Preferential solvational effects on the Cr(VI) oxidation of benzylamines in benzene/2-methylpropan-2-ol mixtures

Article abstract of DOI:10.1002/kin.20464

Imidazolium fluorochromate (IFC) oxidation of 11 meta-and para-substituted benzylamines, in varying mole fractions of benzene/2-methylpropan-2-ol binary mixtures, is first order in IFC and acid and zero order in substrate. The Hammett correlation yielded

Full text of DOI:10.1002/kin.20464

Preferential Solvational
Effects on the Cr(VI)
Oxidation of Benzylamines
in Benzene/2-Methyl-
propan-2-ol Mixtures
A. THIRUMOORTHI,1 D. S. BHUVANESHWARI,2 K. P. ELANGO1
1Department of Chemistry, Gandhigram Rural University, Gandhigram 624 302, India
2Department of Chemistry, Thiagarajar College, Madurai 625 009, India
Received 9 March 2009; revised 2 September 2009; accepted 5 September 2009
DOI 10.1002/kin.20464
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Imidazolium fluorochromate (IFC) oxidation of 11 meta- and para-substituted
benzylamines, in varying mole fractions of benzene/2-methylpropan-2-ol binary mixtures, is first
order in IFC and acid and zero order in substrate. The Hammett correlation yielded a U-shaped
curve, indicating a change in the relative importance of bond formation and bond fission in the
transition state. The rate data failed to correlate with macroscopic solvent parameters such as
εr and ETN. The correlation of kobs with Kamlet–Taft solvatochromic parameters suggests that
H-bonding between the reacting species and the solvent plays a major role in governing the
reactivity. ꢀ 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 159–167, 2010
C
Studies on the kinetics of oxidation of organic com-
pounds in nonaqueous and aquo-organic solvent mix-
tures have revealed the important role of nonspecific
and specific solvent effects on reactivity [2–9]. It has
been shown that the reactivity is influenced by the pref-
erential solvation of the reactants and/or transition state
through such effects, and has been established that the
technique of correlation analysis may be well applied
to separate and quantify these effects on the reactivity.
Furthermore, the important tool in deciding the
mechanism of reactions is the study of substituent ef-
fects and thermodynamic parameters. The Hammett
equation and its modified forms [10], all known as lin-
ear free energy relationships (LFER), have been found
to be useful for correlating reaction rates and equilib-
rium constants for side chain reactions of meta- and
INTRODUCTION
The study of solute–solvent interactions in binary sol-
vent mixtures is more complex than in pure solvents.
In a pure solvent, the composition of the microsphere
of a solvated solute, the so-called cybotatic region, is
the same as in the bulk solvent, whereas in binary mix-
tures this can be distinct which leads to preferential
solvation phenomena. This is due to the interactions of
the solute to a different extent with each of the mixture
components, which in turn affect the composition of
the microsphere of solvation [1].
Correspondence to: K. P. Elango; e-mail: drkpelango
@rediffmail.com.
c
ꢀ
2010 Wiley Periodicals, Inc.

160
THIRUMOORTHI, BHUVANESHWARI, AND ELANGO
para-substituted derivatives. Thus, a systematic study
of various LFERs and isokinetic relationship has been
made to establish the role of solvent and substituents
on reactivity and to decide the nature of the mecha-
nism being followed in the oxidation of several meta-
and para-substituted benzylamines by imidazolium
fluorochromate (IFC, a mild and selective oxidant, re-
ported only recently [11]) in benzene/2-methylpropan-
2-ol mixtures of varying compositions. This mixture
is chosen since by varying the mole fraction of the
constituent solvents, the hydrogen bonding proper-
ties of the medium can be varied gradually since 2-
methylpropan-2-ol is classified as a typical hydrogen
bond donor (HBD) and benzene as a typical non-
hydrogen bond donor (non-HBD) solvent [12].
Product Analysis
The product analysis was carried out, under kinetic
conditions, by employing GC-MS technique. The re-
sults revealed that the oxidation products were ben-
zaldehyde and N-benzylidene(phenyl)methanamine.
The following major fragments with corresponding
m/z values were obtained in the GC-MS analysis. In
addition to these, as reported earlier [14], two more
fractions corresponding to complex intermediates were
also obtained.
O
m/z = 77
m/z = 106
EXPERIMENTAL
Materials
N
N
All the chemicals and solvents used were of analytical
grade. The solvents 2-methylpropan-2-ol and benzene
are of analytical grade and were purified by conven-
tional methods. The benzylamines (Aldrich or Merck,
Bangalore, India) used were with substituents H,
p-Me, p-OMe, p-Ph, p-Cl, p-F, p-COOH, m-Me,
m-F, m-Cl, and m-OMe. The solid benzylamines were
used as such, and the liquid benzylamines were used
after vacuum distillation. IFC was prepared by the re-
ported method [11], and its purity was checked by the
iodometric method.
m/z = 118
m/z = 195
N
m/z= 104
m/z = 91
RESULTS AND DISCUSSION
Kinetic Measurements
The kinetics of oxidation of benzylamine and meta-
(Me, OMe, Cl, F)- and para-(Me, OMe, Cl, F, Ph,
COOH)-substituted benzylamines was carried out un-
der pseudo-first-order conditions with [substrate] >
[IFC] in varying mole fractions of benzene (0.13,
0.15, 0.18, 0.32, 0.42, 0.52, 0.67, and 0.86) in
2-methylpropan-2-ol. The mole fraction values se-
lected as the solvatochromic parameters for these mole
fractions are available in the literature [1].
The first-order dependence of the reaction on IFC
is obvious from the linearity of the plots of log [IFC]
versus time. Furthermore, the pseudo-first-order rate
constants, kobs, do not depend on the initial concentra-
tion 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]. The oxidation process is remark-
ably slow, but found to be catalyzed in the presence
of p-toluenesulfonic acid and the reaction proceeds
at a comfortable rate. A plot of log [acid] versus log
kobs was found to be linear (r = 0.998) with a slope
of unity, indicating that the order with respect to acid
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 unconsumed oxidant iodometrically at 299,
307, 315, and 322 K. The rate constants were deter-
mined by the least-squares method, from the linear
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 a fit was discussed using the correlation coefficient
(r in the case of simple linear regression and R in the
case of multiple linear regressions) and standard devi-
ation. The percentage contribution (Px) of a parameter
to the total effect on reactivity was computed using the
regression coefficient of each parameter as reported
earlier [13].
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PREFERENTIAL SOLVATIONAL EFFECTS ON THE Cr(VI) OXIDATION OF BENZYLAMINES
161
Table I Pseudo-First-Order Rate Constants for the
Oxidation of Benzylamine by IFC at 299 K in 0.67 Mole
Fraction of Benzene in 2-Methylpropan-2-ol
102 [BA]
(M)
103 [IFC]
(M)
102 [TsOH]
(M)
104 kobs
(s−1
20
10
0
)
3.0
4.0
5.0
6.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
3.0
4.0
5.0
6.0
2.0
2.0
2.0
2.0
1.0
1.5
2.0
2.5
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
2.0
3.0
4.0
–
4.17
4.21
4.19
4.08
4.17
4.22
4.21
4.17
1.82
4.21
6.45
8.24
1.86
1.92
1.88
1.89
0
4
8
12
16
105 kobs, s–1 at 299 K
–
–
–
Figure 1 Isokinetic plot for the oxidation of benzylamines
by IFC in 0.32 mole fraction of benzene in 2-methylpropan-
2-ol.
is also one. Catalysis by p-toluenesulfonic acid sug-
gested protonation of IFC species rather than the ben-
zylamine molecule, which would have resulted in rate
retardation. Parallel observations were made in the ox-
idation of substituted anilines by the same oxidant [4].
The participation of protonated chromium species in
Cr(VI) oxidations is well known [15]. Furthermore,
the reaction did not promote polymerization of acry-
lonitrile, indicating the absence of free radicals.
Table II. The operation of isokinetic relationship is
tested by plotting the logarithms of rate constants at two
temperatures (T2 > T1) against each other according to
Eq. (1) as suggested by Exner [16].
log k(at T2) = a + b log k(at T1)
(1)
A representative plot is shown in Fig. 1 (r = 0.938,
isokinetic temperature = 248 K). In the present study,
a fairly linear plot implied the validity of the isoki-
netic relationship, which revealed the operation of the
compensation effect in the present system [16].
Activation Parameters
The activation parameters were calculated from kobs at
299, 307, 315, and 322 K using the van’t Hoff plot
by the method of least squares and are collected in
The thermodynamic parameters were also calcu-
lated for the oxidation of meta-methyl benzylamine in
Table II Activation Parameters for the Oxidation of Benzylamines by IFC in 0.32 Mole Fraction of Benzene in
2-Methylpropan-2-ol
104kobs (s−1
)
R
299 K
307 K
315 K
322 K
ꢀH# (kJ mol−1
)
−ꢀS# (J K−1 mol−1
)
ꢀG# (kJ mol−1
)
H
4.2
1.2
9.5
16
9.5
7.8
17
14
8.3
17
9
12
1.8
13
25
13
15
21
23
13
22
9.5
17
2.5
20
36
19
24
26
29
19
33
14
44
23
22
24
21
36
10
22
27
20
21
180
242
248
238
238
202
285
245
235
251
254
97.9
95.8
96.6
94.9
96.5
97.0
94.9
95.3
96.9
95.0
97.1
m-Me
m-OMe
m-Cl
m-F
p-Me
p-OMe
p-Cl
p-F
p-Ph
p-COOH
1.4
10.4
17
9.9
10.5
18
16
8.9
18
7.3
7.5
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THIRUMOORTHI, BHUVANESHWARI, AND ELANGO
Table III Activation Parameters for the Oxidation of m-Me Benzylamine by IFC in Different Mole Fractions of Benzene
in 2-Methylpropan-2-ol
104 kobs (s−1
)
Mole Fraction
of Benzene
299 K
307 K
315 K
322 K
ꢀH# (kJ mol−1
)
−ꢀS# (J K−1 mol−1
)
ꢀG# (kJ mol−1
)
0.13
0.15
0.18
0.32
0.42
0.52
0.67
0.86
0.13
0.38
0.63
1.18
2.1
2.9
6.2
9.4
0.15
0.46
0.93
1.35
2.53
3.2
0.45
0.71
1.15
1.8
3.5
6
0.90
0.99
1.39
2.52
8
10
22
39
69
32
24
23
40
42
39
37
110
224
244
242
182
172
176
179
101
99
97
96
95
94
92
91
6.4
12
9.3
20
all the mole fractions of benzene in a 2-methylpropan-
2-ol solvent mixture (Table III). The observed, negative
entropies of activation indicated a greater degree of or-
dering in the transition state than in the initial state,
due to an increase in solvation during the activation
process. The existence of a linear relationship (Fig. 2,
r = 0.995, isokinetic temperature = 292 K) between
log kobs at 322 K and log kobs at 299 K indicated that
operation of the compensation effect in all the mole
fractions of the solvent mixture systems studied.
40
30
20
10
Structure–Reactivity Correlation
The effect of substituents on the rate of oxidation
was studied with 11 para- and meta-substituted ben-
zylamines in benzene/2-methylpropan-2-ol solvent
mixtures (Table IV). The results revealed that the
rate constants vary with the substrate in a particular
mole fraction of the solvent mixture, though the rate of
the reaction is independent of [substrate]. This may be
due to the fact that because of difference in polarity of
0
0
2
4
6
8
10
104 kobs, s–1 at 299 K
Figure 2 Isokinetic plot for the oxidation of m-Me com-
pound by IFC in different mole fractions of benzene in 2-
methylpropan-2-ol.
Table IV Pseudo-First-Order Rate Constants (104 kobs, s−1) for the Oxidation of meta- and para-Substituted
Benzylamines by IFC at 299 K
Mole Fractions of Benzene
X
0.13
0.15
0.18
0.32
0.42
0.52
0.67
0.86
H
0.11
0.13
0.32
0.46
0.40
0.25
0.67
0.51
0.48
0.51
0.48
0.14
0.38
0.50
0.54
0.52
0.36
0.79
0.67
0.57
0.75
0.49
0.16
0.63
0.71
0.84
0.57
0.41
0.99
0.85
0.59
0.98
0.57
0.18
1.18
0.95
1.71
0.96
0.77
1.80
1.47
0.83
1.66
0.73
1.56
2.11
1.72
1.72
1.74
0.94
2.70
3.31
1.50
2.95
1.75
2.39
2.96
1.89
2.50
2.89
1.41
3.87
5.69
1.84
3.44
2.09
4.21
6.27
2.57
2.97
3.99
1.95
6.02
8.28
2.47
9.39
2.78
4.89
9.39
3.84
4.76
5.78
2.21
8.25
9.70
2.83
11.6
6.02
m-Me
m-OMe
m-Cl
m-F
p-Me
p-OMe
p-Cl
p-F
p-Ph
p-COOH
X: Substituent in benzylamine moiety.
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PREFERENTIAL SOLVATIONAL EFFECTS ON THE Cr(VI) OXIDATION OF BENZYLAMINES
163
Table V Specific Rate Constants (104 k2, s−1) for the Oxidation of meta- and para-Substituted Benzylamines by IFC at
299 K
Mole Fractions of Benzene
X
0.13
0.15
0.18
0.32
0.42
0.52
0.67
0.86
H
2.7
3.2
7.9
11.6
9.9
6.3
17
3.5
9.6
13
13.6
13
9
20
17
14
4.1
16
18
21
14
11
25
21
15
11
30
24
42
39
19
45
37
21
39
53
43
43
44
24
68
83
37
60
74
47
63
72
36
97
142
46
105
157
64
74
100
49
150
207
62
122
235
96
119
144
55
206
243
71
m-Me
m-OMe
m-Cl
m-F
p-Me
p-OMe
p-Cl
p-F
13
12
X: Substituent in benzylamine moiety.
different benzylamines, the extent of solvation should
be different and hence the experimental values of rate
constants (as well as the values of ꢀS#) may be dif-
ferent for different benzylamines as observed by us in
the present study. The rate data failed to conform to
either the usual Hammett equation or its modified ones
[17]; plots of log kobs versus substituent constants are
nonlinear.
in nonaqueous media. The p-Ph and p-COOH sub-
stituents are not included due to the nonavailability of
pKa values. In the reactions of benzylamines in acid
medium, as the free bases are the nucleophiles; the
specific reaction rates of benzylamines are to be ob-
tained using the concentrations of the free bases but
not the total concentrations of benzylamines. The spe-
cific rates of the oxidant-molecular benzylamine reac-
tions are collected in Table V. The rate constants (k2)
calculated through the modified method are analyzed
using the usual Hammett equation. A representative
Hammett plot (in 0.13 mole fraction of benzene in
2-methylpropan-2-ol) is shown in Fig. 3. The Ham-
mett plot has a concave shape with a minimum for the
unsubstituted benzylamine, and all other benzylamines
either with electron-donating or electron-withdrawing
Benzylamines in basic and neutral media are present
as free bases, but in acid medium they exist in dual
forms: the free bases and the conjugate acids. And, the
ratio of the concentrations of the free bases to the con-
jugate acid [XC6H4CH2NH2]/[XC6H4CH2NH+3 ]) de-
pends on the pKa of the benzylamines and the acidity of
the medium. The reported oxidation of benzylamines,
in the present study, was carried out under pseudo-
first-order conditions ([benzylamine] > [IFC]), and
the concentration of IFC at different reaction times
was determined by titrimetry. The pseudo-first-order
rate constants (kobs) were obtained from the least-
squares slopes of log [IFC] versus time plots, and the
second-order rate constants were calculated by divid-
ing kobs values with [benzylamine]T, where [benzyl-
amine]T is the total concentration of benzylamine.
Since the molecular benzylamine is the reactive species
(nucleophile), the reported kobs/[benzylamine]T values
are not the rate constants of the oxidant-molecular
benzylamine reactions. And hence the analysis of
rate constants in terms of the Hammett equation is
erroneous, i.e., it yields a nonlinear plot.
–2.8
–3.0
–3.2
–3.4
–3.6
Hence, the specific reaction rates of molecular ben-
zylamine with IFC have been obtained from the mea-
sured pH and the reported pKa values of corresponding
benzylamines [18]. The measured pH varies linearly
with the pKa of benzylammonium ion. This clearly
revealed the operation of a linear free-energy rela-
tionship in the dissociation of benzylammonium ions
–0.3
–0.2
–0.1
0.0
0.1
0.2
0.3
0.4
Figure 3 Hammett plot for the Cr(VI) oxidation of benzyl-
amines at 299 K.
International Journal of Chemical Kinetics DOI 10.1002/kin

164
THIRUMOORTHI, BHUVANESHWARI, AND ELANGO
substituents react faster than the parent. The observed
U-shaped Hammett plot may be due to a change in
the relative importance of bond formation and bond
breaking in the transition state [19].
donor/electron pair acceptor interactions), is likely in
the present case [12].
To obtain a deeper insight into the various solvent–
solvent–solute interactions, which influence reactivity,
we have tried to adopt the solvatochromic compari-
son method developed by Kamlet and Taft [21]. This
method may be used to unravel, quantify, correlate,
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
linear solvation energy relationship (LSER):
Solvent–Reactivity Correlation
The Cr(VI) oxidation of benzylamines has been
studied, in varying mole fractions of benzene in
2-methylpropan-2-ol, with a view to understand the
effect of preferential solvation on the reaction. Solvent
mixtures are very useful for studying solvent effects on
reactions since the properties of the medium can be ad-
justed continuously by changing the composition of the
mixture. The rate constants (Table IV) were correlated
with solvent macroscopic parameters, viz. relative per-
mittivity [20] and Dimroth–Reichardt’s ETN [12]. The
plots of log kobs versus the above-mentioned solvent
parameters were nonlinear, and a representative plot is
shown in Fig. 4. This nonlinear dependence of the rate
with solvent macroscopic parameters indicated that no
single macroscopic physical parameter could possibly
account for the multitude of solute–solvent interac-
tions on the molecular microscopic level. These bulk
solvent properties have poorly described the microen-
vironment around the reacting species, which governs
the stability of the transition state and hence the rate
of the reaction. Hence, the operation of selective or
preferential solvation, which includes both nonspe-
cific solute–solvent association (caused by dielectric
enrichment in the solvation shell of solute ions or
dipolar solute molecules) and specific solute–solvent
association (such as hydrogen bonding or electron pair
log k = Ao + sπ∗ + aα + bβ
(2)
where π∗ in an index of solvent dipolarity/polariza-
bility, which measures the ability of the solvent to
stabilize a charge or a dipole by virtue of its dielectric
effect, α is the solvent HBD acidity, β is the solvent
HBA (hydrogen bond acceptor) basicity, and Ao is the
regression value of the solute property in the refer-
ence solvent cyclohexane. The regression coefficients
s, a, and b measure the relative susceptibilities of the
solvent-dependent solute property log kobs to the in-
dicated solvent parameter. The rates of oxidation for
all the compounds studied showed an excellent cor-
relation with solvent via the above-mentioned LSER
(Eq. (2)) with an explained variance of ca. 98%. Such
an excellent correlation indicated the existence of both
specific and nonspecific solute–solvent interactions in
the present study.
From the values of the regression coefficients, the
contributions of each parameter, on a percentage basis,
to reactivity were calculated and are listed in Table VI.
The weighted percentage contributions of these sol-
vatochromic parameters indicated that (i) the rate of
the reaction is strongly influenced by specific solute–
solvent interactions as indicated by the percentage con-
tributions of α and β parameters. (ii) The solvent HBD
acidity, as indicated by the α term, plays a dominant
role in governing the reactivity. It alone explains more
than 50% of the observed solvent effect. The nega-
tive sign of the coefficients of the α term suggests that
the specific interaction between the reactants and the
solvent (through the HBD property) is relatively more
than that between the transition state and the solvent.
Since 2-methylpropan-2-ol is a typical HBD solvent, it
forms a sheath of solvent shell around the lone pair of
electrons residing on N-atom (as shown below) through
H-bond donation or, in other words, acts as a very mild
form of a blocking reagent [4,7]. Thus restricting the
approach of the oxidant molecule to form the tran-
sition state and consequently retarded the rate of the
oxidation.
1/εr
ET
0.36
0.32
0.28
0.24
0.25
0.20
0.15
0.10
N
–5
–4
–3
log kobs
Figure 4 Plot of log kobs versus solvent macroscopic pa-
rameters.
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PREFERENTIAL SOLVATIONAL EFFECTS ON THE Cr(VI) OXIDATION OF BENZYLAMINES
165
Table VI Statistical Results and Weighted Percentage Contributions for the Correlation of kobs for the Cr(VI)
Oxidation of Substituted Benzylamines in Benzene/2-Methylpropan-2-ol with Kamlet–Taft’s Solvatochromic
Parameters α, β, and π∗
R2
Standard Deviation
a
b
s
Pα
Pβ
Pπ
∗
X
H
0.993
0.932
0.951
0.957
0.985
0.968
0.988
0.983
0.995
0.984
0.984
0.08
0.22
0.10
0.12
0.07
0.08
0.05
0.09
0.03
0.07
0.07
−10
−8.5
−4.8
−2.7
−4.8
−5.4
−5
0.8
−0.3
−0.1
2.3
−0.3
−0.1
−0.4
0.4
6
60
83
72
31
61
92
76
57
55
68
34
04
03
01
26
04
01
06
03
01
08
12
36
14
27
43
35
07
18
40
44
24
54
m-Me
m-OMe
m-Cl
m-F
p-Me
p-OMe
p-Cl
p-F
p-Ph
p-COOH
1.5
1.8
−3.7
2.8
0.4
1.2
4.9
3
−6.9
−3.7
−5.3
−2.3
0.1
−0.6
1.9
3.6
−0.8
X: Substituent in benzylamine moiety.
observations were recorded in the Cr(VI) oxidation of
anilines in the binary mixture [5,8].
Mechanism
CH2NH2
Figure 5 shows the solution FT-IR spectrum of the re-
action mixture at different time intervals. The doublet
around 3500–3600 cm−1 corresponds to asymmetric
and symmetric stretching vibrations of the two N–H
bonds of the aromatic primary amine. It is evident from
the figure that with lapse of time the intensity of the
peak decreased, which indicated the participation of the
amine group in the reaction. The result of the structure–
reactivity correlation indicated that molecular benzy-
lamines are the reactive species [24]. A perusal of data
in Table II indicated that the enthalpy of activation of
the oxidation is susceptible to the substituent present
in the ring. This indicated the involvement of substrate
in or prior to the rate-limiting step in such a way that
R = t-Bu
Such a specific solvation of the NH2 moiety by
HBD solvents has already been demonstrated by us
using electronic spectral and cyclic voltammetric tech-
niques [22,23]. When the mole fraction of benzene
(a typical non-HBD solvent) in the mixture is in-
creased, the mild block has been removed progres-
sively, the formation of transition state has increas-
ingly become easier and consequently increased the
rate of oxidation, as observed. Parallel observations
were made in the Cr(VI) oxidation of anilines in neat
organic solvents [4,7] wherein such retardation in rate
in 2-methylpropan-2-ol medium was observed when
compared to that in other nonpolar solvents. (iii) The
solvent polarizability/dipolarity, as indicated by Pπ∗
,
also plays an appreciable role in governing the re-
activity. The positive sign of the coefficients of this
term suggests that with an increase in the polarizabil-
ity/dipolarity of the medium the rate of the oxidation
process will increase. Since benzene is a more polariz-
able solvent (δ = 1) than 2-methylpropan-2-ol (δ = 0)
[1], an increase in its mole fraction in the mixture
increases the polarizability of the medium and con-
sequently increased the rate of the oxidation. Parallel
Figure 5 FT-IR spectra of the reaction mixture with lapse
of time.
International Journal of Chemical Kinetics DOI 10.1002/kin

166
THIRUMOORTHI, BHUVANESHWARI, AND ELANGO
O– ImH+
O– ImH+
O
O
O
K'
+
Cr
H+
Cr
+
F
F
HO
O– ImH+
O–ImH+
O
O
+
Cr
H
C
H
+
Cr
H
N
H
K
CH2NH2
+
F
HO
F
HO
R
R
k
O
–ImH+
+
Cr
H
C
H
H
NOH
+
F
HO
R
Cr(IV)
(I)
Scheme 1
the rate is independent of [substrate] [15]. In nonaque-
ous media, Cr(VI) reagents complex with the substrate
and the reaction exhibits Michaelis–Menten kinetics
with respect to the substrate. If the formation con-
stant (K) of the oxidant–substrate complex is large,
the oxidation is to exhibit a zero-order dependence on
[substrate] [5,8,25]. Based on the foregoing results and
discussions, a plausible mechanism (see Scheme 1) has
been proposed for the IFC oxidation of benzylamines
in benzene/2-methylpropan-2-ol mixtures.
The compound (I) undergoes further reactions to
give the products [9,14]. The above-mentioned mech-
anism leads to, under the condition that the formation
constant (K) of the oxidant–substrate complex is large,
the following rate law:
[substrate]. The rate of the reaction was found to in-
crease with the increase in mole fraction of benzene in
the mixture. The validity of the isokinetic relationship
and various linear free-energy relationships has been
tested and discussed. Both the electron-donating and
electron-withdrawing substituents facilitate the rate of
oxidation and correlation with Hammett substituents
constants yielded a concave upward curve. Solvent
variation studies indicated that the rate of oxidation
is relatively faster in non-HBD solvent-rich mixtures
than in HBD solvent-rich mixtures. The correlation
of the experimental data with solvent parameters sug-
gested that solvent H-bonding plays a major role in
governing the reactivity.
−d[IFC]/dt = k[IFCH+ − Sub]
BIBLIOGRAPHY
[IFCH+ − Sub] = K [IFCH+] [Sub]/1 + K [Sub]
If K [Sub] ꢁ 1
1. Bosch, E.; Rived, F.; Roses, M. J Chem Soc, Perkin
Trans 1996, 2, 2177.
2. Fatima Jeyanthi, G.; Elango, K. P. Int J Chem Kinet
2003, 35, 154.
3. Saravanakanna, S.; Elango, K. P. Int J Chem Kinet 2002,
34, 585.
[IFCH+ − Sub] = [IFCH+] = Kꢂ [IFC] [H+]
Therefore, −d[IFC]/dt = k Kꢂ [IFC] [H+]
4. Bhuvaneshwari, D. S.; Elango, K. P. Int J Chem Kinet
2006, 38, 166.
The above scheme accounts for the observed orders.
5. Bhuvaneshwari, D. S.; Elango, K. P. Z Phys Chem 2006,
220, 697.
6. Bhuvaneshwari, D. S.; Elango, K. P. J Indian Chem Soc
2006, 83, 999.
CONCLUSION
The Cr(VI) oxidation of benzylamines is first order
with respect to [IFC] and [acid] and zero order in
7. Bhuvaneshwari, D. S.; Elango, K. P. Int J Chem Kinet
2006, 38, 657.
International Journal of Chemical Kinetics DOI 10.1002/kin

PREFERENTIAL SOLVATIONAL EFFECTS ON THE Cr(VI) OXIDATION OF BENZYLAMINES
167
8. Bhuvaneshwari, D. S.; Elango, K. P. Z Naturforsch B
2005, 60, 1105.
17. Hansch, C.; Leo, A.; Taft, R. W. Chem Rev 1991, 91,
165.
9. Thirumoorthi, A.; Bhuvaneshwari, D. S.; Elango, K. P.
Int J Chem Kinet 2007, 39, 362.
18. Blackwell, L. F.; Fischer, A.; Miller, I. J.; Topsom,
R. D.; Vaughan, J. J Chem Soc 1964, 3588.
19. Kim, S. H.; Yoh, S. D.; Fujio, M. Bull Korean Chem Soc
1995, 16, 760.
20. Amis, E. S.; Hinton, J. I. Solvent Effect of Chemical
Phenomena; Academic Press: New York, 1973.
21. Kamlet, M. J.; Abboud, J. M.; Abraham, M. H.; Taft,
R. W. J Org Chem 1983, 48, 2877.
10. Shorter, J. Correlation Analysis of Organic Reactivity;
Research Student Press: Letchworth, UK, 1982.
11. Pandurangan, A.; Abraham Rajkumar, G.; Banumathi,
A.; Murugesan, V. Indian J Chem B 1999, 38,
99.
12. Reichardt, C. Solvents and Solvent Effects in Or-
ganic Chemistry; VCH: Weinheim, Federal Republic of
Germany, 1988.
22. Bhuvaneshwari, D. S.; Elango, K. P. J Mol Liq 2008,
143, 147.
13. Reichardt, C. Angew Chem, Int Ed (Eng) 1979, 18, 98.
14. Shechter, H.; Rawalay, S. S.; Tubis, M. J Am Chem Soc
1964, 86, 1701.
15. Wiberg, K. B. Oxidation in Organic Chemistry, Part A;
Academic Press: New York, 1965.
23. Bhuvaneshwari, D. S.; Elango, K. P. Z Naturforsch A
2008, 63, 493.
24. Karunakaran, C.; Kamalam, R. J Chem Soc, Perkin
Trans 2002, 2, 2011.
25. Karunakaran, C.; Chidambaranathan, V. Croatica Chim
Acta 2001, 74, 51.
16. Liu, L.; Guo, Q. Chem Rev 2001, 101, 673.
International Journal of Chemical Kinetics DOI 10.1002/kin