Structural activity of unsaturated alcohols and oxidation towards pyrazinium chlorochromate

Article abstract of DOI:10.13005/ojc/350156

The kinetics of oxidation of allyl, crotonyl, styryl carbinol by pyrazinium chlorochromate (PzCC) has been investigated in 60% acetic acid medium.The order was found to be first order with respect to [oxidant], [substrate] and [H⁺.]. The rate of the oxidant decreased with increase in [PzCC] and increased with increase in the percentage of acetic acid. The addition of sodium perchlorate did not show any change in rate constant. Based on the kinetic results, the mechanism between unsaturated alcohol and chromium (VI) oxidant is proposed. The reactivity of unsaturated alcohols was observed to be allyl alcohol< crotonyl alcohol< styryl carbinol.

Full text of DOI:10.13005/ojc/350156

ISSN: 0970-020 X
CODEN: OJCHEG
2019, Vol. 35, No.(1):
Pg. 436-441
ORIENTAL JOURNAL OF CHEMISTRY
An International Open Access, Peer Reviewed Research Journal
www.orientjchem.org
Structural Activity of Unsaturated Alcohols and OxidationTowards
Pyrazinium Chlorochromate
K. ANbARASU and K. K. ILAvENIL*
Department of Chemistry, Arignar Anna Government Arts College, Affliliated to Bharathidasan University,
Trichy, Musiri-621 211, Tamilnadu, India.
*Corresponding E-mail: arasu007@gmail.com, illavenil81@gmail.com
http://dx.doi.org/10.13005/ojc/350156
(Received: August 16, 2018; Accepted: January 29, 2019)
AbSTRACT
The kinetics of oxidation of allyl, crotonyl, styryl carbinol by pyrazinium chlorochromate
(PzCC) has been investigated in 60% acetic acid medium.The order was found to be first order with
respect to [oxidant], [substrate] and [H+.].The rate of the oxidant decreased with increase in [PzCC]
and increased with increase in the percentage of acetic acid. The addition of sodium perchlorate
did not show any change in rate constant. Based on the kinetic results, the mechanism between
unsaturated alcohol and chromium (VI) oxidant is proposed. The reactivity of unsaturated alcohols
was observed to be allyl alcohol< crotonyl alcohol< styryl carbinol.
Keywords: Unsaturated alcohols, Oxidation, Kinetics, Sodium perchlorate,
Pyrazinium chlorochromate, Mechanism.
INTRODUCTION
used as raw material for the preparation of resins,
plasticizers, pharmaceuticals and many organic
compounds6. Styryl carbinol (Cinnamyl alcohol) is
employed as an additive in cosmetic products, soaps,
toothpaste, deodorants, as food additives in chewing
gum, bakery products, candy and soft drinks.
The alcohols on oxidation give acids,
aldehydes and ketones based on the nature of the
oxidant. They are used largely in perfumes, dye
industries, rubber, polymer industry, pharmaceutical
industry and agrochemical industry. In synthetic
organic chemistry, the oxidation of alcohol to carbonyl
group is of greater importance1, 2. The metal based
oxidants like KMnO4, Cr (VI), RuO4, produce large
amounts of toxic end-products3,4. Chromium (VI)
(Cr+6) are powerful oxidizing agent for the oxidation
of varieties of organic compounds5. Allyl alcohol is
In order to avoid the versatality of these
oxidants, mild Cr+6 derivatives are synthesized from
heterocyclic halochromate. Kinetics of oxidation of
alcohols using pyridinium chlorochromate7, pyridinium
fluorochromate8, bipyridinium chlorochromate9,
halosilanes chromium trioxide10, quinolinium
This is an
Open Access article licensed under a Creative Commons license: Attribution 4.0 International (CC- BY).
Published by Oriental Scientific Publishing Company © 2018

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IlAVENIl et al., Orient. J. Chem., Vol. 35(1), 436-441 (2019)
dichromate11, imidazolium dichromate12, quinaldinium
Stoichiometry and Product analysis
fluorochromate13, quinaldinium dichromate14
and quinolinium fluorochromate15 and benzyl
triethylammonium chlorochromate16 has been
reported in the literature.The kinetics and oxidation
of allyl alcohol, crotonyl alcohol and styryl carbinol
using pyrazinium chlorochromate has not yet been
investigated. The aim of the work is to study the
kinetics of oxidation of unsaturated alcohols by
pyrazinium chlorochromate and explain the structural
behaviour of these compounds.
Pyrazinium chlorochromate (PzCC) was
added in excess to carry out stoichiometric study.The
alcohols and PzCC were mixed in the ratio 1: 2 and
were kept over night at 298 K in perchloric acid, acetic
acidandwatermixture.1moleofanalcoholconsumed
2 moles of PzCC showing 1: 2 stoichiometries. From
iodometric methods, the unreacted pyrazinium
chlorochromate was determined.The reaction is
shown in the equation (2).
Unsaturated alcohol PzCC Aldehyde
(R= -H, -CH3& -C6H5)
MATERIALS AND METHODS
Theproductsacrolein,crotonylaldehydeand
cinnamaldehyde were obtained on oxidation of allyl
alcohol, crotonyl alcohol and styryl carbinol as shown
in the equation 2. The aldehydes were identified by
spot test18 as silver mirror test. The aldehyde shown
in the above reaction was characterised by IR spectra
and Chromatographic method (TlC).
Allyl alcohol, crotonyl alcohol, styryl carbinol
of AR grade were obtained from Sigma Aldrich and
used as such. The solvent glacial acetic acid was
distilled with chromic oxide and the boiled fraction
was collected between 116 – 118 oC.Doubly distilled
water was used throughout the experiment.
RESULTS AND DISCUSSION
PreparationofPyraziniumchlorochromate(PzCC)
C4N2H5CrO3Cl
The kinetics of oxidation of allyl alcohol
was studied in 60% acetic acid and water medium in
presence of perchloric acid at constant temperature.
Pyrazinium chlorochromate (PzCC) was
prepared by literature method17 as shown below.
0.35 g of pyrazine in 0.4 ml of HCl was added along
with 8 ml of water. 0.45 g of CrO3 was dissolved
in the mixture of 0.4 ml HCl and 4 ml of water and
added to the above solution and then it was stirred
in ice bath for one hour. A bright orange coloured
solid was obtained as shown in the reaction (1) below
and recrystallised with acidified water.
Effect of varying [Alcohol]
The reaction rate increased linearly with
increase in the concentration of alcohols (Table.1).
The plot of log k1 versus log [alcohols], (Fig. 1) gave
the unit slope B = 0.876 with linear correlation of
r = 0.994 (allyl alcohol).The experiment was carried
for crotonyl alcohol as mentioned above and
the unit slope B = 0.882 with linear correlation of
r = 0.998 was obtained.Styryl carbinol was taken as
the substrate and the same procedure was followed
as mentioned above.The slope value B = 0.958 was
attained with a linear correlation of r = 0.999.
Kinetic studies
The unsaturated alcohols were maintained
in large excess concentration than the oxidant to
attain pseudo-first order conditions. The reactions
were carried out at constant temperature using
thermostat. The absorbance was recorded using
colorimeter at 470 nm, Elico Cl-63 Photometer.The
pseudo-first order rate constants kobserved were
determined up to 75% completion of the reaction
from the linear plots, r = 0.990 to 0.999 for log
[absorbance] versus time.
Effect of varying [PzCC]
The concentration of pyrazinium
chlorochromate was varied at constant substrate
(alcohol) and perchloric concentration. From the
plot of log absorbance versus time a linear graph
was obtained proving the reaction to be first order
with respect to PzCC. The increase in PzCC
concentration showed decrease in rate constant
value19. From the plot of 1/k1 versus 1/[PzCC]
(Fig. 2) a linear graph was obtained since the total
Cr+6 was in the form of acid chromate in which is the
effective oxidant20 and the two nitrogen atom have
Correlation analysis
linearregression(r)analysiswascarriedusing
computer software, Micro cal origin.The reproducibility
of the rate constants was approximately 2%.

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IlAVENIl et al., Orient. J. Chem., Vol. 35(1), 436-441 (2019)
withdrawn the electrons which have resulted in the
decrease of rate constant values.
Effect of varying the [Perchloric acid]
The [perchloric acid] H+ was varied in the
range of 0.5 × 10-1 to 2.5 × 10-1mol dm-3 and the rate
constants were measured (Table 1).The increase in
[perchloric acid] in the oxidation reaction increases
the rate and confirms to be first order reaction.From
the plot of log k1 against log [H+] a linear graph was
obtained with unit slope (Figure 3).
N
N
HCl
CrO3Cl-
(1)
CrO3
N
N
H
Pyrazine
Pyrazinium chlorochromate
CHO
2CrClO-N2C4H5
2H2CrClO-N2C4H5
OH
+
H+
R
O
(2)
+
O
+
Fig. 3. Plot of logk1 versus log [H+]
Effect of varying Ionic strength [NaClO4]
The effect of sodium perchlorate salt by
Fig. 1. Plot of log k1 versus log [alcohols]
varying concentration from 0.002 to 0.008M shown
in the Table 2 was studied in Debye-Huckel limit.The
rate of the reactions was nearly constant and it implies
the ionic interactions in the rate determining step.
Effect of Acetic acid and Water
The acetic acid composition was varied at
six different combinations with water by having the
other variables constant. The rate increased with
increase in the solvent concentration due to the
interaction of ionic species in the slow step, specified
in theTable 2.The plot of log k1 versus 1/D was linear
with positive slope21 as shown in the Figure 4.
Fig. 2. Plot of 1/ k1 versus 1/ [PzCC]
Table.1: Rate data for the oxidation of allyl alcohol
by PzCC at 303k
103[PzCC]
(mol dm-3)
102[Allyl alcohol]
(mol dm-3)
101[H+]
(mol dm-3)
104k1
(s-1)
0.5
1
1.50
1.50
1.50
1.50
1.50
0.75
2.25
3.00
3.75
1.50
1.50
1.50
1.50
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
1.5
1.8
1.9
31.83
15.23
12.37
10.91
10.04
5.11
13.54
18.17
21.47
6.21
1.5
2.0
2.5
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
Fig. 4. Plot of log k1 versus 1/D
17.29
21.04
22.59
Free radicals Test
The addition of acrylonitrile had no effect on
the rate of oxidation of allyl alcohol, crotonyl alcohol
AcOH:H2O=60:40(v/v)
Temperature= 303K

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IlAVENIl et al., Orient. J. Chem., Vol. 35(1), 436-441 (2019)
and styryl carbinol.One electron transfer of oxidation
is not observed.
= k3 K2K1[S] [PzCC] [H+]
-d[PzCC] = k3 K2K1 [S][O][H+]
dt
Effect of varying MnSO4 concentration
Kobs = k3 K2K1 [S] [H+]
Addition of Mn2+ in the range of 0.002 to
0.008 M retards the oxidation rate indicating two
electron oxidation22 given in the Table 2. This is due
to involvement of chromium (IV) intermediate.
Oxidation of Unsaturated alcohols
The kinetics of oxidation of allyl alcohol,
crotonyl alcohol and styryl carbinol were investigated
at 298 K, 303 K, 313 K, 323 K and rate constants
were calculated as shown in the Table 3. Using
Eyring relation of least square method23 the enthalpy
of activation (ΔH#), entropy of activation (ΔS#), free
energy change (ΔG#) and Energy of activation (Ea)
were evaluated from these temperature range.
The entropy of activation was found to be negative
for alcohols due to the increase in polarity of the
transition state. From the similar values of free
energies of activation we can conclude that the
oxidation mechanism is same for all the alcohols used
as substrate.This is also confirmed from the energy
of activation value Ea, where the value is higher
for the slow reaction, oxidation of styryl carbinol
Mechanism and Rate law
From the kinetic investigations, the following
mechanism has been proposed and suitable rate law
is derived.
H
C
H
C
OH
C
O
O
O-Pz+
Cl
K1
H
C
H
C
H
C
H
Cr
O-Pz+
Cl
O
R
H
+
R
OH
Cr
H
O
C1
H
C
H
C
O
C
H
H
O-Pz+
K2
H H
O
HO
O-Pz+
Cl
R
C
C
C
O H
+
Cr
R
H
Cr
O
Cl
O
H
(Ea = 9.15
1.2). The enthalpies of activation
C2
Cr(IV)
C1
(ΔH#) values are very low indicating a concerted
mechanism as proposed.The iso-kinetic plot24
between ΔH# versus ΔS# gives a straight line
(Fig. 5.) with correlation coefficient, r = 0.972. The
iso-kinetic temperature β obtained from the slope
is 158.38 K. Since the β value is lower than the
experimental temperature, it indicates that this
oxidation is an entropy controlled reaction25,26. The
Exner plot shows, r = 0.999, at 303 K and 313 K
on applying the relation27 (log (k1) T2 = a + b log
(k1) T1) where a-intercept, b-slope, (Temperature)
T2> T1 respectively.This implies that all the alcohols
follow the same scheme of mechanism (Figure 6.)
H
H
C
H
H
k3
Slow step
R
C
C
H+
+
O
R
C
C
H
C
H
O H
H
C2
Aldehyde
R = -H (Allyl), -CH3 ( Crotonyl), -C6H5 ( Styryl)
C2 (Carbanion)
C1
(Intermediate state)
Rate law
Rate = k3 [Product]
= k3 K2 [C1]
Table 3:Thermodynamic and activation parameters for oxidation of unsaturated alcohols by PzCC
Substrate
Order
k1104 (s-1)
303 K
ΔH#
323 K
-ΔS#
ΔG#
Ea
r
293 K
313 K
(kJ mol-1) (Jk-1mol-1) (kJ mol-1) (kJ mol-1)
Allyl
0.876
0.882
0.958
9.88
25.84
256.3
10.91
30.1
12.14
37.11
397.5
13.77
46.6
2.67 0.6 218.78 2.8 68.96 0.8 5.19 0.7
5.63 0.8 205.27 3.5 67.83 1.0 8.15 1.1
6.63 0.9 193.53 4.2 65.27 1.2 9.15 1.2
0.993
0.991
0.998
Crotonyl
Styryl carbinol
316.45
506.2
[Alcohols] = 1.5 x 10-2 mol dm-3; [PzCC] = 2.0 x 10-3 mol dm-3; [H+] = 1.0 x 10-1 mol dm-3
to be in the order of Allyl alcohol< Crotonyl alcohol<
Styryl carbinol. The electron cloud is higher in the
styryl carbinol and hence the activation energy is
also higher (Ea = 9.15 KJ mol-1).
Structural activity of alcohols
The resonance stabilisation and the positive
character of styryl carbinol is higher than crotonyl
and allyl alcohol and hence the reactivity is believed

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CONCLUSION
The reaction was first order with respect
to [Alcohols], [PzCC] and [Perchloric acid]. The
stoichiometry was found to be 1: 2 that is one
mole of substrate consumed two moles of oxidant.
The activated complex formed was rigid due to
the negative ΔS# value. The polymerisation was
not observed due to absence of free radicals. The
rate constant values of the oxidant [PzCC] was
decreased due to the presence of Cr+6 as acidic
chromate and due to the electron withdrawing nature
of two nitrogen groups in the pyrazine ring.The order
of reactivity was [Allyl alcohol] < [Crotonyl alcohol] <
[Styryl carbinol]. From the kinetic data the plausible
mechanism has been proposed and the suitable rate
law is derived.
Fig. 5. Isokinetic plot of ΔH# versus -ΔS#
ACKNOWLEDGEMENT
This research work was carried in the
Center for Research, Arignar Anna Government
Arts College, Musiri, Trichy. I thank all the Faculties,
Research Scholar for their support.
Fig. 6. Exner plot of log k1(313K) versus log k1(303K)
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