Studies on the kinetics of tetraethylammonium bromochromate oxidation of some meta- and para-substituted benzyl alcohols in non-aqueous media

Article abstract of DOI:10.1524/zpch.2011.0044

Tetraethylammonium bromochromate (TEABC) oxidation of 18 meta- and para-substituted benzyl alcohols (BnOH) in dimethylsulphoxide (DMSO) leads to the formation of corresponding aldehydes. The reaction is first order each in TEABC and the alcohols. The reaction is catalyzed by hydrogen ions. The hydrogen ion dependence has the form: k_obs = a+b[H⁺]. The oxidation of αα′-dideuterio benzyl alcohol exhibited a substantial primary kinetic isotope effect (k_H/k_D = 5.71 at 303 K). Oxidation of benzyl alcohol was studied in 17 different organic solvents. The solvent effect has been analyzed using Kamlet's and Swain's multi parametric equation. A correlation of data with Kamlet-Taft solvatochromic parameters (α, β, π*) suggests that the specific solute-solvent interactions play a major role in governing the reactivity. A mechanism involving a hydride ion transfer via chromate ester is proposed. by Oldenbourg Wissenschaftsverlag, Mu?nchen.

Full text of DOI:10.1524/zpch.2011.0044

Z. Phys. Chem. 225 (2011) 249–263 / DOI 10.1524/zpch.2011.0044
© by Oldenbourg Wissenschaftsverlag, München
Studies on the Kinetics of Tetraethylammonium
Bromochromate Oxidation of Some Meta- and
Para-Substituted Benzyl Alcohols in Non-Aqueous
Media
By S. Sheik Mansoor1 ∗ and S. Syed Shafi2
,
1
Post Graduate and Research Department of Chemistry, C. Abdul Hakeem College,
Melvisharam 632509, Tamil Nadu, India
Post Graduate and Research Department of Chemistry, Islamiah College,
2
Vaniyambadi 635752, Tamil Nadu, India
(Received August 20, 2010; accepted in revised form December 2, 2010)
Tetraethylammonium / Bromochromate / Solvent Effect / Oxidation / Kinetics
Tetraethylammonium bromochromate (TEABC) oxidation of 18 meta- and para-substituted benzyl
alcohols (BnOH) in dimethylsulphoxide (DMSO) leads to the formation of corresponding aldehydes.
The reaction is first order each in TEABC and the alcohols. The reaction is catalyzed by hydrogen
ions. The hydrogen ion dependence has the form: kobs = a+b[H+]. The oxidation of ααꢀ-dideuterio
benzyl alcohol exhibited a substantial primary kinetic isotope effect (kH/kD = 5.71 at 303 K).
Oxidation of benzyl alcohol was studied in 17 different organic solvents. The solvent effect
has been analyzed using Kamlet’s and Swain’s multi parametric equation. A correlation of data
with Kamlet–Taft solvatochromic parameters (α, β, π∗) suggests that the specific solute-solvent
interactions play a major role in governing the reactivity. A mechanism involving a hydride ion
transfer via chromate ester is proposed.
1. Introduction
Chromium(VI) is established as a versatile oxidant for many types of substrates vary-
ing from metal ions to naturally occurring organic compounds and has a wide range
of applications. Cr(VI) as chromate or dichromate is highly soluble in water and is re-
ported to be highly toxic [1], there are continued interest in the development of new
chromium(VI) reagents for the effective and selective oxidation of organic substrates,
in particular alcohols, under mild conditions. Therefore, the search for new oxidizing
agents are of interests to synthetic organic chemists. Many such reagents have been
developed in recent years with some success, some of the important entries in the list
of reagents are triethylammonium chlorochromate [2], tributylammonium chlorochro-
mate [3], tetramethylammonium fluorochromate [4], tetrahexylammonium fluorochro-
* Corresponding author. E-mail: smansoors2000@yahoo.co.in

250
S. S. Mansoor and S. S. Shafi
mate [5], tetraheptylammonium bromochromate [6], tetrabutylammonium bromochro-
mate [7], tetraethylammonium chlorochromate [8], isoquinilinium bromochromate [9],
benzyltrimethylammonium fluorochromate [10] and tripropyl ammonium fluorochro-
mate [11]. Specific and selective oxidation of organic compounds under non aqueous
conditions is an important reaction in synthetic organic chemistry. In aqueous solutions
primary alcohols are usually oxidized to carboxylic acids [12], while in the absence of
water the oxidation will stop at the aldehyde. TEABC is more efficient and stronger
oxidizing agent. Further this new compound does not react with acetonitrile, which is
suitable medium for studying kinetics and mechanism [6].
Kinetics of oxidation of alcohols by various oxidizing agents have been well
studied [13–19]. Substituted benzyl alcohols are suitable for studying the structure-rate
correlations, as a large number of mono substituted benzyl alcohols are either commer-
cially available or can be easily prepared. There seems to be no report on the oxidation
of benzyl alcohols by TEABC. Therefore, we studied the kinetics of oxidation of benzyl
alcohol and several meta- and para-substituted benzyl alcohols by TEABC in DMSO.
The major objective of this investigation is to study the solvent effect for the substrates
undergoing oxidation.
2. Experimental
Materials. Tetraethylammoniumbromide and chromium trioxide were obtained from
Fluka (Buchs, Switzerland). The benzyl alcohols used were with substituents H, p-
NMe2, p-OMe, p-OEt, p-Me, p-F, p-Cl, p-Br, p-COOEt, p-CN, p-NO2, m-OMe,
m-Me, m-F, m-Cl, m-Br, m-CN and m-NO2. The procedure used for the purification of
alcohols has been described earlier [20]. Due to non-aqueous nature of the solvent, p-
toluenesulfonic acid was used as a source of hydrogen ions. Solvents were purified by
the usual methods [21].
Preparation of tetraethylammonium bromorochromate (C2H5)4N[CrO3Br]. Tetra-
ethylammonium bromochromate is easily prepared as follows: To a solution of 1 g
(10 mmol) CrO3 in 25 ml acetonitrile, placed in a 100 ml beaker, tetraethylammonium
bromide (2.10 g, 10 mmol) is added portion-wise with stirring. A reddish orange solid
is separated and stored in the refrigerator. The solid was washed with hexane and dried
under vacuum for 1 h [6]. Yield (95%); mp 130 ◦C. The chromium(VI) content may be
easily determined iodometrically.
CrO
3
−
N(C2H5)4Br −→
(C2H5)4N+[CrO3Br]
(1)
Tetraethyl amine
Tetraethylammonium bromochromate
Product analysis. Product analysis was carried as follows: In a typical experiment,
benzyl alcohol (5.4 g, 0.05 mol) and tetraethylammonium bromochromate (3.00 g,
0.01 mol) were made up to 50 cm3 in DMSO and kept in the dark for 24 h to ensure
completion of the reaction. The solution was then treated with an excess (150 cm3) of
a saturated solution of 2,4-dinitro phenyl hydrazine in 2 mol dm−3 HCl and kept over

Studies on the Kinetics of Tetraethylammonium Bromochromate Oxidation Ni
251
night in a refrigerator. The precipitated 2,4-dinitro phenyl hydrazone (DNP) was fil-
tered off, dried and recrystallised from ethanol. The DNP was found identical (m.pt
and mixed m.pt.) with DNP of benzaldehyde. (m.pt 234–236 ◦C, lit 237 ◦C). Similar
experiments were performed with other alcohols also.
Kinetic measurements. The pseudo-first-order conditions were attained by maintaining
a large excess (×15 or more) of alcohol over TEABC. The solvent was DMSO, unless
specified otherwise. The reactions were followed, at constant temperatures ( 0.01 K),
by monitoring the decrease in [TEABC] spectrophotometrically at 360 nm. No other
reactant or product has any significant absorption at this wavelength. In a spectropho-
tometer, monochromatic plane-parallel light enters a sample at right angles to the plane
surface of the sample. In these conditions, the transmittance and absorbance of a sample
depends on the molar concentration (c), light path length in centimeters (L), and mo-
lar extinction coefficient (ε) for the dissolved substance at the specified wavelength (λ).
Therefore,
Aλ = εcL
Absorbance is an extensive property of a substance whereas absorptivity (molar ex-
tinction coefficient) is its intensive property. If there is a change of concentration and
in the thickness of the container, the value of molar extinction coefficient will remain
constant within Beer’s law range but the absorbance will change significantly. The
value of molar extinction coefficient will be different at different wavelengths. Simi-
larly, absorbance also varies with wavelength. Because transmittance and absorbance
are unitless, the units for molar extinction coefficient must cancel with units of meas-
ure in concentration and path length. Therefore, molar extinction coefficient have units
of M−1 cm−1. Standard laboratory spectrophotometers are fitted for use with 1 cm width
sample cuvettes.
The pseudo-first-order rate constant. kobs, was evaluated from the linear (r = 0.990
to 0.999) plots of log [TEABC] against time. The second order rate constant k2, was
obtained from the relation k2 = kobs/[alcohol]. All experiments, other than those for
studying the effect of hydrogen ions, were carried out in the absence of p-toluene sul-
phonic acid (TsOH).
3. Results and discussion
The rate and other experimental data were obtained for all the alcohols. Since the results
are similar, only representative data are reproduced here.
Stoichiometric studies. The stoichiomety of the reaction was determined by carrying
out several sets of experiments with varying amounts of TEABC largely in excess over
benzyl alcohol. The estimation of unreacted TEABC showed that 1 mol of TEABC
reacts with 1 mol of benzyl alcohol.
C6H5CH2OH+O2CrBrO−N+(C2H5)4 → C6H5CHO+OCrBrO−N+(C2H5)4 +H2O
(2)

252
S. S. Mansoor and S. S. Shafi
Table 1. Effect of varying the concentration of [BnOH], [TEABC] and [H+] on the rate of reaction at
303 K.
103 [TEABC]
(M)
102 [BnOH]
(M)
[TsOH]
(M)
105 kobs
(s−1
)
0.5
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
4.0
6.0
8.0
10.0
2.0
2.0
2.0
2.0
2.0
2.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.1
0.2
0.4
0.6
0.8
0.0
15.84
15.88
15.82
15.88
15.80
31.60
47.40
62.28
78.90
17.16
20.60
24.03
28.18
32.85
15.08a
a
Contained 0.001 mol dm−3 acrylonitrile.
Order of reaction. The reaction was carried out at different concentrations of TEABC.
The near constancy in the values of kobs irrespective of the concentration of the TEABC
confirms the first order dependence on TEABC (Table 1). The rate of oxidation in-
creased progressively on increasing the concentration of benzyl alcohol, indicating first
order dependence with benzyl alcohol (Table 1). A plot of log kobs against log [BnOH] is
linear with a slope value of 0.994 (Fig. 1). The second order rate constant k2 is invariant
supporting the first order dependence on [BnOH].
Induced polymerization of acrylonitrile. Vinyl monomers like acrylonitrile are added
to the reaction mixture under nitrogen atmosphere to find out whether the reaction under
investigation involves the formation of free radicals as the reaction intermediates or
not. In the present study, freshly distilled acrylonitrile is added to the reaction mixture.
The oxidation of benzyl alcohol by TEABC, in an atmosphere of nitrogen failed to in-
duce the polymerization of acrylonitrile. Further, an addition of a radical scavenger,
acrylonitrile, has no effect on the rate (Table 1).
Effect of acidity. The reaction is catalyzed by hydrogen ions (Table 1). The hydrogen
ion dependence has the following form kobs = a+b [H+]. The values of a and b, for ben-
zyl alcohol at 303 K are 15.68 0.32×10−5 s−1 and 21.24 0.27×10−5 mol−1 dm3 s−1
respectively (r2 = 0.997).
Kinetic isotope effect. To ascertain the importance of the cleavage of the α-C–H bond
in the rate-determining step, oxidation of α,αꢀ-dideuterio benzyl alcohol was studied.
Results showed the presence of a substantial primary kinetic isotope effect (Table 2).

Studies on the Kinetics of Tetraethylammonium Bromochromate Oxidation Ni
253
Fig. 1. Order plot for the oxidation of benzyl alcohol by tetraethylammonium bromochromate at 303 K.
Table 2. Kinetic isotope effect on the oxidation of benzyl alcohol by TEABC.
105 ×k1, (s−1
)
Substrate
298 K
303 K
308 K
313 K
H
10.16
1.96
5.18
15.88
2.78
5.71
24.60
4.24
5.80
37.90
6.21
6.10
α,αꢀ-BnOH
kH/kD
[BnOH] = 2.0×10−2 M; [TEABC] = 1.0×10−3 M.
Effect of temperature. The rates of oxidation of all benzyl alcohols were determined at
four different temperatures and activation parameters were calculated (Table 3).
Effect of solvent. The rate of oxidation of benzyl alcohols were determined in 17 dif-
ferent organic solvents. The choice of the solvents was limited by the solubility of
TEABFC and its reactivity with primary and secondary alcohols. There was no notice-
able reaction with the solvents chosen. The kinetics were similar in all the solvents. The
values of k2 at 303 K are recorded in Table 4.
Structure–reactivity correlation. As the study of influence of substituents on the rate
of oxidation often provides an insight into the nature of the transition state, the rates
of oxidation of 18 meta- and para-mono substituted benzyl alcohols by TEABC were

254
S. S. Mansoor and S. S. Shafi
Table 3. Activation parameters and second order rate constants for the oxidation of para- and meta-
substituted benzyl alcohols by TEABC.
Substrate
103 ×k2, dm3 mol−1 s−1
ΔH#
−ΔS#
ΔG#
298 K
303 K
308 K
313 K
(kJ mol−1
)
(J K−1 mol−1
)
(kJ mol−1
)
H
5.39
22.60
11.00
4.60
3.62
0.60
0.33
124.4
21.40
2.90
1.06
9.16
9.24
2.08
1.98
0.46
0.25
1.56
7.94
33.74
16.79
7.26
5.60
0.90
12.20
50.16
24.32
10.35
8.49
1.47
0.80
308.5
47.03
6.28
18.50
71.89
37.99
15.95
13.04
2.23
61.47 1.5
57.43 0.3
60.82 2.0
60.80 2.0
63.56 0.8
66.05 2.0
64.77 1.3
68.07 1.0
59.81 1.5
58.30 1.3
66.36 2.0
55.52 1.0
56.48 0.7
65.69 2.6
63.62 1.1
68.97 1.0
72.72 1.7
62.52 1.3
82.24 5.0
84.17 1.0
78.37 6.0
85.58 6.0
78.56 2.0
84.88 6.0
94.31 4.0
33.85 4.0
76.25 5.0
97.91 4.8
79.36 6.5
97.17 3.0
94.14 2.4
76.03 8.5
88.82 3.6
77.53 3.6
70.44 5.5
89.08 4.5
86.38 3.0
82.94 0.6
84.56 3.8
86.73 3.5
87.37 1.5
91.76 3.8
93.54 2.5
78.32 2.2
82.91 3.0
87.96 2.7
90.40 4.0
84.96 2.0
85.00 1.4
88.72 5.0
90.53 2.2
92.46 2.0
94.06 3.4
89.51 2.5
p-OMe
p-Me
p-F
p-Cl
p-CN
p-NO2
p-NMe2
p-OEt
p-Br
p-COOEt
m-OMe
m-Me
m-F
0.50
1.20
199.0
32.10
4.36
490.8
72.05
9.50
1.60
2.54
4.00
13.70
13.86
3.12
2.96
0.72
19.51
19.94
4.84
4.60
1.17
28.44
29.14
7.80
6.84
1.82
m-Cl
m-CN
m-NO2
m-Br
0.40
2.40
0.65
3.57
1.07
5.53
[BnOH] = 2.0×10−2 M; [TEABC]= 1.0×10−3 M; ΔG# at 303 K.
Table 4. Second-order rate constants for the oxidation of benzyl alcohol by TEABC at 303 K in various
solvents.
104 ×k2
×104k2
Solvents
(dm−3 mol−1 s−1
)
Solvents
(dm−3 mol−1 s−1
)
Chloroform
1,2-Dichloroethane
Dichloromethane
DMSO
Acetone
DMF
Butanone
Nitrobenzene
Benzene
21.8
18.6
26.0
79.4
23.0
35.3
18.3
33.1
8.60
Toluene
Acetophenone
THF
tert-butylalcohol
1,4-Dioxane
1,2-Dimethoxyethane
Ethylacetate
Cyclohexane
7.28
35.6
11.0
9.98
13.6
5.88
9.20
0.82
determined at four different temperatures viz., 298, 303, 308 and 313 K in DMSO. The
activation parameters for the oxidations have been evaluated from the slope and inter-
cept of Eyring’s plot of log (k2/T) against 1/T, from k2 at 298, 303, 308 and 313 K by
the method of least square and presented in Table 3. A representative Eyring’s plot for
the oxidation of benzyl alcohol by TEABC is given in Fig. 2. The least square method
gives the values and standard errors of enthalpy and entropy of activation respectively.
Statistical analysis of the Eyring equation clearly confirms that the standard errors of

Studies on the Kinetics of Tetraethylammonium Bromochromate Oxidation Ni
255
Fig. 2. Eyring’s plot for the oxidation of benzyl alcohol by tetraethylammonium bromochromate.
ΔH# and ΔS# correlate.
(ΔS#) = 1/Tav(ΔH#),
where Tav is the centre of the temperature range used.
It follows that in most solution phase studies (ΔS#) ≈ (ΔH#)×0.003 K−1 [22]. The
negative entropy of activation suggests a definite orientation in the transition state and
this may be in part due to the solvation of the activated complex.
Isokinetic relationship. The reaction is neither isoenthalpic nor isoentropic but com-
plies with the compensation law also known as the isokinetic relationship.
ΔH# = ΔHo +βΔS#
(3)
The isokinetic temperature β 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 reaction, 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 [23].
An isokinetic plot between activation enthalpies and entropies of oxidation of 18
alcohols is not very good (r2 = 0.9456). The value of isokinetic temperature is 544
16 K. However, according to Exner [23] an isokinetic relationship between the calcu-
lated values of enthalpies and entropies of activation is often vitiated by the random

256
S. S. Mansoor and S. S. Shafi
Fig. 3. Exner’s plot for the oxidation of benzyl alcohol by tetraethylammonium bromochromate between
4+log k2 (at 308 K) and 4+log k2 (at 303 K).
experimental errors associated with them. Exner [23] suggested an alternative method
of testing the validity of isokinetic relationship. The isokinetic relationship is tested by
plotting the logarithms of rate constants at two different temperatures (T2 > T1) against
each other according to Eq. (4).
log k(at T2) = a+b log k(at T1) .
(4)
The linear relationship in Exner plots [24,25] at 4+ log k2 (303 K) and 4 + log k2
(308 K) observed in the present study imply the validity of the isokinetic relationship
(Fig. 3). Isokinetic temperature obtained is 418 12 K. The linear isokinetic correla-
tion implies that all the benzyl alcohols are oxidized by the same mechanism and the
changes in the rate are governed by the changes in both the enthalpy and entropy of
activation [26].
Hammett plot. A linear Hammett’s plot is obtained when substituent constant (σ) for
different substituents were plotted against log k2 for the oxidation of benzyl alcohols
by TEABC at various temperatures. The value of slope of Hammett plot is known as
reaction constant (ρ). Reaction constant values at different temperatures are given in
Table 5. A linear Hammett’s plot is given in Fig. 4. According to Hammett, the reac-
tion with positive ρ values are accelerated by electron withdrawal from benzene ring,
whereas those with negative ρ values are retarded by electron withdrawal from benzene

Studies on the Kinetics of Tetraethylammonium Bromochromate Oxidation Ni
257
Table 5. Reaction constant values for the oxidation of benzyl alcohol by TEABC at Different Temperaturesa.
Temperature
(K)
Reaction
Correlation
coefficient
Standard
deviation
constant (ρ)b
298
303
308
313
−1.71 0.7
−1.68 0.3
−1.63 0.5
−1.55 0.9
0.998
0.992
0.993
0.997
0.09
0.02
0.07
0.08
[BnOH] = 2.0×10−2 M; [TEABC] = 1.0×10−3 M.
a
σp Values were taken from reported works [28].
The values were obtained by correlating log k2 with σp for the reac-
b
tions of oxidations.
Fig. 4. Hammett plot for the oxidation of benzyl alcohol by tetraethylammonium bromochromate at 303 K.
ring [27,28]. In this oxidation reactions, the electron withdrawing groups decreases the
rate and the electron donating groups increases the rate. These observations supporting
the negative ρ values obtained from the Hammett Plot.
Solvent–reactivity correlation. The influence of solvent on the rate of any reaction can
be described in terms of solvation which is a stabilization process. Two view points
have been established on the solvation phenomenon. According to the first, a solvent is
considered as a homogeneous continuum which surrounds the solute molecules and ex-
erts long range interactions. The strength of these interactions with the solute molecules

258
S. S. Mansoor and S. S. Shafi
is described in terms of macroscopic physical properties of the solvent like dielectric
constant (ε) and refractive index (η). According to the second view point, a solvent is
considered to be anisotropic and inhomogeneous which exerts short range forces on the
solute molecules. These forces are chemical in nature, and result in the formation of
solvation complexes through donor–acceptor bonds which are localized and directed
in space. The strength of these interactions is described in terms of solvation parame-
ters namely hydrogen bond donor acidity (α), hydrogen bond acceptor basicity (β), etc.,
Thus the solvent can solvate the solute by exhibiting any of these interactions with the
specific sites in the solute. Hence, the effect of solvent on the rate of the reaction can
be described in terms of multiparameteric equation which involves different solvation
parameters given by Kamlet–Taft.
The Kamlet–Taft method for the examination of solvent effect. In order to obtain
a deeper insight into the various solvent–solvent–solute interactions, which influence
reactivity, the solvatochromic comparison method developed by Kamlet [29] has been
used. This method may be used to unravel, quantify, correlate and rationalize multiple
interacting solvent effects on reactivity. The kinetic data were correlated with the sol-
vatochromic parameters α, β and π∗ characteristic of different solvents in the form of
following LSER:
log k2 = Ao + pπ∗ +bβ +aα
(5)
where π∗ is an index of solvent dipolarity/polarizability, 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 (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
intercept term.
The rate constants of oxidation, k2, in 17 solvents were correlated in terms of the
linear solvation energy relationship (5) of Kamlet et al. [29]. It may be mentioned here
that out of the 17 solvents, 12 have a value of zero for α.
Kamlet et al. [30] established that the effect of a solvent on the reaction rate should
be given in terms of the following properties: i) the behaviour of the solvent as a dielec-
tric, facilitating the separation of opposite charges in the transition state, ii) the ability
of the solvent to donate a proton in a solvent-to-solute hydrogen bond and thus stabilize
the anion in transition state and iii) the ability of the solvent to donate an electron pair
and therefore stabilize the initial benzyl alcohol, by way of a hydrogen bond between
the alcoholic proton and the solvent electron pair. The parameter π∗ is an appropriate
measure of the first property, while the second and third properties are governed by
the parameters α and β respectively. The solvent parameters (π∗, α and β) are taken
from the literature [29] and are given in Table 6. The linear dependence (LSER) on the
solvent properties were used to correlate and predict a wide variety of solvent effect.
In order to explain the kinetic results through the solvent polarity and basicity or
acidity, the rate constants were correlated with he solvatochromic parameters π∗, α and
β using total solvatochromic equation, Eq. (5). The correlation of kinetic data were real-
ized by means of multiple linear regression analysis. It was found that the rate constants
in 17 solvents showed satisfactory correlation with the π∗, α and β solvent parameters.

Studies on the Kinetics of Tetraethylammonium Bromochromate Oxidation Ni
259
Table 6. Solvent parameters [29].
Solvent
π∗
α
β
Chloroform
1,2-Dichloroethane
Dichloromethane
DMSO
Acetone
DMF
Butanone
Nitrobenzene
Benzene
Cyclohexane
Toluene
Acetophenone
THF
0.58
0.81
0.82
1.00
0.71
0.88
0.67
1.01
0.59
0.00
0.54
0.90
0.58
0.41
0.55
0.53
0.55
0.44
0.00
0.30
0.00
0.08
0.00
0.06
0.00
0.00
0.00
0.00
0.00
0.00
0.68
0.00
0.00
0.00
0.00
0.00
0.00
0.76
0.48
0.69
0.48
0.39
0.10
0.00
0.11
0.49
0.55
1.01
0.37
0.41
0.45
tert-butylalcohol
1,4-Dioxane
1,2-Dimethoxyethane
Ethylacetate
The results of correlation analysis in terms of Eq. (5), a biparametric equation involving
π∗ and β are given below in Eqs. (6)–(9).
log k2 = −4.03+(1.60 0.18)π∗ +(0.19 0.09)β −(0.13 0.08)α
R2 = 0.8656; SD = 0.11; n = 17; ψ = 0.43
log k2 = −4.06+(1.74 0.13)π∗ +(0.15 0.07)β
R2 = 0.8640; SD = 0.10; n = 17; ψ = 0.33
log k2 = −4.07+(1.79 0.17)π∗
(6)
(7)
(8)
(9)
r2 = 0.8822; SD = 0.11; n = 17; ψ = 0.20
log k2 = −3.08+(0.45 0.22)β
r2 = 0.0792; SD = 0.33; n = 17; ψ = 0.75
Here n is the number of data points and ψ is the Exner’s statistical parameter [31].
Kamlet’s [29] triparametric equation explains ca. 86% of the effect of solvent on on
the oxidation. However, by Exner’s criterion [31] the correlation is not even satisfactory
(cf. (9)). The major contribution is of solvent polarity. It alone accounted for ca. 88% of
the data. Both α and β play relatively minor roles.
The Swain’s method for the examination of solvent effect. The data on solvent effect
were analysed in terms of Swain’s equation [32] of cation- and anion-solvating concept
of the solvents also.
log k2 = aA +bB +C
(10)

260
S. S. Mansoor and S. S. Shafi
Here A represents the anion-solvating power of the solvent and B the cation-solvating
power. C is the intercept term. (A + B) is postulated to represent the solvent polarity.
The rates in different solvents were analysed in terms of Eq. (10), separately with A and
B and with (A + B).
log k2 = (0.64 0.03)A +(1.74 0.02)B −4.25
R2 = 0.9870; SD = 0.14; n = 17; ψ = 0.32
log k2 = 0.46( 0.29)A −3.04
(11)
(12)
(13)
(14)
r2 = 0.3052; SD = 0.08; n = 17; ψ = 0.76
log k2 = 1.70( 0.16)B −4.27
r2 = 0.9490; SD = 0.15; n = 17; ψ = 0.33
log k2 = 1.42 0.16(A + B)−4.21
r2 = 0.9070; SD = 0.25; n = 17; ψ = 0.55
The rates of oxidation of benzyl alcohol in different solvents showed an excellent cor-
relation in Swain’s equation with the cation-solvating power playing the major role. In
fact, the cation solvation alone accounts for ca. 95% of the data. The correlation with
anion- solvating power was very poor. The solvent polarity, represented by (A + B),
also accounted for ca. 90% of the data.
Mechanism of oxidation. From the product analysis, DNP was confirmed. Hence, it
shows that under the experimental conditions employed in the present study, ben-
zyl alcohols were oxidized to the corresponding aldehydes stage only. Based on the
above kinetic observations the following mechanism is proposed for the reaction. Ab-
sence of any effect of added acrylonitrile on the reaction discounts the possibility of
a one-electron oxidation, leading to the formation of free radicals. The presence of
a substantial kinetic isotope effect in the oxidation of benzyl alcohol confirms the cleav-
age of the α-C–H bond in the rate-determining step. Therefore, a hydride-ion transfer
in the rate determining step is suggested (Scheme 1). The hydride ion transfer may
take place either by a cyclic process via an ester intermediate or by an acyclic one-
step bimolecular process. Negative reaction constants are traditionally associated with
an electron deficient centre in transition states: a convention originally developed from
the analysis of substituent effects in nucleophilic displacement reactions. Negative reac-
tion constants have been used by Banerji [33–35] as supporting evidence for oxidation
mechanisms involving a hydride-ion transfer in the rate determining step.
Kwart and Nickel [36] have shown that a dependence of kinetic isotope ef-
fect on temperature can be gainfully employed to determine whether the loss of
hydrogen proceeds through a concerted cyclic process or by an acylic one. The
data for protio- and deuterio-benzyl alcohols, fitted to the expression: kH/kD =
AH/AD(−ΔH∗/RT) [37,38] show a direct correspondence with the properties of
a symmetrical transition state in which activation energy difference for protio and deu-
terio compounds is equal to the difference in the zero-point energy for the respective
C–H and C–D bonds (≈ 4.5 kJ mol−1) and the entropies of activation of the respective
reactions are almost equal.

Studies on the Kinetics of Tetraethylammonium Bromochromate Oxidation Ni
261
Scheme 1. Acid-independent path.
Scheme 2. Acid-dependent path.
It is well-established that intrinsically concerted sigmatropic reactions, character-
ized by transfer of hydrogen in a cyclic transition state, are the only truly symmetrical
processes involving a linear hydrogen transfer [39]. Littler [40] has shown that a cyclic
hydride transfer, in which the oxidation of alcohols by Cr(VI), involving six electrons
and, being a Huckel-type system, is an allowed process. Thus, a transition state having
a planar, cyclic and symmetrical structure can be envisaged for the decomposition of the
ester intermediate. Hence, the overall mechanism is proposed to involve the formation
of a chromate ester in a fast pre-equilibrium step and then a decomposition of the ester

262
S. S. Mansoor and S. S. Shafi
in a subsequent slow step via a cyclic concerted symmetrical transition state leading to
the product.
The mechanism depicted in Scheme 1 leads to the following equation:
Rate = kK [BnOH] [TEABC]
It can be shown that
(19)
(20)
(21)
[TEABC] = [TEABC]T/(1+ K [BnOH])
Therefore,
−d[TEABC]/dt = kK [BnOH] [TEABC]T/(1+ K [BnOH])
Since the reaction is the first order with respect to each the alcohol and TEABC, one can
assume that 1 ꢁ K [BnOH]. The assumption is made because the concentration of al-
cohol is in the order of 0.01 to 0.02 mol dm−3 and which is very small compared with 1.
Equation (22) can therefore, be written as
−d[TEABC]/dt = kK [BnOH] [TEABC]
(22)
This rate equation is in accord with the experimental results.
The observed dependence on the hydrogen–ion concentration in the reaction shows
that there is an additional acid-catalyzed pathway. The acid-dependent path way is
given in Scheme 2.
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