Studies on the kinetics of imidazolium fluorochromate oxidation of some meta- and para-substituted anilines in nonaqueous media

Article abstract of DOI:10.1002/kin.20150

The imidazolium fluorochromate (IFC) oxidation of meta- and para-substituted anilines, in seven organic solvents, in the presence of p-toluenesulfonic acid (TsOH) is first order in IFC and TsOH and is zero order with respect to substrate. The IFC oxidation of 15 meta- and para-substituted anilines at 299-322 K complies with the isokinetic relationship but not to any of the linear free energy relationships; the isokinetic temperature lies within the experimental range. The specific rate of oxidizing species-anilines reaction (k₂) correlates with substituent constants affording negative reaction constants. The rate data failed to correlate with macro-scopic solvent parameters such as ε_r and E_T^N. A correlation of rate data with Kamlet-Taft solvatochromic parameters (α, β, π*) suggests that the specific solute-solvent interactions play a major role in governing the reactivity, and the observed solvent effects have been explained on the basis of solute-solvent complexation.

Full text of DOI:10.1002/kin.20150

Studies on the Kinetics
of Imidazolium
Fluorochromate Oxidation
of Some meta- and
para-Substituted Anilines
in Nonaqueous Media
D. S. BHUVANESHWARI, K. P. ELANGO
Department of Chemistry, Gandhigram Rural Institute (Deemed University), Gandhigram 624 302, India
Received 23 September 2004; revised 27 September 2005; accepted 4 October 2005
DOI 10.1002/kin.20150
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The imidazolium fluorochromate (IFC) oxidation of meta- and para-substituted ani-
lines, in seven organic solvents, in the presence of p-toluenesulfonic acid (TsOH) is first order
in IFC and TsOH and is zero order with respect to substrate. The IFC oxidation of 15 meta- and
para-substituted anilines at 299–322 K complies with the isokinetic relationship but not to any
of the linear free energy relationships; the isokinetic temperature lies within the experimental
range. The specific rate of oxidizing species-anilines reaction (k2) correlates with substituent
constants affording negative reaction constants. The rate data failed to correlate with macro-
scopic solvent parameters such as εr and ETN. A correlation of rate data with Kamlet–Taft sol-
vatochromic parameters (α, β, π∗) suggests that the specific solute–solvent interactions play
a major role in governing the reactivity, and the observed solvent effects have been explained
on the basis of solute–solvent complexation. ꢀ 2006 Wiley Periodicals, Inc. Int J Chem Kinet
C
38: 166–175, 2006
reactants and/or the transition state through nonspe-
cific and specific solvent–solvent–solute interactions.
Furthermore, it has been established that the technique
of correlation analysis may well be used to separate,
quantify, and rationalize such solvent–solvent–solute
interactions on reactivity [1–5].
Further, one of the important tools in deciding the
mechanism of reactions is the study of substituent ef-
fects and thermodynamic parameters. The Hammett
equation and its modified forms [6], all known as lin-
ear free energy relationships (LFER), have been found
INTRODUCTION
The kinetics of oxidation of organic compounds in non-
aqueous and aquo-organic solvent media have revealed
the important role of nonspecific and specific solvent
effects on the reactivity. It has been shown that the reac-
tivity is influenced by the preferential solvation of the
Correspondence to: K. P. Elango; E-mail: drkpelango@
rediffmail.com.
c
ꢀ
2006 Wiley Periodicals, Inc.

STUDIES ON THE KINETICS OF IMIDAZOLIUM FLUOROCHROMATE OXIDATION
167
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
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 be-
ing followed in the imidazolium fluorochromate (IFC,
a mild and selective oxidant, reported only recently [7])
oxidation of some meta- and para-substituted anilines.
This article focuses on the study of kinetics and
mechanism of oxidation of substituted anilines by
IFC in nonaqueous media. Anilines (aromatic amines)
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 class of synthetic chemicals. The reaction
pathways of aromatic amines in natural systems are
dominated by redox reactions with soil and sediment
constituents. Better understanding of the mechanism of
oxidation of such compounds/contaminants to harm-
less products is the important goal for basic research
and industrial applications, hence, the present study.
plots (r > 0.97) of log [IFC] versus time. Replicate
runs showed that the rate constants were reproducible
to within 3%.
Data Analysis
Correlation analyses were carried out using Microcal
origin (version 6) computer software. The goodness of
the fit was discussed using correlation coefficient (r in
the case of simple linear regression and R in the case
of multiple linear regression), standard deviation, sd,
and Exner’s statistical parameter, ꢀ. The percentage
contribution (Px) of a parameter to the total effect on
reactivity was computed using the regression coeffi-
cient of each parameter as reported earlier [8].
Stoichiometry
The stoichiometry of the reaction was determined by
carrying out several sets of experiments with vary-
ing amounts of [IFC] largely in excess over [aniline].
The estimation of unreacted IFC showed that 1 mol of
aniline reacts with 1 mol of IFC.
Product Analysis
The oxidation product was analyzed using prepara-
tive TLC on silica gel, which yielded the following
fractions:
EXPERIMENTAL SECTION
Materials
(1) Azobenzene mp 66◦C (lit. 68◦C), UV (EtOH)
λmax 320 nm.
All the chemicals and solvents used were of analytical
grade. The solvents tert-butanol (t-BuOH), dimethyl
formamide (DMF), dimethyl sulfoxide (DMSO), ace-
tonitrile (MeCN), nitrobenzene (NB), chlorobenzene
(CB), and 1,4-dioxane (Diox) are of analytical grade
and were purified by conventional methods. The ani-
lines used were with substituents 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.
Imidazoliumfluorochromate(IFC)waspreparedbythe
reported method [7], and its purity was checked by the
iodometric method.
(2) p-Benzoquinone mp 111◦C (lit. 114◦C), UV
(EtOH) λmax 246 nm.
RESULTS AND DISCUSSION
The kinetic studies were carried out under pseudo-first-
orderconditionswiththe[substrate] ꢁ [IFC]. Thefirst-
order dependence of the reaction on IFC is obvious
from the linearity of the plots of log [IFC] versus time.
Further, the pseudo-first-order rate constants, kobs, do
not depend on the initial concentration of IFC (Table I).
The oxidation is zero order in the substrate, both in the
presence and absence of acid; the pseudo-first-order
rate constant remains constant at different [substrate]o.
The oxidation of anilines by IFC in acetonitrile is re-
markably slow, but is catalyzed in the presence of p-
toluene sulfonic acid, and the reaction proceeds at a
comfortable rate. The kobs values varied with variation
in the initial concentration of TsOH and dependence
on acid was also observed to be unity, as seen from
linear plot (r = 0.996, sd = 0.02, ꢀ = 0.12, slope =
0.99 0.05) of log kobs versus log [TsOH]. Catalysis
Kinetic Measurements
The reactions were carried out under pseudo-first-order
conditions by keeping an excess of substrate over IFC.
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

168
BHUVANESHWARI AND ELANGO
Table I Pseudo-First-Order Rate Constants for the
Oxidation of Aniline by IFC at 299 K in Acetonitrile
Medium
also known as isokinetic relationship (Eq. (1)):
ꢁH# = ꢁH0 + βꢁS#
(1)
102[aniline]
(M)
103[IFC]
(M)
102[H+]
(M)
105 kobs (s−1
)
The isokinetic temperature is the temperature at which
all the compounds of the series react equally fast. Also,
at the isokinetic temperature the variation of substituent
has no influence on the free energy of activation. 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
isoenthalpic series, and the reactivity is determined by
the entropy of activation [11]. In the present oxidation,
the activation enthalpy is linearly related to activation
entropy (in acetonitrile medium, r = 0.993, sd = 3.16,
ꢀ = 0.13, isokinetic temperature = 314 0.01 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 or-
der of magnitude. However, because of difference in
the polarity of different anilines, the extent of solva-
tion should be different and hence the experimental
value of ꢁS# may be different for different anilines, as
observed by us in the present case.
3.0
4.0
5.0
6.0
3.0
4.0
5.0
6.0
3.0
4.0
5.0
6.0
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
3.0
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
1.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.0
1.5
2.0
2.5
3.0
–
13.7
13.3
13.6
13.5
63.2a
58.9a
63.8a
62.5a
14.2b
13.5b
13.8b
13.6b
13.6
12.9
13.4
13.3
6.3
10.0
13.6
15.5
18.9
0.69
0.66
0.66
0.68
10.3c
–
–
–
2.0
The activation energies and the entropies and en-
thalpies of activation were also calculated for the ox-
idation of p-COMe-substituted aniline in the seven
organicsolventmedia(TableIII). Theexistenceofalin-
earrelationship(r = 0.998, sd=2.29, ψ = 0.07, isoki-
a p-Me-substituted aniline.
bIn DMF.
cIn the presence of 0.003 M Mn(II).
netic temperature = 315
0.01 K) between ꢁH# and
ꢁS# indicates that a single mechanism is operating in
all the solvent systems studied. A perusal of data in
Table III indicates that values of ꢁS# vary from 201 to
−173 J K−1 mol−1 which clearly suggests that the ex-
tent of solvation of the activated complex is different in
different solvents. A solvent change from tert-butanol
to 1,4-dioxane causes a 66-fold rate acceleration for
the reaction which corresponds to a decrease in ꢁG# of
10.2 kJ mol−1. 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 results in Table III show
that the largest kobs values are obtained in the apolar sol-
vents correspond to the largest decrease in activation
entropy. This observation can be rationalized because
polar solvents will have some structure corresponding
to the orientation of the dipolar solvent molecules due
to intermolecular solvent–solvent interactions. In less
polar solvents, however, which have only a small or no
dipole moment, the solvent molecules will be relatively
unoriented and consequently have higher entropy.
by p-toluene sulfonic acid suggests protonation of the
IFC (pH of a 0.01 M solution; IFC 2.62, PCC 1.75,
PFC 2.45 [7]) species rather than the aniline molecule,
which would have resulted in retardation. The proto-
nated IFC species is probably difficult to visualize, but
participation of protonated chromium species in Cr(VI)
oxidations is well known [9].
The reaction did not promote polymerization of
acrylonitrile indicating absence of free radicals. How-
ever, addition of Mn(II) retards the rate of the oxidation
process indicating two electron oxidation [10].
Activation Parameters
The activation parameters were calculated from kobs at
299, 307, 315, and 322 K using the Eyring relation-
ship by the method of least squares and are collected
in Table II. The oxidation is neither isoenthalpic nor
isoentropic but complies with the compensation law

STUDIES ON THE KINETICS OF IMIDAZOLIUM FLUOROCHROMATE OXIDATION
169
Table II Effect of Temperature on the Rate of Oxidation of Substituted Anilines by IFC in Acetonitrile and Activation
Parameters for the Oxidation
Rate Constants 105kobs (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
13.6
63.6
1000
5.96
1.55
1.54
6.88
3.48
24.0
19.9
2.00
1.44
20.5
22.2
20.3
54.6
103
69.5
274
106
452
fast
052
057
028
095
128
116
089
062
057
043
074
086
126
030
006
51
55
26
93
67
56
80
60
56
41
72
84
11
28
04
142
120
195
007
093
131
053
126
127
179
086
047
280
219
301
93.8
90.8
84.5
95.4
94.3
95.4
95.6
97.2
93.1
94.0
97.5
98.5
94.1
93.8
94.3
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
1150
27.3
25.2
10.5
29.1
14.2
46.3
25.8
18.6
11.0
26.1
29.4
21.4
1970
89.1
59.9
30.4
75.1
17.6
94.4
59.9
27.5
22.2
29.4
47.2
23.5
183
194
111
180
38.3
187
82.8
33.9
36.4
32.6
64.3
25.4
[Substrate] = 5 × 10−2 M; [IFC] = 1 × 10−3 M; [H+] = 2 × 10−2 M.
Table III Thermodynamic Parameters and Relative Rate Constants at 299 K for the Oxidation of p-COMe-Substituted
Aniline by IFC in Different Solvents
105kobs
(s−1
ꢁH#
(kJ mol−1
ꢁS#
(J K−1 mol−1
ꢁG#
(kJ mol−1
)
rel
obs
Solvent
εr
)
k
)
)
t-BuOH
DMF
DMSO
MeCN
NB
12.47
36.71
46.68
37.50
34.82
5.62
1.33
2.55
5.33
5.96
38.4
42.2
88.1
1
2
4
4.5
29
32
66
161
107
50
93
41
201
36
101
96.9
96.3
95.4
92.7
91.3
90.8
−154
−7
−173
−66
−161
CB
Diox
72
43
2.209
Thus, nonpolar solvents will have a greater entropy
loss as a result of increased solvation during activation
process. Hence, it is presumed that, in the present study,
solvent–solvent interactions play a significant role, in
addition to, solute–solvent interactions in governing
the rate of the reaction [12].
constants may be different for different anilines as ob-
served by us in the present studies. The rate data fail
to conform to the usual Hammett equation; log kobs
(para- and meta-collectively and separately) versus σ
plot is a scatter gram (Table V). The oxidation rates
of para- and meta-substituted anilines are correlated
separately with Hammett σ− and Brown–Okamoto σ+
but also without success (Table V). A typical represen-
tative plot is shown in Fig. 1. The failure of the single
parameter equation to correlate the rate data leads to
the possibility of operation of dual substituent param-
eter (DSP) equations. The biparametric equations fail
to correlate the rate data with the substituent (Table V).
Structure–Reactivity Correlation
The effect of substituents on the oxidation rate was
studied with 15 para- and meta-substituted anilines
in seven organic solvent media (Table IV). The re-
sults in Table IV reveal that the rate constants vary
with substrate in a particular solvent, though the rate
of the reaction is independent of [substrate]. This may
be due to the fact that because of difference in polar-
ity of different anilines, the extent of solvation should
be different and hence the experimental values of rate
The ꢀ and σR values used are those reported by Dayal
I
et al. [13]. The F and R values are those of Swain
et al. [14]. The possible reason for the lack of any linear
free energy relationship is that the isokinetic temper-
ature falls within the experimental temperature (299–
322 K); the isokinetic temperature calculated from the

170
BHUVANESHWARI AND ELANGO
Table IV Pseudo-First-Order Rate Constants [105kobs, (s−1)] for the oxidation of meta- and para-substituted anilines
by IFC at 26 0.1◦C
Substituents in Aniline Moiety
para-Position
meta-Position
Solvents
H
Me OMe COMe NHCOMe NO2 Cl
Br
F
Me COOH NO2 Et OMe COMe
DMSO
MeCN
DMF
NB
0.61 3.22 8.62
13.6 63.6 1000 5.96
13.8 1.59 10.7
117 520 65.2
5.33
5.43
1.55
10.2
42.9
97.6
1,052
141
6.21 3.98 0.77 0.61 0.32
1.54 6.88 3.48 24.0 19.9
2.51 0.93 1.15 0.43 9.97
143 29.6 43.0 41.2 62.4
80.5 0.44 3.09 2.98 5.38
108 25.8 242 81.2 172
169 91.9 36.3 204 109
0.16
2.00
2.47
50.3
0.59
91.8
31.4
1.97 5.59 23.7
1.44 20.3 22.2
0.48 440 29.6
272 30.0 322
267 96.2 377
21.4 24.7 412
24.7 64.5 94.2
15.7
20.3
119
75.7
5.64
18.8
25.8
2.55
38.4
1.33
42.2
88.1
t-BuOH 1.54 15.0 102
CB
Diox
12.2 248 101
5.65 242 253
[Substrate] = 5 × 10−2 M; [IFC] = 1 × 10−3 M; [H+] = 2 × 10−2 M.
Table V Results of Simple and Multiple Linear
Correlation of the Rate Data (kobs) of IFC–Aniline
Reaction at 299 K in All the Organic Solvents
Investigated
Explanatory Variable
100 r2a
n
para- and meta-Substituents
ꢀ
3–72
3–68
6–52
15
12
15
ꢀ+
ꢀ−p and ꢀm
para-Substituents Only
ꢀ
ꢀ
ꢀ
5–71
7–62
1–46
9
8
9
+
–
meta-Substituents Only
ꢀ
ꢀ
6–78
17–95
7
5
+
para-Substituents
Figure 1 Plot of log kobs(DMF) versus σ+.
ꢀI, ꢀR
F, R
2–47
3–52
8–46
9
9
9
F, R∗
meta-Substituents
25–80
study, were carried out under pseudo-first-order con-
ditions ([anilines] ꢁ [IFC]), and the concentration
of IFC at different reaction times was determined by
ꢀI, ꢀR
F, R
7
7
7
62–79
57–72
F, R∗
titrimetry. The pseudo-first-order rate constants (kobs
)
a100 R2 in the case of multiple linear regression analysis.
∗Improved F and R.
were obtained from the least-squares slopes of log
[IFC] 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 molec-
ular anilines 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 of kobs and kobs/[aniline]T in terms
of the Hammett and modified Hammett equations is
erroneous.
isokinetic plot is 314 0.01 K. It is pertinent to note
that the compensation law may lead to artifact [15].
Anilines in basic and neutral medias 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.
The reported oxidations of anilines, in the present
Hence, the specific reaction rates of molecular ani-
lines with the oxidant {k2 = kꢂ(Ka + [H+])/Ka} have

STUDIES ON THE KINETICS OF IMIDAZOLIUM FLUOROCHROMATE OXIDATION
171
Table VI Specific Rate (× 103 k2, dm3 mol−1
s
−1) of the Oxidant–Molecular Aniline Reaction at 299 K
Substituents in Aniline Moiety
para-Position
COMe NO2
meta-Position
COOH NO2 OMe
Solvents
H
Me
282
5,564
139
OMe
Cl
45
79
11
338
5
Br
Me
12
779
391
COMe
DMSO
MeCN
DMF
NB
t-BuOH
CB
17
369
374
1,469
170,491
1,815
5
5,824
2
2
7
0.4
6
3
2
408
382
92
118
695
442
33
0.5
0.9
51
29
38
60
31
10
7
140
2
255
2
0.7
413
406
33
509
3,186 45,542
42 1,311
331 21,718
153 21,185
11,111
17,378
17,188
43,132
38
1
41
86
384 2,446
28 211
5,553
6,487
7,088
1,623
296 2,160 6,741
10,524 324 4,254
110
150
Diox
38
been obtained, as reported earlier for the oxidation of
anilines by percarbonate [16] and perborate [11], 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 col-
lected in Table VI. p-NHCOMe, p-F, and m-Et are not
included due to nonavailability of their pKa values.
The specific rates of the rate-limiting step of para-
and meta-substituted anilines correlate satisfactorily
with the Brown–Okamoto equation (Table VII), and a
representative plot is given in Fig. 2. Regression anal-
ysis of para- and meta-substituents, both collectively
andseparately, showsanimprovedcorrelationwithvar-
ious substituent constants (Table VII), when compared
to the same using kobs. Multiple correlation analysis
reveals that dual substituent parameter (DSP) equation
with explanatory variables ꢀI and σ+R governs fairly
the variation of k2 with meta substituents. The reaction
coefficients of field and resonance effects are negative,
and the percentage contributions of these two terms are
almost equal [11]. Parallel to simple linear correlation
results, the correlation of k2 with substituent constants,
through multiple correlation equations, are better than
those with kobs (Table VII).
Table VII Results of Simple and Multiple Linear
Correlation of the specific Rate (k2) of
Oxidant–Molecular Aniline Reaction at 299 K in All the
Organic Solvents Investigated
Explanatory Variable
100 r2a
n
para- and meta-Substituents
ꢀ
29–80
47–85
27–80
12
10
12
ꢀ+
ꢀp− and ꢀm
para-Substituents Only
ꢀ
ꢀ
ꢀ
56–92
64–99
41–81
7
6
7
+
−
meta-Substituents Only
ꢀ
ꢀ
14–82
27–79
5
5
+
para-Substituents
53–87
ꢀI, ꢀR
ꢀI, ꢀ+R
ꢀI, ꢀ−R
F, R
7
7
7
7
66–90
44–83
58–93
meta-Substituents
38–83
ꢀI, ꢀR
ꢀI, ꢀ+R
F, R
6
6
6
61–91
41–88
Figure 2 Correlation of log k2 (DMF) with σ+.
a100 R2 in the case of multiple linear regression analysis.

172
BHUVANESHWARI AND ELANGO
Solvent–Reactivity Correlation
εr and ETN will poorly describe the microenvironment
around the reacting species, which governs the stability
of the transition state and hence the rate of the reaction.
Hence, there have been a variety of attempts to quan-
tify different aspects of solvent polarity and then use the
resultant parameters to interpret solvent effects on reac-
tivity through multiple regression. Various treatments
for the above solvent–solvent–solute interactions based
on linear solvation energy relationships (LSER) have
been developed [6].
Swain et al. [19] believed that the specific solvation
is determined principally by the acidity and basicity
of the solvent. The rate data are analyzed using a two-
parameterequationinvolvinganion-solvatingtendency
(A) and cation-solvating tendency (B).
The reaction has been studied in seven organic sol-
vents viz. DMF, DMSO, MeCN, NB (all hydrogen
bond acceptor (HBA), aprotic, and dipolar), CB, Diox
(both HBA, aprotic, and apolar) and t-BuOH (hydro-
gen bond donor (HBD) and protic) with a range of
ca. 44 units of relative permittivity. The pseudo-first-
order rate constants are given in Table IV. The cor-
relation of these rate constants with inverse of rela-
tive permittivity through Laidler–Eyring [17] equation
(0.663 > r > 0.074, 1.17 > sd > 0.48, 1.18 > ꢀ >
0.80) and with Reichardt’s [12] normalized micropo-
larity parameter ETN (0.885 > r > 0.074, 1.11 > sd >
0.45, 1.18 > ꢀ > 0.61) does not yield any meaningful
equation. Khurana et al. made parallel observation for
the oxidation of formic and oxalic acid by quinolinium
fluorochromate, QFC, in organic solvent media, and
the effect of solvent on the reaction was satisfactorily
explained by multiparameter equations [18].
From idealized theories, the solvent parameters εr
and ETN are often predicted to serve as a quantitative
measure of solvent polarity. However, this approach
is often inadequate since these theories regard sol-
vents as a nonstructured continuum, not composed of
individual solvent molecules with their own solvent–
solvent interactions and they do not take into account
specific solute–solvent interactions, such as hydrogen
bonding and electron pair donor–electron pair accep-
tor interactions, which often play a dominating role
in solute–solvent interactions. No single macroscopic
physical parameter could possibly account for the mul-
titude of solute–solvent interactions on the molecular
microscopic level. Thus, bulk solvent properties like
log kobs = aA + bB + c
(2)
where the regression coefficients a and b measure the
susceptibility of the solvent-dependent solute property
log kobs to the indicated solvent parameter and c is the
regression value of the solute property in the reference
solvent. The rates of oxidation for all the compounds
investigatedshowasatisfactorycorrelationintheabove
LSER(Eq. (2))withanexplainedvarianceofover90%.
Results in Table VIII suggest that the contribution of
anion-solvating tendency to reactivity was found to be
dominant, as indicated by PA. The sign of the coeffi-
cients of both these terms are negative indicating that
the reactants are solvated, through specific anion- and
cation-solvating tendencies, to a greater extent than the
transition state and further may be the operation of ap-
preciable solvent–solvent interactions.
Table VIII Statistical Results and Weighted Percentage Contributions for the Correlation of Rate of Oxidation (log
obs) of Substituted Anilines by IFC with Swain’s Specific Solvatochromic Parameters A and B
k
Substituent
R2
sd
ꢀ
a
b
PA
PB
H
p-Me
0.877
0.890
0.881
0.967
0.955
0.939
0.949
0.966
0.931
0.954
0.961
0.891
0.881
0.951
0.685
0.83
0.69
0.63
0.41
0.44
0.53
0.55
0.44
0.63
0.48
0.50
0.78
0.67
0.39
0.42
0.41
0.39
0.41
0.21
0.25
0.29
0.27
0.22
0.31
0.25
0.23
0.39
0.41
0.26
0.66
−6.795
−5.374
−2.069
−8.514
−4.282
−3.962
−9.593
−7.032
−6.586
−6.287
−8.968
−0.782
−3.057
−2.104
−3.701
−2.190
−2.214
−2.947
−1.529
−2.838
−3.020
−1.329
−2.414
−2.524
−2.363
−1.972
−4.479
−2.775
−2.883
−0.214
76
71
41
85
60
57
88
74
72
73
82
15
52
42
95
24
29
59
15
40
43
12
26
28
27
18
85
48
58
05
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

STUDIES ON THE KINETICS OF IMIDAZOLIUM FLUOROCHROMATE OXIDATION
173
In order to obtain a deeper insight into the various
solvent–solvent–solute interactions, which influence
reactivity, we have tried to adopt the solvatochromic
comparisonmethoddevelopedbyKamletandTaft[20].
This method may be used to unravel, quantify, corre-
late, and rationalize multiple interacting solvent effects
on reactivity. The kinetic data were correlated with the
solvatochromic parameters α, β, and π∗ characteristic
of the different solvents in the form of the following
LSER:
The observation of this systematic multiple regression
analysis leads us to the following preliminary conclu-
sions: (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 suggests that the specific interactions between
the reactants and the solvent, through HBD and HBA
properties, are more than that between the transition
state and the solvent. (iii) tert-Butanol, a typical hy-
droxylic solvent, can form relatively strong complexes
with solutes by hydrogen bonding, and such a complex-
ing solvent acts as a very mild form of blocking reagent
[21]; hence, the approach of the oxidizing species dur-
ing activation is relatively difficult, 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 oxidation would
be faster in nonpolar solvents than in polar solvents
and the observed results (Table IV, a solvent change
from tert-BuOH to Diox causes a 66-fold rate acceler-
ation) are in the same lines. (iv) 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 max-
imum in apolar solvents as observed from Table IV.
log k = Ao + sπ∗ + aα + bβ
(3)
where π∗ in an index of solvent dipolarity/
polarizability, which measures the ability of the sol-
vent to stabilize a charge or a dipole by virtue of its
dielectric effect, α is the solvent HBD (hydrogen bond
donor) acidity, β is the solvent HBA (hydrogen bond
acceptor) basicity of the solvent in a solute to solvent
hydrogen bond, and Ao is the regression value of the so-
lute property in the reference solvent cyclohexane. The
regression coefficients s, a, and b measure the relative
susceptibilities of the solvent-dependent solute prop-
erty log kobs to the indicated solvent parameter. The
rates of oxidation for all the compounds studied show
a satisfactory correlation with solvent via the above
LSER (Eq. (3)) with an explained variance of about
92%. Such a correlation indicates the existence of both
specific and nonspecific solute–solvent interactions in
the present study.
Mechanism
The correlations of log kobs versus pKa [22] and log kobs
versus ꢁpKa (= pKa-substituted aniline – pKa aniline)
[23] are nonlinear. This observation along with the re-
sults of structure–reactivity correlation indicates that
From the values of the regression coefficients, the
contribution of each parameter, on a percentage ba-
sis, to reactivity was calculated and listed in Table IX.
Table IX Statistical Results and Weighted Percentage Contributions for the Correlation of Rate of Oxidation (log
obs) of Substituted Anilines by IFC with Kamlet–Taft’s Solvatochromic Parameters α, β, and π∗
k
Substituent
R2
sd
ꢀ
a
b
s
Pα
Pβ
Pπ∗
H
p-Me
0.860
0.936
0.934
0.960
0.911
0.889
0.951
0.967
0.969
0.968
0.961
0.817
0.890
0.892
0.872
0.95
0.57
0.51
0.49
0.67
0.77
0.58
0.47
0.45
0.44
0.54
1.09
0.69
0.62
0.78
0.44
0.29
0.30
0.23
0.35
0.39
0.26
0.21
0.20
0.21
0.23
0.50
0.39
0.38
0.42
−3.011
0.264
1.086
−1.976
−3.282
−2.067
−1.368
−0.988
−0.891
−1.757
−2.763
−3.169
−2.673
−2.828
−1.449
0.772
−3.185
−2.124
−2.612
−3.491
−3.573
−3.687
−3.457
−3.098
−2.954
−2.912
−3.219
−3.771
−4.141
−2.892
−3.576
37
05
19
50
37
38
49
25
10
20
35
01
47
22
61
24
58
36
14
14
12
17
35
47
38
30
28
08
20
00
39
45
45
36
49
50
34
40
43
42
35
71
45
58
39
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
−4.799
−2.749
−2.739
−4.887
−1.932
−0.660
−1.365
−3.231
−0.037
−4.327
−1.092
−5.685
−1.026
0.035

174
BHUVANESHWARI AND ELANGO
molecular anilines are the reactive species [24]. 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 chromium(VI) reagents complex
with the aniline, and the reaction exhibits Michaelis–
Menten kinetics with respect to the substrate. If the for-
mation constant (K) of the oxidant–substrate complex
is large, the oxidation is to exhibit zero-order depen-
dence on [substrate] [25] (Scheme 1).
K
←
→
IFC + Substrate
Complex
(rapid)
(slow)
k
Complex → Product
scheme 3 In the absence of acid.
The mechanism shown in Scheme 3 yields, if the
formation constant (K) of the oxidant–substrate com-
plex is large, the following rate law:
−d[IFC]/dt = k[IFC]
Kꢂ
IFC + H+
IFCH+
←
→
The rate law in its final form accounts for the observed
kinetics.
K
+
←
→
IFCH + Substrate
Complex
(rapid)
(slow)
k
Complex → Product
BIBLIOGRAPHY
Scheme 1 In the presence of acid.
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O
O
ArNH2 → ArNHOH → ArNO
ArNH2 + ArNO → Ar N N Ar
Rearrangement, H+
ArNHOH
→
p-Aminophenol
O
p-Aminophenol → Benzoquinone
Scheme 2
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is zero (see Scheme 3). This suggests that protonated
IFC is the active species and better electrophile.
19. Swain, C. G.; Swain, M. S.; Powel, A. L.; Alunni, S. J
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STUDIES ON THE KINETICS OF IMIDAZOLIUM FLUOROCHROMATE OXIDATION
175
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