not included
due to nonavailability of their pKa values.
Hence, the specific reaction rates of molecular ani-
lines with the oxidant {k2 = kꢂ(Ka + [H+])/Ka} have
been obtained, as reported earlier for the oxidation of
anilines by percarbonate [14] and perborate [15], and
correlated in terms of the structure–reactivity relation-
ships. In the reactions of anilines in acid medium, as
the free bases are the nucleophiles, the specific reaction
rates of anilines are to be obtained using the concentra-
tions of the free bases but not the total concentrations
of anilines. The concentrations of the free bases, in the
present study, have been deduced as reported earlier by
adopting the similar approximations. The specific rates
of the oxidant-molecular aniline reactions are collected
The specific rates of the rate-limiting step of para-
and meta-substituted anilines correlate satisfactorily
with the Hammett’s substituent constants, and a rep-
resentative plot is given in Fig. 3. The statistical results
and the reaction constant values in different solvents
are given in Table VI. The negative reaction constant
indicates the formation of a positively charged acti-
vated complex with extensive bond formation between
the reactants [6]. The reaction constant is comparable
to that of the percarbonate oxidation of molecular ani-
line in glacial acetic acid (ρ = −2.7 0.1 [14]), perbo-
rate oxidation of molecular aniline in glacial acetic
acid (ρ = −3.0 [15]), pyridinium chlorochromate ox-
idation of anilines in chlorobenzene-nitrobenzene
mixtures (ρ = −3.75 [16]), and imidazolium fluo-
rochromate oxidation of anilines in DMF (ρ = −2.22
[17]). The specific rate constants for the meta-
and para-substituted anilines correlate satisfacto-
rily with σ+(0.841 <r < 0.936, 0.31 < SD < 0.62,
−1.33 0.21 < slope < −2.03 0.41) and σ−p and
σm (0.900 <r < 0.966, 0.26 < SD < 0.47, −1.51
0.19 < slope < −2.28 0.32) also.
It is clear that ρ values, like rate constant values
for given reactants, are influenced by solvent effects.
Hammett predicted that, in general, the reaction con-
stant appears to increase with decreasing relative per-
mittivity of the medium. In the present study, however,
there is a very marked deviation from the relationship
between ρ and εr (Fig. 4). This suggests that ρ val-
ues are influenced both by nonspecific and by specific
solvent effects [6].
Figure 1 The isokinetic plot for the oxidation of anilines in
DMF.

CORRELATION ANALYSIS OF REACTIVITY IN THE OXIDATION OF ANILINES
661
Table III Effect of Temperature on the Rate of Oxidation of Aniline by NDC in Different Solvents and Activation
Parameters for the Oxidation
Rate Constants × 105 kobs (s−1
)
Ea
ꢀH#
(kJ mol−1
−ꢀS#
ꢀG#
(kJ mol−1
)
Solvents
299 K
307 K
315 K
322 K
(kJ mol−1 K−1
)
)
(J K−1 mol−1
)
Diox
27.8
19.6
35.7
49.4
30.4
123.5
12.1
23.8
44.4
25.5
52.7
51.1
58.9
143.9
16.1
30.1
86.3
35.6
64.6
137.4
51.6
89.8
117.6
163.5
253.2
25.0
454
269
247
248
485
201
205
287
43
25
23
23
46
18
18
27
165
231
233
231
153
239
257
223
92.9
94.2
92.5
92.1
92.4
89.7
95.3
93.6
AcOH
t-BuOH
EMK
MeCOMe
NB
75.0
129.3
186.9
21.4
DMF
MeCN
48.6
64.2
(PF), where PF = |F|∗ 100/(|F| + |R|), andpercentres-
onance factor (PR), where PR = |R|∗ 100/(|F| + |R|)
[10]. In all the solvents studied, the meta-substituted
anilines show PF values of ca. 77% and the PR values
are ca. 23%. In meta-compounds, the inductive effect
predominates and the resonance effect is secondary.
In the case of the para-substituted compound, the ob-
served results (i.e., inductive and resonance effects
operating almost equally) are as expected. Electron
release favors radical-cation formation, and the neg-
ative reaction constants are in consonance with the
mechanism.
Solvent-Reactivity Correlation
Figure 2 The isokinetic plot for the oxidation of aniline in
all solvents.
The reaction has been studied in eight organic solvents
with a range of ca. 35 units of relative permittivity. The
rate data (Table IV) failed to correlate either with rela-
tive permittivity of the medium, εr (explained variance,
100r2 < 50), or with Reichard’s polarity parameters
ETN(100r2 < 60). This suggests the operation of se-
lective or preferential solvation, which includes both
nonspecific and specific solute–solvent interactions, is
Multiple correlation analysis of the rate of para- and
meta-substituted anilines using the biparametric equa-
tions including Swain et al.’s equation reveals that that
substituent constants F and R explain better than oth-
ers the variation of rate with substituents (Table VII).
A better description of the composition of the elec-
trical effects is given in terms of percent field factor
Table IV Pseudo-First-Order Rate Constants [× 105 kobs (s−1)] for the Oxidation of Meta- and Para-Substituted
Anilines by NDC at 26 0.1◦C
Substituents in Aniline Moiety
Para Position
Meta Position
Me COOH NO2 Et OMe COMe
Solvent
H
Me OMe COMe NHCOMe NO2
Cl
Br
F
Diox
AcOH
27.8 25.0 46.8 40.4
19.6 22.6 96.1 26.3
216
124
111 53.6
59.7 16.4
22.8 56.9 40.5 42.4 37.1 54.8 42.5 40.2
19.5 18.3 23.1 34.2 17.5 15.2 38.2 24.5
t-BuOH 35.7 5.20 7.17 5.72
6.27
76.7
73.6
77.7
5.06
112
25.5
5.72
6.21 7.85 22.0
29.1 19.2 25.0 27.6 31.8 39.2 49.1 23.8
23.2 17.7 18.2 18.5 24.2 23.8 40.0 22.7
18.7 25.4 84.1 19.8 15.4 43.5 33.2
7.36 7.35 5.18 2.76 6.82 12.3 12.2
19.2 18.3 18.1 19.4 34.7 52.3 46.4 22.1
8.75 6.73 9.95 14.2 19.3
EMK
MeCOMe 30.4 18.5
NB
49.4 30.4
113 51.9
145 53.6
191 77.7
23.1 39.4
16.1 30.9
17.5 50.0
3.78 5.75 11.3
14.7 12.5
12.3 77.7
9.69
DMF
MeCN
12.1 7.36 9.60 4.44
23.8 19.2 35.6 19.5
[Substrate] = 2 × 10−2 M; [NDC] = 1 × 10−3 M; [H+] = 5 × 10−2 M.

662
BHUVANESHWARI AND ELANGO
Table V Specific Rate (× 103k2, dm3 mol−1
s
−1) of the Oxidant-Molecular Aniline Reaction at 299 K
Substituents in Aniline Moiety
Para Position
Meta Position
COOH NO2 OMe COMe
Solvent
H
Me
OMe
COMe
NO2
Cl
Br
Me
Diox
3774
2659
4854
6718
4128
16787
1642
3233
10950
9910
2275
13296
8114
33989
3220
8412
39890
8188
6115
96678
123527
162655
8179
197
128
28
254
262
380
22
197
106
45
3070 1021
7942
4532
4313
4907
4532
16488
1441
354
586
475
121
383
257
276
72
283
133
51
242
184
117
21
3664
3290
123
4230
3183
2862
1057
3997
1172
715
564
695
664
283
357
646
AcOH
t-BuOH
EMK
MeCOMe
NB
942
328
872
277
41 2256 1302
28 1769 1038
31 2863
7
26
838
504
858
DMF
MeCN
329
717
30361
95
269
264
Table VI Results of Simple Linear Correlation of the
Specific Rate (k2) of Oxidant-Molecular Aniline Reaction
at 299 K in All the Organic Solvents Investigated
likely in the present case [18]. This is in line with the
results of the relation between ρ and εr that the reac-
tivity is influenced both by nonspecific and by specific
solvent effects.
Solvent
εr
100r2
SD
ρ
To explain this type of dual dependency of rate on
solvent, the kinetic data were correlated with Kamlet–
Taft’s [19] solvatochromic parameters α, β, and π∗ in
the form of the following LSER:
Diox
2.21
6.15
91
91
85
93
90
95
94
90
0.23
0.27
0.34
0.26
0.31
0.27
0.25
0.29
−2.11 0.21
−2.45 0.25
−2.33 0.31
−2.65 0.23
−2.70 0.28
−3.28 0.25
−2.81 0.23
−2.47 0.27
AcOH
t-BuOH
EMK
MeCOMe
NB
12.47
15.45
20.70
34.82
36.71
37.50
log k = Ao + sπ∗ + aα + bβ
(2)
DMF
MeCN
where π∗ is solvent dipolarity/polarizability, α is sol-
vent hydrogen bond donor acidity (HBD), and β is
solvent hydrogen bond acceptor basicity.
The regression coefficients s, a, and b measure the
relative susceptibilities of the solvent-dependent solute
propertylogkobs totheindicatedsolventparameter. The
rates of oxidation for all the compounds studied show
a satisfactory correlation with solvent via the above
LSER (Eq. (2)) with an explained variance of about
85%. Such a correlation indicates the existence of both
specific and nonspecific solute–solvent interactions in
the present study.
Figure 3 Hammett plot for the oxidation of anilines in DMF
Figure 4 Effect of relative permittivity of the medium on
at 299 K.
the reaction constant of the reaction.

CORRELATION ANALYSIS OF REACTIVITY IN THE OXIDATION OF ANILINES
663
Table VII Statistical Results of Multiple Linear Correlation Analysis for the Oxidation of Anilines by NDC in Different
Solvents
Solvent
100R2
SD
ρF
ρR
PF
PR
Para-substitutents
−0.62 0.24
−0.69 0.25
−1.17 0.45
−1.07 0.11
−0.92 0.21
−1.60 0.22
−1.23 0.26
−1.13 0.07
Diox
96
97
88
99
98
98
97
99
0.22
0.23
0.40
0.10
0.19
0.20
0.23
0.07
−0.84 0.10
−1.01 0.11
−0.73 0.19
−1.04 0.05
−1.11 0.09
−1.06 0.09
−0.99 0.11
−0.97 0.03
42
41
62
51
45
60
55
54
58
59
38
49
55
40
45
46
AcOH
t-BuOH
EMK
MeCOMe
NB
DMF
MeCN
Meta-substitutents
−0.96 0.25
−0.98 0.17
−1.63 0.46
−0.99 0.32
−0.99 0.34
−1.85 0.49
−1.37 0.46
−0.69 0.32
Diox
92
97
87
90
89
90
87
87
0.20
0.14
0.37
0.25
0.27
0.40
0.37
0.26
−0.28 0.01
−0.33 0.06
−0.27 0.17
−0.35 0.12
−0.34 0.13
−0.37 0.18
−0.35 0.17
−0.37 0.12
77
75
86
74
74
83
80
65
23
25
14
26
26
17
20
35
AcOH
t-BuOH
EMK
MeCOMe
NB
DMF
MeCN
From the values of the regression coefficients, the
contribution of each parameter, on a percentage basis,
to reactivity was calculated and is listed in Table VIII.
The observation of this multiple regression analysis
leads us to the following preliminary conclusions:
(i) The rate of the reaction is strongly influenced by
specific solute–solvent interactions as indicated by
the percentage contributions of α and β parameters.
(ii) The negative sign of the coefficients of α and β
terms suggest that the specific interaction between the
reactants and the solvent is more than that between the
transition state and the solvent. (iii) The solvent HBA
basicity, as indicated by the β term, plays a dominant
role in governing the reactivity. It alone explains more
than 50% of the observed solvent effect. The rate of the
reaction is very much slower in tert-butanol, a typical
HBA solvent, than in other solvents. Since it can form
relatively strong complexes with solutes by hydrogen
bonding and such a complexing solvent acts as a very
mild form of blocking reagent [20], the approach of the
oxidizing species during activation is relatively diffi-
cult; consequently, the rate of the reaction is very slow
in t-BuOH. On the basis of complex formation between
the anilines and solvents, it would be expected that the
Table VIII Statistical Results and Weighted Percentage Contributions for the Correlation of Rate of Oxidation (log
obs) of Substituted Anilines by NDC with Kamlet–Taft’s Solvatochromic Parameters α, β, and π∗
k
Substituent
R2
SD
a
b
s
Pꢀ
Pꢁ
Pꢂ∗
H
p-Me
0.712
0.806
0.726
0.768
0.993
0.828
0.794
0.805
0.893
0.713
0.942
0.887
0.839
0.874
0.891
0.36
0.21
0.38
0.29
0.07
0.24
0.24
0.13
0.12
0.33
0.10
0.17
0.18
0.11
0.08
0.175
0.024
0.204
−0.426
−1.482
−2.371
−2.161
−3.392
−1.661
−1.868
−1.022
−1.475
−0.935
−1.608
−2.061
−1.663
−1.225
−0.637
0.048
0.223
27
1
8
4
2
5
10
4
6
4
4
3
11
0
3
66
86
88
74
66
42
63
76
58
69
57
55
57
66
35
7
13
4
p-OMe
p-COMe
p-NHCOMe
p-NO2
p-Cl
p-Br
p-F
m-Me
m-COOH
m-NO2
m-Et
m-OMe
m-COMe
−0.120
−0.635
−1.645
−2.135
−0.823
−0.269
−0.925
−0.366
−1.099
−1.596
−0.929
−0.627
−1.146
−0.115
22
32
53
27
20
36
27
39
42
32
34
62
0.133
0.200
−0.293
−0.049
−0.161
−0.046
0.115
−0.112
−0.329
0.009
−0.051

664
BHUVANESHWARI AND ELANGO
Scheme 1
oxidation would be faster in nonpolar solvents than
in polar solvents and the observed results are approx-
imately in the same lines. Parallel observations were
recorded by us in the imidazolium fluorochromate ox-
idation of anilines in neat organic solvents [17]. (iv).
The solvent polarizability/dipolarity, as indicated by
Conflicting ideas have been prevailing as to the true
mechanism operative in halochromates oxidation of
organic compounds [23]. The two mechanisms pro-
posed are (i) direct hydride ion loss in a concerted
process and (ii) prior formation of an ester in a re-
versible step, preceding the loss of a hydride ion. In
the present study, kinetics and solvent and structural
variation studies favor our choice of the second mech-
anism, i.e., reversible formation of a complex (very
likely concerted transfer of oxygen from the oxidant to
N-atom and electron pair from N-atom to Cr(VI) lead-
ing to the formation of a complex) preceding the H−
loss. The negative ρ value is indicative of the presence
of a positive nitrogen center, which would mean deple-
tion of lone-pair electron density, and this can be facil-
itated only when the oxidant forms a complex with the
substrate in which the nitrogen lone-pair can be used
up in coordinating with an electron-deficient center,
preferably a metal ion. Similar mechanism has been
postulated for the oxidation of anilines by pyridinium
chlorochromate in chlorobenzene–nitrobenzene mix-
ture[16]andbynicotiniumdichromateinbenzene-tert-
butanol mixtures [22]. Further, such a complex forma-
tion is supported by the negative entropy of activation
observed.
∗
Pꢂ , also plays an appreciable role in governing the re-
activity. The negative sign of the coefficient of this term
suggests that with decrease in polarizability/dipolarity
of the medium the rate of the oxidation process will
increase and the rate is maximum in apolar solvents as
observed from Table IV.
Mechanism
Under the reported anhydrous condition, hydrolysis of
the dichromate to the chromate anion is unlikely. A pe-
rusal of data in Table II indicates that the energy of acti-
vation of the oxidation is susceptible to the substituent
present in the benzene ring. This indicates the involve-
ment of substrate in or prior to the rate-limiting step in
such a way that the rate is independent of [substrate].
In nonaqueous media, Cr(VI) reagents complex with
the aniline and the reaction exhibits Michaelis–Menten
kinetics with respect to the substrate. If the formation
constant (K) of the oxidant–substrate complex is large,
the oxidation is to exhibit zero-order dependence on
[substrate] [21,22] (Scheme 1).
BIBLIOGRAPHY
The above mechanism leads to, under the condition
that the formation constant (K) of the oxidant–substrate
complex is large, the following rate law:
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−d[NDC]/dt = k[NDC] [TsOH]
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The above scheme accounts for the observed orders.
The formation of the reaction product azobenzene may
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O
O
ArNH2 → ArNHOH → ArNO + H2O
ArNH2 + ArNO → Ar N N Ar + H2O
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Scheme 2

CORRELATION ANALYSIS OF REACTIVITY IN THE OXIDATION OF ANILINES
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