Mechanistic Aspects of Oxidation of P-Bromoacetophen one by Hexacyanoferrate (III) in Alkaline Medium

Renu Gupta. Int. Journal of Engineering Research and Application www.ijera.com
ISSN : 2248-9622, Vol. 6, Issue 3, ( Part -5) March 2016, pp.01-03
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Mechanistic Aspects of Oxidation of P-Bromoacetophen one by
Hexacyanoferrate (III) in Alkaline Medium
Renu Gupta*
Department of Chemistry, Lucknow Christian P.G. College, Lucknow-226018, India
ABSTRACT
The kinetics of oxidation of p-bromoacetophenone by hexacyanoferrate (III) has been studied in alkaline
medium. The order of reaction with respect of both acetophenone and hexacynoferrate (III) has been found to be
unity. The rate of reaction increases with increase in the concentration of sodium hydroxide.On addition of
neutral KCl, reaction rate increases. The effects of solvent and temperature have been also studied. The product
p-bromophenyl glyoxal have been characterized by IR studies.
Keywords:p-bromoacetophenone; Hexacyanoferrate; Oxidation; Mechanism;Kinetics
I. INTRODUCTION
Aromatic ketones are widely used in the
synthesis of a large number of fine chemicals such
as drugs, fragrances, dyes and pesticides [1-
3].Friedal-Craft acylation is one of the most
important methods for the synthesis of aromatic
ketones. Aromatic ketones are mainly prepared by
acylation of aromatics with acid chlorides,
carboxylic acids and their anhydrides in the
presence of acid catalysts. p-bromoacetophenone is
an aromatic chemical compound with an aroma. p-
bromoacetophenone was synthesised from
bromobenzene via friedal craft acylation [4] The
13
NMR spectrum of p-bromoacetophenone is very
interesting in several point of views .Note
particularly that six carbon absorption are observed
,even though the molecule has eight carbon [5]
.Various thermodynamics parameters like entropy,
enthalpy etc. was studied by Jaspal etal.[6].
Hexacyanoferrate(III) has been proven to
be an efficient oxidant for a wide variety of organic
substrates, because the CN−
ligands are resistant to
substitution reactions and thereby outer-sphere
electron transfer is the preferred oxidation pathway
[7].Kinetics of oxidation of
ketones [8,9] have been studied in alkaline
medium by hexacyanoferrate (III),which
isclassified as an oxidising agent in which the
oxidising species is a complex electron attracting
ion and the reactions are brought to proceed by a
radical formation [10,11]. We report here the
kinetics and mechanism of oxidation of p-
bromoacetophenone by hexacyanoferrate(III) in
alkaline medium.
II. EXPERIMENTAL
2.1. Materials and Methods
p-bromoacetophenone(Fluka) and all other
chemicals of A.R., B.D.H. grade were used. In a 50
ml flask freshly prepared standard solution of
acetophenone in methanol-water (w/w) and in
another flask desired solution of
hexacyanoferrate(III) and NaOH were taken and
placed in a thermostat maintained at± 0.1o
C
accuracy.
After half an hour both the reactants were
mixed. At different intervals of time,5 ml aliquot
was taken out and poured in a flask containing 5ml
of 2N H2SO4 and 1 gm of KI. The
unreacted K3Fe(CN)6 was estimated by titrating the
liberated iodine against standard sodium
thiosulphate solution, using starch as an indicator.
The result of stoichiometeric runs under
conditions,[K3Fe(CN)6]>>[acetophenone] keeping
for 15 to 16 days at room temperature(25-300
C)
showed that one mole of acetophenone consumed
36 moles of K3Fe(CN)6 for its oxidation. The
liberation of bromide ion is confirmed by adding
AgNO3 solution.
2.2
2.3 Stoichiometery and product analysis:
However, under experimental conditions
[acetophenone]>>[K3Fe(CN)6], the product p-
methoxyphenylglyoxal has been separated by
distillation and characterized by preparing its 2,4
dinitrophenylhydrazone derivative [12,13]
m.p.1320
C (lit. value-130.50
C)followed by
stretching frequencies at 1630 cm-1
for C=O
recorded by I.R. spectra (in KBr). Thus
stoichiometeric equation can be shown as
BrC6H4COCH3+4Fe(CN)6
3-
+4O H-
BrCH3C6H4COCHO +4Fe(CN)6
4-
+3H2O….(i)
This difference in observation indicates that
oxidation takes place in stages.
III. RESULT AND DISCUSSION
Underpseudo conditions
[substrate]>>[Fe(CN)6]3-
, the data collected at
RESEARCH ARTICLE OPEN ACCESS

varying concentration of hexacyanoferrate(III)
from 1.43 to 3.33x 10-3
at 250
C ; [methanol] 30%
(w/w); [NaOH] =0.166 M; [p-bromoaceto]
=1.25x10-2
M and constant ionic strength(µ=0.4M)
gave uniform pseudo first order velocity constants
k1= (3.12+
-0.035) x 10-4
s-1
indicating first order
dependence of the reaction rate on oxidant. Under
similar conditions,k1 values calculated at varying
concentration of p-bromoacetophenone from 1.66
x10-2
M to 1.11 x 10-2
M; gave uniform ratio;
k1/[aceto] =2.58 x10-2
mole-1
s-1
in each set
confirming the first order dependence of the
reaction rate on substrate concentration.The
reaction rate increases proportionality with the
increase in concentration of NaOH from 0.11 to
0.25 M. The ratio; k1/[NaOH] =1.85 x10-3
is fairly
uniform in each set showing thereby that the
reaction is base catalysed in nature.
On addition of KCl from 0.2 to 0.6 M the
reaction rate increases from 1.83 to5.32 s-1
at 250
C. The linear plots passing through origin
between log k1/k0 (where k0 = 4.17 x10-5
s-1
) and √µ
with unit slope indicate ion-ion interaction [14] in
the rate determining step.The data collected at
different dielectric constants (D) from 69.99 to
56.28 by varying weight percentage of methanol in
methanol-water mixture.(20 to 50% w/W) at
250
C;[K3Fe(CN)6] = 2.0 x 10-3
M;[NaOH] = 0.166
M; [p-bromoacetophenone] =1.25 x 10-2
M ;
µ=0.4M show that the reaction rate decreases from
5.31 to 0.63 x 10-4
s-1
with decrease in dielectric
constant of the medium . The linear plot between
log k1 and 1/D with negative slope further indicates
interaction between simply charged ions[15].
Effect Of Temperature:
The reaction rates are enhanced on
enhancing the temperature from 200
C to 350
C of the
reaction mixture. The energy of activation (Ea) has
been determinedfrom the slope of linear plots
between log k1 and 1/T and all others activation
parameters have been evaluated at 250
C as:
Kr= 15.8 x10-2
sec-1
l2
mole-2
, Ea=68.9 kJ mole-1
,
ΔH#
=66.5 kJ mole-1
, ΔS#
=- 39.2kJ mole-1
and
ΔF#
=78.1 kJ mole-1

IV. MECHANISM OF REACTION
Kinetically it appears that at first the
enolate anion is formed due to interaction between
the enolateanion is formed due to interaction
between acetophenone and OH-
ion, which interacts
slowly with Fe(CN)6
3-
and as a result of an electron
transfer, it is converted into a radical [16], which is
subsequently oxidized into p-bromophenylglyoxal
in a fast process.
4.1. Rate Law
The rate of disappearance of [Fe (CN6)3-
] is given
by step 2 as :
- d [Fe (CN6)3-
] / dt = k1 [anion] [Fe
(CN6)3-
]
From step 1 taking activity of water as unity:
[anion] = K1 [acetophenone] [OH-
]
And then final rate law becomes:
- d [Fe (CN6)3-
] / dt = K1.k1
[acetophenone] [OH-
] [Fe (CN6)3-
]
V. CONCLUSION
The derived rate law is fully justified by
observed kinetics. The produced free radical is quite
weak, as it is ineffective to polymerization of
monomer acrylamide.
REFERENCES
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(2007). Process Chemistry in the
Pharmaceutical Industry, Volume 2.
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[3]. Itsuo Furuoya, Catalysis Surveys from
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[4]. Francis A Corey. Organic Chemistry vii
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Mechanistic Aspects of Oxidation of P-Bromoacetophen one by Hexacyanoferrate (III) in Alkaline Medium

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Mechanistic Aspects of Oxidation of P-Bromoacetophen one by Hexacyanoferrate (III) in Alkaline Medium