Kinetics and correlation analysis of reactivity in the oxidation of aliphatic primary alcohols by isoquinolinium dichromate in non-aqueous medium

Article abstract of DOI:10.1016/j.jics.2021.100009

Mild oxidation in dimethyl sulfoxide (DMSO) medium by isoquinolinium dichromate (IQDC) of aliphatic primary alcohols produces corresponding carbonyl compounds. A Michaelis-Menten kind kinetics noticed as for alcohols while unit dependency on rate observed as for IQDC. At non-identical temperatures the formation constants and the rates of decomposition of alcohol-IQDC complexes have been evaluated. Thermodynamic parameters and activation parameters for formation of the complex and break down of the complexes have been determined respectively. The oxidation process accelerates with increase in proton concentration. An α-C-H bond fisson in the rate-controlling step suggested by the deuterium isotope effect. For oxidation of ethanol, k_H/k_D = 5.82 at 293 K, was observed. The oxidation rates have been evaluated in 19 organic solvents and greater role of solvating power of the cation is observed. Depended on the kinetic parameters, solvent effect analysis and the outcome of thermodynamic parameters, a mechanism in which rate-controlling break down of the complex is suggested, to give the resulting product through hydride-ion transfer with a cyclic transition state.

Full text of DOI:10.1016/j.jics.2021.100009

Journal of the Indian Chemical Society 98 (2021) 100009
Contents lists available at ScienceDirect
Journal of the Indian Chemical Society
journal homepage: www.editorialmanager.com/JINCS/default.aspx
Kinetics and correlation analysis of reactivity in the oxidation of aliphatic
primary alcohols by isoquinolinium dichromate in non-aqueous medium
Reena Kalal, Dinesh Panday *
Department of Chemistry, M. L. Sukhadia University, Udaipur, 313001, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Kinetics
Mild oxidation in dimethyl sulfoxide (DMSO) medium by isoquinolinium dichromate (IQDC) of aliphatic primary
alcohols produces corresponding carbonyl compounds. A Michaelis-Menten kind kinetics noticed as for alcohols
while unit dependency on rate observed as for IQDC. At non-identical temperatures the formation constants and
the rates of decomposition of alcohol-IQDC complexes have been evaluated. Thermodynamic parameters and
activation parameters for formation of the complex and break down of the complexes have been determined
Alcohols
Oxidation
Correlation
Mechanism
IQDC
respectively. The oxidation process accelerates with increase in proton concentration. An α-C-H bond fisson in the
rate-controlling step suggested by the deuterium isotope effect. For oxidation of ethanol, kH/kD ¼ 5.82 at 293 K,
was observed. The oxidation rates have been evaluated in 19 organic solvents and greater role of solvating power
of the cation is observed. Depended on the kinetic parameters, solvent effect analysis and the outcome of ther-
modynamic parameters, a mechanism in which rate-controlling break down of the complex is suggested, to give
the resulting product through hydride-ion transfer with a cyclic transition state.
1. Introduction
were purified by usual methods. [1,1–2H2] Ethanol was formed by the
described procedure [4] and its purity was 92.4% which is determined by
In synthetic organic chemistry, under non-aqueous medium partic-
ular oxidation of organic compounds is a significant conversion. Due to
this reason various Cr(VI) complexes have been documented [1,2]. Iso-
quinolinium dichromate [IQDC, (C9H7NHþ)2Cr2O7-2] is a complex of
Cr(VI), reported in 1998 [3]. Since IQDC is neither light sensitive nor
hygroscopic, thus, it is stored easily and more stable rather than other
derivatives of Cr(VI). Conversion of primary and secondary alcohols by
IQDC to their respective carbonyl compounds with 60 to 90% yield is
well documented3. We get focused on mechanistic investigations with
kinetic aspects of the oxidation by IQDC. A study of literature appears
that there is no work reported on the oxidation kinetics of aliphatic
primary alcohols by IQDC. Here we investigate the oxidation kinetics of
aliphatic primary alcohols by IQDC in solvent DMSO.
proton NMR spectra. Purification of solvents was determined by the
described methods [5]. Amongst the solvents, CS2 is a combustible and
toxic liquid in the midst of solvents, and as resource of hydrogen ions
TsOH was utilized. The standard alkali solution was used for standardi-
zation of TsOH solution.
Product analysis: Under kinetic conditions i.e. excess of alcohol over
oxidant, product analyses were executed. In this experiment, ethanol
(0.5 mol), TsOH (0.1 mol), and IQDC (0.02 mol) in DMSO were compose
to 100 ml and it was then left in absence of light for a day to make sure
the reaction accomplishment. The solution with 200 ml of 2,4-dinitro-
phenylhydrazine (saturated) solution in 2 M hydrochloric acid was
then reacted by above solution and kept in a chiller for whole night, and
obtained product corresponding hydrazone was percolated, parched,
weighed and with ethyl alcohol recrystallized it and again weighed. The
formation of 2,4-dinitrophenylhydrazone (DNP), previously and subse-
quently recrystallization was 6.05 g (90%) and 5.44 g (81%) corre-
spondingly. It was perceived analogus identical (mixed mp and mp) to an
authentic sample of DNP of acetaldehyde. Similar experiment was
repeated with other alcohols the yield of DNP of the related aldehyde
later recrystallization was around 78–85%.
2. Materials and methods
From the described procedure [3] oxidant IQDC was composed and
its IR spectrum (KBr) showed bands at 930, 875, 765 and 730 cmꢀ1
which are the properties of dichromate-ion. Its immaculateness was
checked iodometrically. The alcohols were commercial products and
* Corresponding author. Flat No. 03, Krystal Plaaza Apartment, Sector-9, Savina, Udaipur, 313001, India.
E-mail addresses: reenakalal71@gmail.com (R. Kalal), dpanday26@gmail.com (D. Panday).
https://doi.org/10.1016/j.jics.2021.100009
Received 5 December 2020; Received in revised form 17 January 2021; Accepted 17 January 2021
0019-4522/© 2021 Indian Chemical Society. Published by Elsevier B.V. All rights reserved.

R. Kalal, D. Panday
Journal of the Indian Chemical Society 98 (2021) 100009
is for a day. Spectrophotometrically at 370 nm the residual IQDC was
measured. Various observations indicate that 3 mol of alcohols react with
2 mol of IQDC at non-identical alcohol concentrations. The outcomes
with ethanol are shown in Table 1. IQDC is reduced to Cr(III) and
participating in the reaction as a 3-electron oxidant.
Table 1
Stoichiometry data for the oxidation by IQDC of ethanol.
103 [IQDC]
103 [Ethanol]
103 [Unused
[Ethanol] [Used up
IQDC]
(mol dmꢀ3
)
(mol dmꢀ3
)
IQDC] (mol dmꢀ3
)
10.0
10.0
10.0
2.0
4.0
6.0
8.74
7.30
6.03
1.55
1.48
1.51
Kinetic measurements: The pseudo-first-order state were executed for
all kinetic runs at constant temperature (ꢁ0.1), by sustaining over
abundance of alcohols (15 fold or greater) on IQDC. DMSO was used as
solvent in the reaction, unless specified. The progress of reaction up to
75% was studied by monitoring the consumption of IQDC at 370 nm. In
this investigation for all concentrations Beer's law is applicable. A graphic
between log [IQDC] and time was linear with r2 > 0.99 and velocity
constant (kobs) was obtained from it and for more than two runs repli-
cability ꢁ3% was observed. In correlation analyses coefficient of deter-
Mean ¼ 1.51 ꢁ 0.03
Table 2
Rate data at 313 K for alcohols oxidation by IQDC.
103[IQDC] (mol
[Alcohol] (mol
10 [Hþ] (mol
103 kobs(sꢀ1
)
dmꢀ3
)
dmꢀ3
)
dmꢀ3
)
ethanol
n-
mination (R2 or r2), Exner's parameter [6] (
(sd) was used.
ψ) and standard deviation
propanol
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
0.8
1.5
2.0
5.0
1.0
0.05
0.1
0.2
0.3
0.4
0.6
1.0
0.6
0.6
0.6
0.6
0.6
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
5.4
7.9
11.69
16.73
21.32
23.47
24.7
26.1
27.3
26.31
25.95
26.05
26.2
3. Results
10.3
11.4
12.1
12.8
13.6
12.96
12.6
12.4
13.2
13.04
13.42a
The stoichiometric ratios, Δ[Alcohols]/Δ[IQDC], confirmed the
following overall reactions for the oxidation of alcohols respectively.
3RCH2OH þ 2Cr2O7-2 þ 22Hþ → 3RCHO þ 2Crþ6 þ 2Crþ3 þ 14H2O (1)
25.83
27.51a
3.1. Test for free radical
Oxidations of alcohols by IQDC, under nitrogen ambience, the acry-
lonitrile induce polymerization was not observed. No considerable IQDC
consumption, in absence of alcohols, was noticed. The oxidation rate was
unaffected on acrylonitrile addition (Table 1). For additional confirma-
tion of the absence of one electron oxidation during course of the
Stoichiometry: The oxidation of alcohols prompts the development of
relating oxoacids. For stoichiometry determination, IQDC (0.05 mol) and
alcohol (0.002 mol) in DMSO were composed to 200 ml with 1.0 M
TsOH. To ensure the completion of the reaction the solution was kept as it
Fig. 1. A plot of (103 kobs) against [Alcohol] at 313 K
[TsOH] ¼ 0.2 mol dmꢀ3; [IQDC] ¼ 0.001 mol dmꢀ3
.
2

R. Kalal, D. Panday
Journal of the Indian Chemical Society 98 (2021) 100009
Fig. 2. A plot of 1/(103 kobs) against 1/[ Alcohol] at 313 K
[TsOH] ¼ 0.2 mol dmꢀ3; [IQDC] ¼ 0.001 mol dmꢀ3
.
Table 3
RCH2OH-IQDC complex formation constants and their parameters of thermodynamics in DMSO.
Alcohol (R)
K# (molꢀ1dm3)
ΔHf
ΔSf
ΔGf
293 K
303 K
313 K
323 K
kJ molꢀ1
J molꢀ1 Kꢀ1
kJ molꢀ1
H
Me
Et
Pr
22.3
22.1
24.8
24.5
23.9
24.9
21.2
22.7
23.2
22.8
16.3
15.8
16.9
18.1
16.3
17.2
14.8
14.9
15.7
16.5
12.9
11.6
13.2
12.8
12.1
12.3
11.8
12.1
11.2
13.5
11.2
10.05
10.8
10.6
9.9
10.1
9.5
10.9
9.3
ꢀ20.67 ꢁ 1.2
ꢀ23.6 ꢁ 1.4
ꢀ24.14 ꢁ 1.3
ꢀ25.03 ꢁ 1
ꢀ25.7 ꢁ 1.3
ꢀ26.5 ꢁ 1.2
ꢀ23.28 ꢁ 1
ꢀ20.9 ꢁ 0.5
ꢀ26.8 ꢁ 1.4
ꢀ21.32 ꢁ 0.8
ꢀ37 ꢁ 3.8
ꢀ47 ꢁ 4.6
ꢀ48 ꢁ 4.1
ꢀ51 ꢁ 3.2
ꢀ54 ꢁ 3.9
ꢀ56 ꢁ 3.8
ꢀ46 ꢁ 3.2
ꢀ38 ꢁ 1.8
ꢀ58 ꢁ 4.6
ꢀ39 ꢁ 2.8
ꢀ9.79 ꢁ 0.9
ꢀ9.3 ꢁ 1.1
ꢀ9.98 ꢁ 1.0
ꢀ10 ꢁ 0.78
ꢀ9.88 ꢁ 0.94
ꢀ9.9 ꢁ 0.92
ꢀ9.6 ꢁ 0.8
ꢀ9.84 ꢁ 0.4
ꢀ9.8 ꢁ 1.1
ꢀ9.85 ꢁ 0.7
Bu
Pri
ClCH2
MeOCH2
But
MeCD2OH
11.0
reaction, the reaction was executed with 0.005 mol dmꢀ3 of butylated
hydroxytoluene (BHT). The BHT did not show any considerable change
and observed nearly quantitatively.
therefore graphic was straight line (r2 > 0.996) between logarithms of
[IQDC] and time. The kobs (pseudo-first-order velocity constant) do not
alter appreciably on IQDC beginning concentration (Table 2). The frac-
tional dependency (0<order<1) was observed for alcohols (Table 2). In
Fig. 1, a graphic between kobs and [Alcohol] is exhibited. The curvature of
the plots leading towards a lower level reveals the development of a
complex. Double reciprocal graphic between 1/kobs and 1/[ Alcohol] was
3.2. Rate laws
In this oxidation reactions unit dependency exhibited as for BIDC
Table 4
RCH2OH-IQDC complex decomposition constants and their parameters of activation in DMSO.
Alcohol (R)
103 k2 (sꢀ1
)
ΔH*
ΔS*
ΔG*
293 K
303 K
313 K
323 K
kJ molꢀ1
J molꢀ1 Kꢀ1
kJ molꢀ1
H
Me
Et
Pr
0.0074
3.2
0.022
6.8
13.6
32.2
37.2
63
0.064
0.82
1830
1.28
5.31
0.068
14.7
29.4
63.6
73.2
119.1
0.186
1.97
2960
2.92
5.03
0.18
31.1
58.6
125
147
230
0.48
4.54
4596
6.45
4.82
81.7 ꢁ 0.8
57.2 ꢁ 0.8
54.9 ꢁ 0.8
50.7 ꢁ 0.7
49.5 ꢁ 2.7
47.9 ꢁ 0.9
76.5 ꢁ 1.1
64.8 ꢁ 0.8
33.5 ꢁ 0.6
62.06 ꢁ 0.5
ꢀ65 ꢁ 2.7
ꢀ98 ꢁ 2.7
ꢀ100 ꢁ 2.5
ꢀ107 ꢁ 2.4
ꢀ110 ꢁ 8.8
ꢀ110 ꢁ 3
100 ꢁ 0.6
86.2 ꢁ 0.6
84.5 ꢁ 0.6
82.4 ꢁ 0.6
82. ꢁ2.1
6.65
16.4
19.3
33.5
0.024
0.35
1172
0.55
5.82
Bu
Pri
80.6 ꢁ 0.7
98.1 ꢁ 0.9
91.7 ꢁ 0.6
72 ꢁ 0.4
ClCH2
MeOCH2
But
ꢀ73 ꢁ 3.7
ꢀ91 ꢁ 2.7
ꢀ130 ꢁ 1.8
ꢀ96 ꢁ 1.5
MeCD2OH
90.5 ꢁ 0.4
kH/kD
3

R. Kalal, D. Panday
Journal of the Indian Chemical Society 98 (2021) 100009
present study, the value of kH/kD, increases with decrease in temperature.
Table 5
Rate
of
reaction
reliance
on
concentration
of
hydrogen-ion
[IQDC] ¼ 0.001 mol dmꢀ3, [Alcohol] ¼ 0.5 mol dmꢀ3 at 313 K.
3.4. Effect of hydrogen-ion concentration
[TsOH] (mol dmꢀ3
ethanol, 103 kobs (sꢀ1
n-propanol, 103 kobs (sꢀ1
)
0.1
8.1
18.4
0.2
13.6
29.5
0.4
0.6
1.0
1.5
)
25.2
45.6
31.2
51.8
39.9
60.6
46.4
66.3
The rates of oxidation of alcohols are catalyzed by hydrogen-ion
concentration (Table 5). A graphic between log kobs and log [Hþ] is
straight line with positive slope which is less than one. This suggests the
dependency with hydrogen-ion concentration on rate is between zero to
one. Thus, a graphic between 1/kobs and 1/[Hþ] was undertaken and
with positive intercept a straight line was obtained (Fig. 3). No particular
reaction was observed without toluene p-sulphonic acid (TsOH).
)
straight line with positive intercept (Fig. 2). Hence, for all Alcohols, a
Michaelis-Menten kind kinetics was noticed. This suggests the existence
of following rate law and mechanism:
#
Alcohol þ IQDCK⇌½Complexꢂ
(2)
3.5. Solvent effect
k2
½Complexꢂ→Product
(3)
(4)
Nineteen organic solvents were used to study the oxidation of alco-
hols by IQDC. The reaction of IQDC with 1ꢃ and 2ꢃ alcohols and reactant's
solubility restricted the solvent's selectivity. No reaction was observed
Rate ¼ k2K#½Alcoholꢂ½IQDCꢂt = ð1 þ K#½AlcoholꢂÞ
Or,
Table 6
1 = ðRate = ½IQDCꢂtÞ ¼ 1 = kobs ¼ 1 = k2K#½Alcoholꢂ þ 1 = k2
Here, ½IQDCꢂt ¼ ½IQDCꢂ þ ½Complexꢂ
(5)
Solvent's effect at 313 K on the ethanol oxidation of by IQDC.
Solvent
K# (molꢀ1dm3)
103k2 (sꢀ1
)
Chloroform
12.1
10.6
11.3
12.4
11.6
10.8
11.5
11.1
11.0
10.2
12.3
11.8
10.3
12.2
10.8
11.9
12.0
11.7
11.1
4.28
0.83
5.36
5.12
14.7
4.92
8.34
3.70
6.18
1.94
0.23
1.57
2.05
6.79
2.68
1.84
2.73
1.46
0.72
At non-identical temperatures, the kobs dependency on alcohol con-
centrations were calculated and using eq. (5), k2 and K# values with the
help of double reciprocal graphic, were determined. For establishment of
complex, the thermodynamic parameters (Table 3) and breaking down of
them, the activation parameters (Table 4) were determined at non-
identical temperatures, from k2 and K# values respectively.
Carbon disulfide
1,2-Dichloroethane
Dichloromethane
DMSO
Acetone
DMF
Butanone
Nitrobenzene
Benzene
Cyclohexane
Toluene
Ethyl acetate
Acetophenone
Tetrahydrofurane
tert-Butyl alcohol
1,4-Dioxane
1,2-Dimethoxy ethane
Acetic acid
3.3. Kinetic isotope effect
The oxidation of [1,1–2H2] ethanol was analyzed with IQDC for
established the significance of
α-CꢀH bond fission in the rate-controlling
pathway. The findings (Table 3) revealed that complex formation con-
stants of ethanol and deuterated ethanol are nearly close but decompo-
sition rates of them (Table 4) indicated a deuterium isotope effect (kH/
kD ¼ 5.82 at 293 K). Primary kinetic isotope effect value was found
similar in case of benzyl alcohol oxidation by BIDC [7]. Further, in
Fig. 3. A plot of 1/(103 kobs) against 1/[Hþ] at 313 K
[Alcohol] ¼ 0.2 mol dmꢀ3; [IQDC] ¼ 0.001 mol dmꢀ3
.
4

R. Kalal, D. Panday
Journal of the Indian Chemical Society 98 (2021) 100009
with the selected solvents. The kinetics was nearly same with these sol-
Table 7
vents. In Table 6, corresponding K# and k2 values are mentioned.
Correlation of the rates of oxidation of alcohols in terms of Pavelich- Taft
equation.
T (K)
ρ
*
δ
R2
sd
ψ
n
4. Discussion
293
303
313
323
ꢀ2.30 ꢁ 0.01
ꢀ2.19 ꢁ 0.02
ꢀ2.07 ꢁ 0.02
ꢀ1.97 ꢁ 0.01
ꢀ1.21 ꢁ 0.02
ꢀ1.14 ꢁ 0.03
ꢀ1.08 ꢁ 0.01
ꢀ1.03 ꢁ 0.01
0.9998
0.9999
0.9999
0.9998
0.0042
0.0041
0.0094
0.0089
0.016
0.011
0.011
0.016
9
9
9
9
The enthalpies and entropies of the oxidation of nine alcohols
revealed adequate correlation (r2 ¼ 0.9979) with an isokinetic temper-
ature 750.14 ꢁ 1.8 K. The correlation was checked and observed
authentic with Exner's criterion [8]. The Exner's graphic between the
values of log k2 at 293 K and at 323 K, for the nine alcohols, was linear
(slope ¼ 0.8504 ꢁ 2.03, r2 ¼ 0.9999) and value of isokinetic temperature
was 773 ꢁ 17 K. The two values showed perfect agreement. For validity
of linear free energy relationships8, a linear isokinetic relationship is an
essential requirement. It specifies that all the reactions which are
correlated, oxidized through common mechanism.
n ¼ number of data points.
log k2 ¼ 0.42 ꢁ 0.01 A þ 1.65 ꢁ 0.03 B–3.75
R2 ¼ 0.9999, sd ¼ 0.003, n ¼ 19,
(9)
(10)
(11)
(12)
ψ ¼ 0.01, T ¼ 313 K
log k2 ¼ 0.18 ꢁ 0.54 A - 2.61
Protonated oxidant (IQDCHþ) appears to be an ionic compound. In
our reaction ambience the nature of IQDC is determined at 313 K with
conductivity measurements. Low conductivity of DMSO and insignificant
variation in conductivity value due to the inclusion of IQDC in DMSO was
observed; therefore IQDC does not break up as isoquinolinium and di-
chromate ions thus IQDC remain non-ionised in our reaction system. The
rate of oxidation does not alters on addition of isoquinolinium ion, also
favours the postulation that IQDC remain as non-ionised.
It is observed, after protonation IQDC (X) produce a protonated
Cr(VI) species (Y) which is active electrophile and stronger oxidant (eq.
(6)). Thus, oxidation process through IQDC depends upon proton
concentration.
r2 ¼ 0.0068, sd ¼ 0.44, n ¼ 19,
log k2 ¼ 1.62 ꢁ 0.07 B–3.61
r2 ¼ 0.9654, sd ¼ 0.08, n ¼ 19,
ψ
ψ
¼ 1.02, T ¼ 313 K
¼ 0.19, T ¼ 313 K
log k2 ¼ 1.24 ꢁ 0.16 (A þ B) – 3.71
r2 ¼ 0.7861, sd ¼ 0.20, n ¼ 19,
ψ ¼ 0.47, T ¼ 313 K
The observed results of solvent effect revealed a perfect correlation
regarding Swain's equation with both solvating power of cation- and
anion-participating with perceived effect of solvent. Although, the
contribution of solvating power of cation is greater, it alone contributes
for 96.54% of the data. (A þ B), represents the polarity of the solvent,
explains 78.61% of the data. Relative permittivities of the solvents were
correlated with the data by considering that 78.61% of the data is
contributed by polarity of the solvent. The above mentioned data was
K1
ðIQOHÞ2Cr2O5 þ Hþ⇌ðIQOHÞ2OCrðOHÞOCrO2
(6)
ðXÞ
ðYÞ
Here, BI represents benzimidazole. Further, effect of proton concentra-
tion on rate could be expressed as -
correlated with the Kirkwood function, (ε-1)/(2εþ1), here ‘ε’ is dielectric
1/ kobs ¼ a þ b / [Hþ]
(7)
constant of the medium. A graphic between log k2 and Kirkwood func-
tion, though, It is not linear (r2 ¼ 0.4758). This showed that the relative
permittivity and polarity of solvent do not represent the same solvent
characteristics, defined by Swain et al. [14].
Generation of a protonated Cr(VI) complex has been before suggested
in the reactions of other Cr(VI) complexes [9,10]. Magnified reactivity
obtained due to the internal electron transfer from hydroxy acid to IQDC
which promotes by the protonation of IQDC. Further, hydrogen ion de-
pendency shows fast equilibrium amid (X) and (Y), magnitude of equi-
librium constant, K1, is less and at higher concentrations of [Hþ] the
reaction remains incomplete because at higher hydrogen ion concen-
tration no leveling of velocity constants occurred. No astounding oxida-
tion noticed without TsOH recommends that just the protonated IQDC
(Y) carries on like an oxidizing species. In oxidation of MA by BIDC
similar kind of hydrogen-ion dependency observed [11].
4.2. Correlation analysis of reactivity
Preliminary computation showed that the rate constants (k2) of the
oxidation of alcohols do not reveal acceptable correlation either with the
Taft polar (
σ*) or steric substituent (Es) parameters separately [15].
log k2 ¼ (ꢀ2.86 ꢁ 0.57)
σ
* -
1.47
(13)
2.24
(14)
r2 ¼ 0.7799, sd ¼ 0.73, n ¼ 9,
log k2 ¼ (ꢀ1.68 ꢁ 0.44) Es –
r2 ¼ 0.6792, sd ¼ 0.88, n ¼ 9,
ψ
ψ
¼ 0.50, T ¼ 313 K
¼ 0.60, T ¼ 313 K
4.1. Solvent effect
The described data in Table 6 represents that with change in solvent
the rate constant (k2), varies considerably though equilibrium con-
stant(K#), is quite insensible. Similar kind of oxidation kinetics observed
in aliphatic primary alcohols oxidation by BIDC [12]. Hence, k2 for the
breakdown of complexes, in 18 solvents (CS2 has not been studied
because the absolute range of solvent parameters are unavailable), have
been associated regarding linear solvation energy relationship (LSER) of
Kamlet et al. [13]. Irrelevant correlations were observed.
Regarding to the Swain's [14] eq. (8), data of solvent effect had been
studied where A, B and (A þ B) denotes the solvating power of anion,
solvating power of cation and polarity of solvent respectively and inter-
cept term denoted by C.
Therefore, the rates were evaluated in terms of dual substituent-
parameter (DSP) equation (eq. (15)) of Palvelich and Taft [15].
log k2 ¼
ρ*
σ* þ δ Es þ log ko
(15)
The observed correlation was excellent with negative reaction con-
stants (Table 7). A steric acceleration is indicated by negative steric re-
action constant of the reaction. This may be described based on large
ground state energy of the sterically crowded alcohols. Since the
crowding is relieved in the product aldehyde as well as in the transition
state leading to it, the transition state energies of the crowded and non-
crowded alcohols do not vary much and a steric acceleration, therefore,
results.
log k ¼ aA þ bB þ C
(8)
In respect of Swain's eq. (8) the conclusion of the correlation studies,
separately with A, B and (A þ B) are described as:
5

R. Kalal, D. Panday
Journal of the Indian Chemical Society 98 (2021) 100009
Scheme 1. Mechanism for the oxidation of aliphatic primary alcohol by IQDC.
4.3. Mechanism
complex (Z) recommended by the Michaelis-Menten kind kinetics in the
context of alcohols. Nucleophilic attack of –OH group of alcohol on
BIDCHþ (Y) results in the origination of the complex C(Scheme 1). The
process of oxidation increases by the electron-donating methyl groups
and this is reflected by the reactivity increasing power. It shows that in
the formation of complex the availability of electron in –OH group ex-
pands. It is noticed that the complexes of normal and [1,1–2H2] ethanol
have formation constants which are nearly equal, however a primary
Considering about the non attendance of any impact of acrylonitrile
the reaction rate it is unfavoured that oxidation by one-electron is
operative in the present reaction. BHT is a perfect trapping reagent for
free radicals [16]. The acid-catalysis is considered as earlier IQDC pro-
tonation to produce IQDCHþ(Y). The major role of solvating power of
cation supports it. In a fast dynamic-equilibrium development of a 1:1
6

R. Kalal, D. Panday
Journal of the Indian Chemical Society 98 (2021) 100009
kinetic isotope impact indicated by the rate of decomposition and it
suggests that in the rate controlling step the α-C-H bond is broken. The
Cr(V). In a fast step this Cr(V) species ultimately produces Cr(III) species.
This pattern of reactions in oxidations by Cr(VI) is markedly established
[26].
end product (aldehyde) in Scheme 1 obtained from the complex (Z), due
to the hydride-ion shifting from alcohol to IQDC and mechanism of
shifting of this hydride-ion further supports that solvating power of
cation have major role. A carbocationic nature accessed by a polar
transition status is postulated in oxidation process. It was noticed that the
oxidation of benzyl alcohols by BIDC [7] is also followed the similar
mechanism. However, the generation of a centre of carbonium ion is
suggested in the controlled step in few reports on primary alcohol's
oxidation [17,18].
Transfer of a hydride-ion may occur either by an acyclic process or via
a chromate ester. Kwart and Nickle [19] developed a method to study of
the temperature reliance of the kinetic isotope effect, revealed that a
concerted cyclic mechanism precedes loss of hydrogen. The results for
ordinary and deuterited-alcohols have been expressed in a familiar
manner-
It is interesting here to compare the findings of the oxidation of the
existing reaction with the results of oxidation of alcohols by other Cr(VI)
complexes viz. quinolium dichromate (QDC) [27], BIDC [12], BTPPD
[22]. The oxidation kinetics of alcohols by QDC and in present reaction
exhibited the Michaelis-Menten type kinetics with respect to alcohols
suggesting the formation of an intermediate in pre-equilibrium and its
subsequent disproportionation in the rate-controlling step. However,
with BIDC and BTPPD, the order of reaction with respect to alcohol is
found to be two. The oxidation of alcohols by BTPPD and BIDC suggested
also involving the formation of an intermediate ester, but the second
order dependence on alcohol concentration can be explained by
assuming that the equilibrium constant of the formation of an interme-
diate is very small and not reflected in the rate laws. All four reactions
exhibited a primary kinetic isotope effect confirming the fission of the
α
-C-H bond in rate-controlling step. An analysis of the temperature
(kH/kD) ¼ (AH/AD) exp (- ΔEa /RT)
(16)
dependence of the kinetic isotope effect indicated the presence of a
symmetrical cyclic transition state in all the four oxidation reactions. The
oxidation of alcohols catalyzed by hydrogen-ion, but the dependence is of
first order in case of QDC where as second order for the oxidation by
BTPPD and BIDC. Fractional order dependency on [Hþ] is observed in the
present reaction. Therefore protonated oxidants are reactive oxidizing
species in the oxidation by QDC and IQDC while doubly protonated for
BIDC and BTPPD. The mechanism of the oxidation of alcohol by BIDC
was similar with that of BTPPD except that BTPPD is postulated to get
dissociated to give dichromate ion which on protonation acts as reactive
oxidizing species. The BIDC is postulated to exist as undissociated under
our reaction condition. Though, the data are not conclusive, it appears
that the oxidation process relies on state of the reaction and character of
the oxidant.
The difference of activation energy for kH/kD is ca. 4.89 kJ molꢀ1
which concluded by final results agrees that the activation entropy (ΔS*)
and zero-point energy gap for both C-D and C–H bonds (ca.
4.86 kJ molꢀ1) are almost same in the present reaction. It is directly
agrees with a symmetrical transition state properties [20,21]. Previously,
similar kind of results has been noticed in alcohol's oxidation by
butyltriphenyl-phosphonium dichromate (BTPPD) [22]. Bordwell [23]
has specified strong facts beside the phenomenon of bimolecular
concerted process of hydrogen shifting completed in one-step. In the
present reaction system this is strongly recommended that the shifting of
hydrogen does not undergo through an acyclic bimolecular course of
action. This symmetrical process is really sigmatropic a reaction in which
direct transfer of hydrogen is involved in which a cyclic transition state
shifting is specified [24]. Afterwards, two electrons transfer in a cyclic
scheme was the next in this reaction. In the oxidation of alcohols by IQDC
5. Conclusions
concerned with six electrons which resembles with Hückel's (4nþ2)
π
system, is a permitted process [25].. Hence, it is presumed that
hydride-ion transfer occurring through a cyclic transition state in alco-
hol's oxidation by IQDC.
The oxidation of alcohols by IQDC in DMSO resulted in the formation
of corresponding aldehyde. The reaction is catalyzed by hydrogen ion
and protonated IQDC is proposed to be the reactive oxidizing species.
Michaelis-Menten type kinetics observed with alcohols and first order
Described mechanism in Scheme 1 is based on the experimental
findings. In suggested mechanism (Scheme 1) the rate law could be
discussed below. Cr(VI) can be determined on applying the equilibrium
condition,
dependency observed as for IQDC. An α-C-H bond fission is indicated in
rate controlling step. An excellent correlation between rate constants and
duel substituent parameters (polar and steric) is observed, the reaction
constants being negative. A mechanism taking in the rate-controlling
oxidative breakdown of complex through a hydride-ion transfer from
alcohol to oxidant to give end product is suggested.
ꢀd½CrðVIÞꢂ = dt ¼ k2½Zꢂ ¼ k2K1K2½Alcoholꢂ½Hþꢂ½CrðVIÞꢂ
(17)
ꢀ
k2K1K2½CrðVIÞꢂt½Hþꢂ½Alcoholꢂ
or; ꢀd½CrðVIÞꢂ dt ¼
1 þ K1K2½Hþꢂ½Alcoholꢂ
Acknowledgement
or,
Thanks are due to University Grants Commission, New Delhi, India,
for their financial support and Department of Chemistry, M. L. Sukhadia
University, Udaipur, for provided facilities for this work.
k2K1K2½Hþꢂ½Alcoholꢂ
kobs
¼
(18)
1 þ K1K2½Hþꢂ½Alcoholꢂ
References
Here, [Cr(VI)]t ¼ [Cr(VI)] þ [Z], here K1 is low and produced [Y] which
get react with alcohol.
[1] (a) Corey EJ, Suggs WJ. Tetrahedron Lett. 1975;16:2647–50. https://doi.org/
10.1016/S0040-4039(00)75204-X.(b) Bhattacharjee MN, Chaudhuri MK,
Dasgupta HS, Roy N, Kathing DT. Synthesis 1982:588–90.
[2] (a) Li M, Johnson ME. Synth. Commun. 1995;25:533–7.(b) Firouzabadi H,
Sharifi A. Synthesis 1992:999–1002. https://doi.org/10.1055/s-1992-26288.(c)
Baltroke IM, Sadeghi MM, Mahmoodi N, Kharmesh B. Indian J. Chem., Sect. B
1997;36B:438.[3] Srinivasan R, Akila S, Caroline J, Balasubramanium K. Synth.
Commun. 1998;28:2245–51.
[4] Kaplan L. J. Am. Chem. Soc. 1958;80:2639–42. https://doi.org/10.1021/
ja01544a010.
[5] Perrin DD, Armarego WL, Perrin DR. “Purification of Laboratory Chemicals” II
Edition. Oxford: Pergamon; 1981. p. 74.
or, 1/kobs ¼ 1/k2K1K2 [Hþ] [Alcohol] þ 1/k2 (19)
The reaction system is well characterized by equation (18) or (19).
With the help of equations (5) and (19), we have K# ¼ K1 K2 [Hþ].
The observed loss of entropy (-ΔS) of activation favors proposed
mechanism. In the transition state, two ends become extremely solvated
when the charge segregation occurs and this leads to stabilization of large
number of solvent molecules, which appears as entropy loss. The effect of
solvent also accounts by loss of entropy.
[6] Exner O. Collect. Czech Chem. Commun. 1966;31:3222–51. https://doi.org/
10.1135/cccc19663222.
[7] Kumar P, Pandey D, Kothari S. J. Indian Chem. Soc. 2018;95:1207–15.
In the beginning oxidation state of Chromium metal reduced from
Cr(VI) to Cr(IV) and expected to react with another Cr(VI) to produced
7

R. Kalal, D. Panday
Journal of the Indian Chemical Society 98 (2021) 100009
[8] Exners O. Prog. Phys. Org. Chem. 1973;10:411–82. https://doi.org/10.1002/
9780470171899.ch6.
[9] Kumar R, Pandey D, Kothari S. Oxid. Commun. 2014;37:424–39.
[10] Panigrahi GP, Mahapatro DD. Int. J. Chem. Kinet. 1981;13:85–96. https://doi.org/
10.1002/kin.550130108.
[18] Mongkynrih I, Dkar JC, Kharpuria E, Mahanti MK. Oxid. Commun. 2007;30:82–7.
[19] Kwart H, Nickle JH. J. Am. Chem. Soc. 1973;95:3394–6. https://doi.org/10.1021/
ja00791a059.
[20] Kwart H, Latimer HC. J. Am. Chem. Soc. 1971;93:3770–1. https://doi.org/
10.1021/ja00744a038.
[11] Panday D, Kachawa T, Kothari S. Prog. React. Kinet. Mech. 2018;43:300–14.
https://doi.org/10.3184/146867818X15319903829236.
[12] Pandey D, Kothari S. Oxid. Commun. 2009;32:371–82.
[13] Kamlet MJ, Abboud JLM, Abraham MH, Taft RW. J. Org. Chem. 1983;48:2877–87.
https://doi.org/10.1021/jo00165a018. and references cited therein.
[14] Swain CG, Swain MS, Powell AL, Alunni S. J. Am. Chem. Soc. 1983;105:502–13.
https://doi.org/10.1021/ja00341a033.
[21] Kwart H, Slutsky J. J. Chem. Soc. Chem. Commun. 1972;21:1182–3. https://
doi.org/10.1039/C39720001182.
[22] Kothari A, Kothari S, Banerji KK. Indian J. Chem. 2005;44A:2039–43.
[23] Bordwell FG. Acc. Chem. Res. 1972;5:374–81. https://doi.org/10.1021/
ar50059a003.
[24] Woodward RB, Hoffmann R. Agnew. Chem. Int. Ed. Engl. 1969;8:781–853. https://
doi.org/10.1002/anie.196907811.
[15] Pavelich WA, Taft RW. J. Am. Chem. Soc. 1957;79:4935–40. https://doi.org/
10.1021/ja01575a029.
[25] Littler JS. Tetrahedron 1971;27:81–91. https://doi.org/10.1016/S0040-4020(01)
92399-3.
[16] Mohajer D, Tangetaninejad S. Tetrahedron Lett. 1994;35:945–8. https://doi.org/
10.1016/S0040-4039(00)76007-2.
[26] Milazzo G, Caroli S, Sharma VK. New York: Wiley and Sons; 1978. https://doi.org/
10.1002/bbpc.19780821020.
[17] Dey D, Mahanti MK. J. Org. Chem. 1990;55:5848–50. https://doi.org/10.1021/
jo00310a015.
[27] Dey D, Dkhar JC, Mahanti MK. Oxid. Commun. 1998;21:252–6.
8