Kinetics of the Oxidation of Aliphatic Aldehydes by ...

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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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