OXIDATION OF SOME ALIPHATIC ALDEHYDES BY CHLORAMINE-T
401
Mechanism of Oxidation of some Aliphatic Aldehydes by Chloramine-T
M . C. AGRAWAL and S. P. MUSHRAN
Chemical Laboratories, University of Allahabad, Allahabad, India
(Z. Naturforsch. 27 b, 401—404 [1972] ; received September 20, 1971, revised November 16, 1971)
Kinetic studies on the oxidation of acetaldehyde, propionaldehyde, n- and tso-butyraldehyde
have been made using diloramine-T as an oxidising agent in an alkaline medium. The reactions
followed almost identical kinetics being first order with respect to both aldehyde and chloramine-T. The order with respect to alkali has been found to be almost equal to unity (0.8 to
0 . 9 0 ) . Effect of an increase in the ionic strength was negligible while that of the addition of ethyl
alcohol was negative. A mechanism involving the interaction of the enol anion of the aldehyde with
chloramine-T best fits the kinetics data. Other possibilities have also been examined. Corresponding
acids were f o u n d to be the oxidation products.
The
sodium
commonly
salt
known
of
p-toluenesulfochloramide,
as chloramine-T,
exerts
strong
oxidising action in both acidic and alkaline media
(Ered.
=
1 . 1 3 8 1 at p H 12)
and thus has been
widely used f o r the oxidimetric determination 2 ' 3 of
a large number of inorganic and organic substances.
With regard to the mechanism of oxidations performed with this potent oxiding agent, kinetics of
the oxidations of glycerol in neutral and alkaline
media4,
of
p-cresol
hexacyanoferrate ( I I )
in
6
an acidic
medium5
and
in a feebly acidic medium
(pH 6 to 7 ) have been investigated. The role of
osmium ( V I I I )
as a catalyst during the oxidation
of a-hydroxy acids b y alkaline chloramine-T
has
also been examined 7 .
Oxidation
of some aldehydes b y
chloramine-T
has been reported to take place quantitatively 8 in
an alkaline solution giving the corresponding acid
as
the
end
product.
Both
direct8
and
indirect
methods 9 ' 1 0 have been carried out f o r the estimation of aldehydes by chloramine-T. It has been observed that the oxidations are quite slow even at
position. Although aqueous solutions of chloramine-T
are very s t a b l e u , their strengths were checked from
time to time using iodometric method 12. Aqueous solution of ethanol was prepared from an Anala-R, B.D.H.
sample of the reagent. Due to low solubility of propanal, n- and iso-butanal in water, stock solutions of these
aldehydes were prepared in 50% ethanol (B.D.H.)
using analytical grade samples of the reagents. The
strengths of the aldehyde solutions were determined by
hydroxylamine hydrochloride-pyridine procedure 13 . All
other chemicals used were of analytical reagent grade
and their solutions were prepared in double-distilled
water. Stills were all made up to Jena glass and the
reactions were carried out in reaction bottles darkened
blade from outside.
Kinetics of the oxidation of ethanal was followed
by estimating chloramine-T directly using iodometric
method. The method, however, did not hold in case of
propanal, n- and iso-butanal and thus the kinetics in
these cases were followed by an indirect method using
ascorbic acid 14. Five ml aliquot portion of the reaction
mixture was transferred to a titrating flask containing
excess ascorbic acid (E. Merck) and 5 ml of 1 N hydrochloric acid. The excess ascorbic acid left was back
titrated against standard chloramine-T using KI-starch
indicator.
pH 12 and take a few hours to complete. The kineResults
tics of the oxidation of f o u r aldehydes viz. ethanal,
propanal, n- and iso-butanal have been investigated
in the present paper and the data obtained have
been interpreted mechanistically.
Experimental Section
Aqueous solutions of chloramine-T were prepared from
a E. Merck, pro analysi, sample of the reagent and were
stored in dark bottles to avoid any photochemical decomRequests for reprints should be sent to Dr. S. P . MUSHRAN,
Department of Chemistry, University of Allahabad, Allahabad (Indien).
Kinetics of the oxidation of aldehydes has been
investigated at several initial concentrations of the
oxidising and the reducing agent at different alkali
concentrations. The reactions obey first order dependence in chloramine-T at all concentrations of
the reactants and the log-time plots were linear upto
two half lives (Fig. 1 ) . Pseudo-first order rate constants ( k j ) in chloramine-T were calculated at different
initial
concentrations
of
aldehydes
which
showed a linear increase in rate constants with increase in aldehyde concentration (Table I ) . Second
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M. C. AGAR WAL AND S. P. MUSHRAN
402
1.0
K
tion between aldehydes and chloramine-T is, theref o r e , established.
0.8
The oxidation of aldehydes by chloramine-T was
f o u n d to be strongly dependent on alkali concentra-
Z.0.6
tion. T o study the effect of alkali on the reaction
I5
rate, kinetic studies were made over a wide range
of
0.4
alkali
concentrations.
In order
to avoid
any
variations due to change in ionic strength the runs
0.2
were made in presence of excess NaC10 4 . The re-
0
20
40
60
80
100
Time in min.
sults
120
Fig. 1. Rate of reduction of 2.0 x 1 0 - 3 M chloramine-T at
A ) [ethanal] = 0.04 M, [ N a O H ] = 0.04 M, Temp. = 5 0 ° ,
B) [propanal] = 0.04 M, [ N a O H ] = 0.04 M, Temp. = 4 0 ° .
C) [n-butanal] = 0.04 M, [ N a O H ] = 0.04 M, Temp. = 4 0 °
and D) [iso-butanal] = 0.04 M, [ N a O H ] = 0.01 M, Temp.
=
(Table II)
show almost linear increase with
increase in alkali concentration. The order of the
>-
reactions with respect to alkali has been obtained
f r o m the slope of the plots of l o g k x against log
[OH9]
(Fig. 2 ) as 0 . 8 7 , 0.85, 0 . 9 0 and 0.80 f o r
the oxidation of ethanal, propanal, n- and iso-butanal respectively.
40°.
order constants were, therefore, calculated as k2 —
The oxidations of propanal, n- and iso-butanal
A ^ / f R C H O ] and the average values were obtained
by chloramine-T were studied in 2 5 % ethanol. T o
50°),
find out the effect of solvent on the reaction rate
(9.6 + 0 . 4 ) and (6.2 + 0.3) x 1 0 ~ 3 (in 0.4 M N a O H
different concentrations of ethanol-water mixtures
at 4 0 ° )
as
(5.5 ± 0 . 5 ) x 1 0 - 3
(in 0.4 M NaOH at
(in
were employed and it was observed that an increase
0.1 M NaOH at 4 0 ° ) f o r the oxidation of ethanal,
in ethanol concentration has a slight retarding in-
propanal, n- and iso-butanal respectively. All the
fluence
and
(16.1 + 0 . 5 ) x 1 0 ~ 3 1 m o l e - 1 s " 1
(Table I I I ) .
kinetic runs presented in Table I f o r the oxidation
The influence of neutral salts viz. NaC10 4 and
of propanal, n- and iso-butanal have been carried
NaCl on the reaction rate was investigated. It was
out in presence of 2 5 % ethanal.
observed that a ten-fold variation in the concentra-
Kinetics were also followed in equivalent amounts
of
the
reactants
where
[chloramine-T]varied
linearly with time. The overall second order reac-
tion of added neutral salts has a little or negligible
effect on the rate of
10 3
[Chloramine-T]
10 2
[RCHO]
105 Jd 8-1 f o r t h e o x i d a t i o n o f
ethanala
propanal 1 3
n-butanalb
iso-butanalc
1.2
1.6
2.0
2.8
4.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.2
1.6
2.8
4.0
10.5
10.9
11.3
10.8
10.6
7.33
9.40
14.8
19.4
11.5
12.1
11.8
11.1
9.60
7.40
9.60
17.6
26.9
33.3
33.0
31.4
30.1
29.7
19.9
25.1
46.1
63.0
105
8-1 f o r t h e o x i d a t i o n o f
ethanal
propanal
n-butanal
[NaOH] c
M
10 5 h s " 1 f o r
the oxidation o f
iso-butanal
4.72
6.41
7.52
10.0
13.3
0.006
0.008
0.010
0.014
0.020
28.9
35.3
40.2
52.8
70.7
[NaOH]
M
0.024
0.032
0.040
0.056
0.080
to
13.0
15.7
18.7
24.6
33.7
19.9
19.8
19.2
17.7
15.6
10.9
14.8
27.8
40.0
8.17
10.1
12.3
15.8
22.5
oxidation of
aldehydes
by
chloramine-T.
Table I.
Dependence on Reactants
Concentrations.
a
In presence of 0.04 M NaOH at
5 0 ° . b In presence of 0.04 M NaOH
at 4 0 ° . C l n presence of 0.01 M
N a O H at 4 0 ° .
Alkali Dependence a
at 4 0
'
a
[chloramine-T] = 2 x 1 0 ~ 3 M
[aldehyde] = 0.02 M. b in preseuce of 0.4 M NaC10 4 c i n presence of 0.16 M NaC10 4 .
Table
IL
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OXIDATION OF SOME
ALIPHATICALDEHYDESBY CHLORAMINE-T
403
MOND and WATERS 15 have shown that the rate of
oxidation of aldehydes by manganic pyrophosphate
follows zero order dependence in oxidant. The oxidations of several carbonyl compounds b y alkaline
ferricyanide 16 have also been shown to follow zero
order dependence in oxidant and first order dependence in hydroxide ion and the carbonyl compound.
It is established that the oxidation of aldehydes in
alkaline media proceeds via an enolization step
which may be rate-determining depending upon the
velocity of the subsequent reaction between enol
anion and the oxidant.
In aqueous alkaline solutions chloramine-T
mediately hydrolyses as follows,
-<V-<wy
0.7
0.9
1.1
2+log [0H@]
1.3
*-
Fig. 2. Plot of l o g / c j against log [ O H 0 ] at 4 0 ° ; [chloramineT ] = 2.0 x 1 0 ~ 3 M and [aldehyde] = 0.02 M; A , B, C represent the oxidation of ethanal, propanal and rc-butanal in
0.4 M NaC10 4 and D represents the oxidation of iso-butanal in
0.16 M NaC10 4 .
ethanol
/o
15
20
25
30
35
40
—
11.0
9.15
8.55
7.29
6.49
5.95
im-
CH 3 C 6 H 4 S0 2 N • NaCl + H 2 0 ^
CH 3 C 6 H 4 S0 2 NHC1 + NaOH .
(la)
CH 3 C 6 H 4 S0 2 N • HCl + NaOH ^
CH 3 C 6 H 4 S0 2 NH 2 + NaCIO .
(1 b)
Further, p-toluenesulfochloramide is known to be
a fairly strong acid (pk a = 4 . 5 5 1 8 ) and therefore
in highly alkaline solutions, it will exist in traces
only. The main oxidising species would therefore
be chloramine-T and OCl Q only which will be in
equilibrium according to step ( 2 )
105&i s - 1 for the oxidation of
propanal b
re-butanalb
iso-butanalc
15.1
14.4
14.1
12.4
17
27.7
24.1
21.5
18.7
16.5
CH 3 C 6 H 4 S0 2 N • NaCl + H 2 0 ^
CH 3 C 6 H 4 S0 2 NH 2 + Na ® + CIO 9 .
A)
OCl9
as oxidizing
species.
If
(2)
hypochlorite
( O C l e ) acts as the oxidizing species of chloraminea [chloramine-T] = 2 x 1 0 ~ 3 M, [aldehyde] = 0.02 M. B in
presence of 0.04 M N a O H . c in presence of 0.01 M N a O H .
T a b l e III. Solvent Effects at 3 5 ° a.
T then the rate determining step would involve an
interaction
between
two
negatively
charged
ions
(enol anion and O C l 0 ) which would correspond to
T o obtain important thermodynamic parameters,
a highly negative entropy of activation, high energy
kinetic investigations were made at several tempera-
of activation and a positive salt effect. The activa-
tures between 35 to 5 5 ° and the approximate aver-
tion parameters and salt effects, however, do not
age specific rate constant k = kj [ A l d e h y d e ] [ N a O H ]
point out to such a reaction path. On the other hand
was obtained. The results are presented in Table I V .
if O C l 9 reacts with the aldehyde molecule in the
Discussion
independent of hydroxide ions which is again con-
rate determining step then the rate would become
trary to experimental observations. Thus the possibility of O C l 9 as the reacting species f o r the oxi-
The oxidation of aldehydes, in general, has been
dation of aldehydes is completely ruled out.
found to be either acid or base catalysed. DRUMReducing
Substrate
EA
[kcals/mole]
log A
AH*
[kcals/mole]
Ethanal
Propanal
n-Butanal
iso-Butanal
13.8
12.9
12.6
11.8
8.24
8.32
7.88
8.34
13.2
12.3
12.0
11.2
± 0.3
± 0.1
± 0.1
±0.1
± .04
± . 0 2
± .01
± . 0 2
± 0.3
± 0.1
± 0.1
±0.1
AS*
e. u .
- 2 2
- 2 2
—23
—21
T a b l e I V . T h e r m o d y n a m i c Parameters.
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O X I D A T I O N OF SOME A L I P H A T I C
B) Chloramine-T as oxidizing species. Two schemes may be proposed.
Scheme I: Taking enol anion of the aldehyde as
the reacting species, following steps are proposed:
> C — OH + OH®
1
(enol)
>
Uli
>C^nu
1
(3)
fast
0 H
(enol anion)
+ CH3C6H4SOoN• NaCl —
CH 4 C 6 H 4 S0 2 NH 2 + NaCl + - C O O 0
slow.
(4)
Representing the reducing substrate ^ 0 — OH as S
and applying steady-state conditions to the intermediate S O H e , the rate of reaction is given b y :
-
[chloramine-T] = - ^ [S] =
[chloramine-T] [S] [ O H e ]
(5)
where k' _ i ^ k'2 [chloramine-T] is a reasonable approximation.
Scheme II: Taking aldehyde molecule as the
reacting species the rate determining reaction would
be represented as —
Chloramine-T + S
Products
Slow
(6)
and accordingly the rate law would be
— j - [chloramine-T] = — ^
[S] = k 3 [chloramine-T] [S].
(7)
The derived rate law ( 5 ) , by taking enol anion
as the reacting species, predicts that the rate of oxidation wrould be of first order in chloramine-T, aldehyde and alkali which is in accordance with the
experimental observations.
The derived rate law ( 7 ) , by taking aldehyde
molecule as the reacting species, predicts that the
1
A. R. V. MURTHY and V. S. RAO, Proc. Indian Acad. Sei.
35 A , 69 [1958].
2
W . SMULEK. W i a d o m o s c i C h e m . 9 , 5 0 5
3
A . BERKA, C h e m i e 1 0 , 1 2 1
4
K.
WEBER
and
F.
[1955].
ALDEHYDES BY CHLORAMINE-T 404
rate wrould be independent of the hydroxide ion
concentration which is contrary to the experimental
observations.
It has been observed that during the oxidation of
aldehydes by chloramine-T, the order with respect
to OH® is fractional (0.8 to 0 . 9 ) . As sodium ions
do not show any retarding influence it seems likely
that, although the reaction follows the scheme I,
some of the reaction does proceed via scheme II
which has an independent (rate) nature to hydroxide
ions, and the net outcome is a fractional order in
hydroxide ions. However, the base induced condensation reactions of aldehydes may also be responsible for the slight decrease in the order with respect
to alkali.
The negligible effect of neutral salts, negative
entropy of activation and the retarding effect of
ethanol are consistent with a rate-determining step
involving a negatively charged ion and a neutral
molecule, as proposed in Scheme I. Further, during
the study of the oxidation of substances like formaldehyde and benzaldehyde which cannot enolize it
was observed that chloramine-T does not oxidize
formaldehyde while that of benzaldehyde is exceedingly slow. This is also in accordance with the
fact that the reaction is mainly represented by
Scheme I.
It is, therefore, concluded that the oxidation of
aldehydes by chloramine-T in alkaline media proceeds mainly through a rate determining reaction
between chloramine-T and the enol anion of the
aldehyde.
The authors thank Council of Scientific and Industrial Research, New Delhi for a Senior Research Fellowship to MCA.
11
Z.
12
physik.
6
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C.
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and
S. P .
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J. p h y s i c .
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BERKA,
J.
VULTERIN,
and
J.
ZYKA,
Chemistry,
Newer
Redox
A . Y . DRUMMOND a n d W . A . WATERS, J. chem. S o c .
[Lon-
don] 1953, 440.
[1971].
(B)
16
AIROLDI, A n n .
[Lon-
Pun-
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R.
DIETZEL
and
K.
TAUFEL,
Apotheker-Ztg.
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989
[1929],
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R.
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CARLI and
BOTTGER, Z . analyt. C h e m . 70,
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Part III, Longmans, 1958, p. 734.
S. P . MUSHRAN, M . C . A G R A W A L , a n d B . PRASAD, J . d i e m .
B.
W.
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jab Univ., N 30, 55 [1953].
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K.
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[London]
B