in which the first step is the rapid reversible formation of a Chromate ester followed by a hydride- ion transfer in the rate determining step has been suggested.
B A N D 26 b
ZEITSCHRIFT FÜR
HEFT 5
NATURFORSCHUNG
Kinetics of the Oxidation of Aliphatic Aldehydes by Chromic Acid C . G O S W A M I a n d K . K . BANERJI Department of Chemistry, University of Jodhpur, Jodhpur (Z. Naturforsdi. 26 b, 383—385 [1971] ; received January 23, 1971)
The kinetics of chromic acid oxidation of propanaldehyde, butyraldehyde and isobutyraldehyde in aq. acetic acid have been studied. The product of the oxidation is the corresponding carboxylic acid. The reaction is of first order with respect to each the acid Chromate ion, H C r O 4 0 , and the aldehyde but of second order to hydrogen ion. The rate increases with proportion of acetic acid in the solution. The activation parameters for the oxidation and enolisation reactions have been evaluated. The rate of enolisation under similar condition is less than that of oxidation. A mechanism in which the first step is the rapid reversible formation of a Chromate ester followed by a hydrideion transfer in the rate determining step has been suggested.
Results
Aliphatic ketones are oxidised b y chromic acid via their enol f o r m
2.
There is a possibility that
oxidation of aldehydes may also involve the enol intermediate. C h r o m i c acid oxidation of aldehydes 3 ' 4 and formaldehyde
5
aromatic
has been studied
in detail, but these aldehydes cannot enolise. Some preliminary studies have been made on the oxida-
Product Analysis:
c a r b o x y l i c acid is f o r m e d . T h e over all reaction may b e written as f o l l o w s (eq. 1 ) . 3 RCHO + 2 Cr0 4 2 ® + 10 H® 3 R C O O H + 5 H , 0 + 2 Cr3® .
tion of acetaldehyde 6 ~ 8 . In the present investigation the role of enolisation in the oxidation of propanaldehyde, butyraldehyde and isobutyraldehyde has been studied.
Reprints request to Prof. Dr. K. K. BANERJI, Department of Chemistry, University of Jodhpur, Jodhpur, India. BEST, J. S. LITTLER, a n d W .
A.
W A T E R S , J.
chem.
Soc. [London] 1962, 822. 2
J.
ROCEK
and
A.
REIHL, J. A m e r .
chem.
Soc.
89,
show that the reaction is strictly of first order with respect to acid Chromate ion, H C r 0 4 ® . Since the results of the aldehydes studied are similar, only results of produced
K . B . WIBERG a n d T . MILL, J. A m e r . c h e m . S o c . 8 0 ,
(Table I ) .
[M]
[M]
1.00 2.50 4.00 5.00 10.00
9.24 21.2 31.7 38.0 64.9
The
of
l O 4 ^ [Cr ( V I ) ]
10 2 [sec
are re-
concentrations
- 1
8.84 8.35 7.50 7.12 6.10
]
[HCr04©] 9.46 9.45 9.46 9.50 9.40
Table I. Oxidant concentration dependence of the reaction rate. [C 2 H 5 CHO] 8 . 0 X 1 0 - 2 M , [ H © ] 0.2 M, Temp. 30 ° C . 5
6
8
3022 9
G . T . E . GRAHAM a n d F . H . WESTHEIMER, J. A m e r . c h e m .
Soc. 80, 3030 [1958].
reactions with propanaldehyde here
10 3 [Cr ( V I ) ] 1 0 4 [ H C r 0 4 © ]
6691
[1958], 4
dis-
appearance of C r ( V I ) f o l l o w e d first order rate laws
7
[1967], 3
and hydrogen ion are in excess, the rate of
in the concentration of chromium ( V I ) . The results
Aldehydes (B.D.H.) were purified by usual method. Perchloric acid (60%, Merck) was used a source of hydrogen ions; sodium Perchlorate (Riedel) was used to adjust ionic strength. Acetic acid (B.D.H., 99.5%) was distilled over chromic oxide before use. All other chemicals were chemically pure. Product Analysis: Oxidation of these aldehydes by aqueous chromic acid yields the corresponding carboxylic acids, as identified by their characteristic spot tests 9 . For quantitative estimation, the completely reduced reaction mixture was shaken by ether and the amount of the carboxylic acid present in the ether solution was determined colorimetrically as ferric-hydroximate 10 . All other experimental procedures have been reported earlier 8 .
P. A.
(1)
Rate Laws: W h e n concentrations of the aldehyde
but values of rate constants decreased with increase
Experimental
1
It is observed that f o r every
mole of C r ( V I ) consumed, nearly 1.5 moles of the
10
T . J. KEMP a n d W . A . W A T E R S , P r o c . R o y . S o c . , S e r . A 2 7 4 ,
480 [1963]. J. ROCEK, Tetrahedron Letters [London] No. 5, 1 [1959]. A . C . CHATTERJEE a n d V . A N T O N Y , Z . p h y s i k . C h e m . 2 1 0 ,
103 [ 1 9 5 9 ] . C. GOSWAMI and K. K. BANERJI, Bull. chem. Soc. Japan 43, 2644 [ 1 9 7 0 ] . F. FEIGL, "Spot tests in Organic Chemistry", Elsevier Publishing Co., Amsterdam 1966, pag. 212. J. MITCHELL, JR., Organic Analysis, Vol. ILL Interscience Publishers Inc,. New York 1953, 63.
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384
C. GOSWAMI AND K. K. BANERJI
HCr04e
were
calculated
assuming
a
value
2 . 3 X 1 0 - 2 f o r association constant f o r
of
dichromate
k(l3 m o l e - 3 sec -
Compound Temp. [°C]
formation [HCr04eJ2
(K—
[Cr 2 O 7 2 0
])
at 30 ° C » .
T h e reactions are also of first o r d e r with respect to the aldehyde ( T a b l e I I ) .
Et. CHO Pr. CHO Pr'. CHO
1
)
25
30
35
40
45
0.15 0.16 0.20
0.21 0.22 0.27
0.27 0.30 0.37
0.36 0.41 0.50
0.47 0.55 0.67
Table V. Effect of temperature on the oxidation rate. 10 2 [C 2 H 5 CHO] M 104 kt (sec" 1 ) 10 2 &i/[C 2 H 5 CHO]
1.0 5.0 5.0
2.0 10.0 5.0
3.0 15.3 5.1
4.0 21.0 5.2
5.0 25.3 51.
ture g a v e a straight line. T h e specific rate constant, k, is defined as hi [Aldehyde] [H]2
Table II. Substrate concentration dependence of the reaction. [Cr(VI)] 1 x 1 0 - 3 M, [H©] 0.5 M, Temp. 30 °C.
U n d e r the c o n d i t i o n s of constant i o n i c the rate is p r o p o r t i o n a l to the square of ion concentration
(Table III)
strength hydrogen
T h e activation
parameters were evaluated
(Table
VII). T h e rate of enolization w a s
Rates of Enolization:
and there is also a
measured
m a r k e d positive i o n i c salt effect.
'
by
bromination
method.
The
rate
of
b r o m i n a t i o n of the aldehydes is of first o r d e r with [H©] M 104 kx (sec - 1 ) kj[ H © ] 2
0.25 1.00 1.60
0.50 3.80 1.52
0.75 8.60 1.54
1.00 15.1 1.71
1.25 24.5 1.57
1.50 36.0 1.60
Table III. Acidity dependence of the reaction velocity. [C 2 H 5 CHO] 5 x 1 0 - 3 M, [Cr (VI)] 1 x 1 0 - 3 M, Temp. 30 °C. Ionic strength (ft) = 2.0 M.
Effect
of Solvent
Composition:
Increase
respect to
the a l d e h y d e and h y d r o g e n
ions
indi-
vidually but of z e r o - o r d e r to b r o m i n e . T h e rate o f b r o m i n a t i o n d i v i d e d b y concentrations of the aldeh y d e and H® gives k2 ( Z - m o l e - 1 s _ 1 ) . T h e T a b l e V I
104 k2 (I. m o l e - 1 s e c - 1 )
Compound
in the
Temp. [°C]
25
30
35
40
45
2.3 1.7 3.0
3.3 2.8 4.4
5.0 4.5 6.2
7.1 7.5 9.3
9.8 11.0 13.0
p r o p o r t i o n o f a c e t i c acid in the solvent m i x t u r e increases the rate of o x i d a t i o n because o f
lowering of
(Table I V ) ,
probably
dielectric constant o f
m e d i u m 1 2 , which f a v o u r s r e a c t i o n s i n v o l v i n g
the
Et. CHO Pr. CHO Pr'. CHO
pro-
tonation. In acetic a c i d , c h r o m i c a c i d exists as aceto-
Table VI.
Rates of bromination at different temperatures.
c h r o m i c acid, C H 3 C 0 0 C r 0 3 H , which is a s s u m e d to b e a stronger a c i d and a much p o w e r f u l o x i d i s i n g
r e c o r d s i n the values of k2 f o r different aldehydes at
agent4'13
different temperatures.
and hence the rate increases.
The
activation
parameters
are r e c o r d e d in T a b l e V I I . Acetic acid [%]
20
40
50
60
70
80
k(P m o l e - 3 s e c - 1 ) Et. CHO Pr. CHO Pr>. CHO Table IV.
0.15 0.16 0.20
0.22 0.24 0.31
0.32 0.31 0.46
0.46 0.45 0.67
0.94 0.98 1.7
1.5 1.7 —
Solvent composition dependence of the reaction rate. Temp. 25 °C.
Effect of temperature:
in
T a b l e V . A plot of l o g k against inverse o f tempera-
F . H . WESTHEIMER a n d A . N O V I C K , J . d i e m . P h y s i c s
II,
12
AE*
AS*
AF*
AE*
e.u.
298 ° K kcal / mole
kcal/ mole
10.1 11.5 11.4
-29.1 -24.1 -24.2
18.8 18.7 18.6
14.9 17.3 14.2
/IS* e.u.
-26 -23 -28
AF* 298 ° K kcal/ mole 25.4 24.1 22.6
Table VII. Activation parameters for oxidation and enolisation.
13
K . B. WIBERG and H .
927 [1969].
506 [1943].
Enolisation
kcal/ mole
Et. CHO Pr. CHO Pr' • CHO
Data o n the effect o f tem-
perature o n the r e a c t i o n rate a r e s u m m a r i z e d
11
Oxidation
Compound
M . COHEN and F . H . WESTHEIMER, J. A m e r . chem. S o c . 7 4 ,
4387 [1952].
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SCHÄFER, J. A m e r . chem. S o c .
91,
ALIPHATIC ALDEHYDES AND CHROMIC ACID
Discussion
the Chromate esters
T h e c o m p a r i s o n of T a b l e s V and V I s h o w s that in all cases the o x i d a t i o n is much faster than enolisat i o n . M o r e o v e r the activation energy is l o w e r in the c h r o m i c a c i d o x i d a t i o n than in enolization. O n these g r o u n d s , w e are f o r c e d to c o n c l u d e that enolization c a n n o t b e an essential step in the o x i d a t i o n reaction. R e c e n t l y WIBERG and SCHÄFER established that o x i d a t i o n o f a l c o h o l s b y c h r o m i c acid p r o c e e d s via 0.25
14
385 and w e have s h o w n e a r l i e r 8
that o x i d a t i o n o f a l d e h y d e is also best represented b y assuming the f o r m a t i o n o f a similar intermediate which then d e c o m p o s e s in the rate determining step. T h e activation parameters suggest that C — H b o n d is ruptured i n the rate determining s t e p 1 5 ' 1 6
and
this r e a s o n i n g is m a d e clear b y the fact that b o t h formaldehyde
and a c e t a l d e h y d e 1 7 s h o w a p r i m a r y
5
i s o t o p e effect. It is, h o w e v e r , farless c l e a r whether h y d r o g e n leaves as a p r o t o n , an atom o r an a n i o n . WESTHEIMER4 p r e f e r r e d the abstraction of
proton
in c y c l i c p r o c e s s b y
oxygen
one of
the Chromate
atoms. H o w e v e r , w h e n the rate constants o f v a r i o u s 0.20
aldehydes
investigated
TAFT'S a*
values18,
here
are
the c u r v e
plotted
against
is a straight
line
( F i g . 1 ) with the values o f q* b e i n g a p p r o x i m a t e l y — 1 . 1 . T h i s indicates that the o x i d a t i o n is facilitated
0.15
b y a h i g h e r electron density at the C — H b o n d and hence
W e s t h e i m e r's
mechanism
is
untenable.
A l l experimental data are f u l l y explained if o n e as-
0.10
sumes the f o l l o w i n g mechanism f o r the sition
of
the Chromate ester i n v o l v i n g a
decompohydride
transfer. T h i s also s u p p o r t s the v i e w of ROCEK o n
0.05
the o x i d a t i o n of a c e t a l d e h y d e 6 . 0
0
0.05
0.10
-a*
0.15
OH I R - O - O
020
—>•
Fig. 1. Dependence of the relative rates on T a f t's o* values. (The rate data of Me • CHO are taken from ref. 8 .) 14
K.
B . WIBERG a n d H .
SCHÄFER, J . A m e r . c h e m . S o c .
91,
H
16
G.
V.
BAKORE a n d
S.
N A R A I N , J. chem.
1963,
3419.
G.
BAKORE a n d S . N A R A I N , Z . p h y s i k .
V.
Soc.
[London]
Chem. 227,
8
/ O
17
933 [1969]. 15
OH
' ^ \
18
> R C O O H + HaCrOs©
C
\ OH
J. W . CORNIFORTH a n d G . POPJAK, N a t u r e [ L o n d o n ]
164,
1053 [1949]. R. W . TAFT, "Steric Effect in Organic Chemistry", ed. by M. S. NEWMAN, John Wiley and Co., New York 1956, pag. 556.
[1964].
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