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OXIDATION OF SOME ALIPHATIC ALDEHYDES BY CHLORAMINE-T 4 0 1

Mechanism of Oxidation of some Aliphatic Aldehydes by Chloramine-T M . C . A G R A W A L a n d S . P . M U S H R A N

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 chlor-amine-T. The order with respect to alkali has been found to be almost equal to unity (0.8 to 0.90) . 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 found to be the oxidation products.

The sodium salt of p-toluenesulfochloramide, commonly known as chloramine-T, exerts strong oxidising action in both acidic and alkaline media (Ered. = 1 .138 1 at pH 12) and thus has been widely used for the oxidimetric determination 2 ' 3 of a large number of inorganic and organic substances. With regard to the mechanism of oxidations per-formed with this potent oxiding agent, kinetics of the oxidations of glycerol in neutral and alkaline media 4 , of p-cresol in an acidic medium 5 and hexacyanoferrate (II) 6 in a feebly acidic medium (pH 6 to 7) have been investigated. The role of osmium (VIII) as a catalyst during the oxidation of a-hydroxy acids by alkaline chloramine-T has also been examined 7 .

Oxidation of some aldehydes by chloramine-T has been reported to take place quantitatively8 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 or the estima-tion of aldehydes by chloramine-T. It has been ob-served that the oxidations are quite slow even at pH 12 and take a few hours to complete. The kine-tics of the oxidation of four 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 decom-

Requests for reprints should be sent to Dr. S. P. MUSHRAN, Department of Chemistry, University of Allahabad, Alla-habad (Indien).

position. Although aqueous solutions of chloramine-T are very stableu , their strengths were checked from time to time using iodometric method 12. Aqueous solu-tion of ethanol was prepared from an Anala-R, B.D.H. sample of the reagent. Due to low solubility of propa-nal, 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 hydro-chloric acid. The excess ascorbic acid left was back titrated against standard chloramine-T using KI-starch indicator.

Results

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 de-pendence 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 con-stants (kj ) in chloramine-T were calculated at dif-ferent initial concentrations of aldehydes which showed a linear increase in rate constants with in-crease in aldehyde concentration (Table I ) . Second

4 0 2 M. C. AGAR WAL AND S. P. MUSHRAN

1.0

0.8 K

Z.0.6 I5

0.4

0.2

0 20 40 60 80 100 120 Time in min. >-

Fig. 1. Rate of reduction of 2.0 x 1 0 - 3 M chloramine-T at A) [ethanal] = 0.04 M, [NaOH] = 0.04 M, Temp. = 50° , B) [propanal] = 0.04 M, [NaOH] = 0.04 M, Temp. = 40° . C) [n-butanal] = 0.04 M, [NaOH] = 0.04 M, Temp. = 40° and D) [iso-butanal] = 0.04 M, [NaOH] = 0.01 M, Temp.

= 4 0 ° .

order constants were, therefore, calculated as k2 — A^/fRCHO] and the average values were obtained as (5.5 ± 0.5) x 1 0 - 3 (in 0.4 M NaOH at 5 0 ° ) , (9.6 + 0.4) and (6.2 + 0.3) x 10~ 3 (in 0.4 M NaOH at 4 0 ° ) and (16.1 + 0.5) x 10~ 31 m o l e - 1 s " 1 (in 0.1 M NaOH at 4 0 ° ) for the oxidation of ethanal, propanal, n- and iso-butanal respectively. All the kinetic runs presented in Table I for the oxidation of propanal, n- and iso-butanal have been carried out in presence of 25% ethanal.

Kinetics were also followed in equivalent amounts of the reactants where [ c h l o r a m i n e - T ] v a r i e d linearly with time. The overall second order reac-

tion between aldehydes and chloramine-T is, there-fore, established.

The oxidation of aldehydes by chloramine-T was found to be strongly dependent on alkali concentra-tion. To study the effect of alkali on the reaction rate, kinetic studies were made over a wide range of alkali concentrations. In order to avoid any variations due to change in ionic strength the runs were made in presence of excess NaC104 . The re-sults (Table II) show almost linear increase with increase in alkali concentration. The order of the reactions with respect to alkali has been obtained from the slope of the plots of l ogk x against log [ O H 9 ] (Fig. 2) as 0.87, 0.85, 0.90 and 0.80 for the oxidation of ethanal, propanal, n- and iso-buta-nal respectively.

The oxidations of propanal, n- and iso-butanal by chloramine-T were studied in 25% ethanol. To find out the effect of solvent on the reaction rate different concentrations of ethanol-water mixtures were employed and it was observed that an increase in ethanol concentration has a slight retarding in-fluence (Table I I I ) .

The influence of neutral salts viz. NaC104 and NaCl on the reaction rate was investigated. It was observed that a ten-fold variation in the concentra-tion of added neutral salts has a little or negligible effect on the rate of oxidation of aldehydes by chloramine-T.

103 102 105 Jd 8-1 for the oxidat ion o f [Chlora- [ R C H O ] ethanal a propanal13 n - b u t a n a l b i so -butanal c

mine-T]

1.2 2.0 10.5 19.9 11.5 33.3 1.6 2.0 10.9 19.8 12.1 33.0 2.0 2.0 11.3 19.2 11.8 31.4 2.8 2.0 10.8 17.7 11.1 30.1 Table I. Dependence on Reactants 4.0 2.0 10.6 15.6 9.60 29.7 Concentrations. 2.0 1.2 7.33 10.9 7.40 19.9 a In presence of 0.04 M NaOH at 2.0 1.6 9.40 14.8 9 .60 25.1 50° . b In presence of 0.04 M NaOH 2.0 2.8 14.8 27.8 17.6 46.1 at 40° . C l n presence of 0.01 M 2.0 4 .0 19.4 40.0 26.9 63.0 NaOH at 40° .

[ N a O H ] to 105 8-1 f o r the oxidation o f [ N a O H ] c 105 h s " 1 for M ethanal propanal n-butanal M the ox idat ion o f

iso-butanal

0.024 4.72 13.0 8.17 0.006 28.9 T a b l e I L Alkali Dependence a 0.032 6.41 15.7 10.1 0.008 35.3 a t 4 0 ' 0.040 7.52 18.7 12.3 0.010 40.2 a [chloramine-T] = 2 x 10~ 3 M 0.056 10.0 24.6 15.8 0.014 52.8 [aldehyde] = 0.02 M. b in pre-0.080 13.3 33.7 22.5 0.020 70.7 seuce of 0.4 M NaC104 c i n pre-

sence of 0.16 M NaC104 .

OXIDATION OF SOME ALIPHATIC

B) Chloramine-T as oxidizing species. Two sche-mes may be proposed.

Scheme I: Taking enol anion of the aldehyde as the reacting species, following steps are proposed:

> C — OH + OH® 1 > C ^ n u f a s t ( 3 ) (enol) 1 0 H

(enol anion)

> + CH3C6H4SOoN• NaCl — U l i

CH4C6H4S02NH2 + NaCl + - C O O 0 slow. (4)

Representing the reducing substrate ^ 0 — OH as S and applying steady-state conditions to the inter-mediate SOH e , the rate of reaction is given by:

- [chloramine-T] = - ^ [S] =

[chloramine-T] [S] [OH e ] (5)

where k' _ i ^ k'2 [chloramine-T] is a reasonable ap-proximation.

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 oxi-dation wrould be of first order in chloramine-T, al-dehyde 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 [ 1 9 5 5 ] . 3 A . BERKA, C h e m i e 1 0 , 1 2 1 [ 1 9 5 8 ] . 4 K . WEBER a n d F . V A L I C , Z . p h y s i k . C h e m . 2 3 8 , 3 5 3

[1968]. 5 T . HIGUCHI a n d A . HUSSAIN, J . d i e m . S o c . [ L o n d o n ] ( B )

1 9 6 7 , 5 4 9 . 6 M . C . A G R A W A L a n d S . P . MUSHRAN, J . p h y s i c . C h e m .

7 5 , 8 3 8 [ 1 9 7 1 ] . 7 S . P . MUSHRAN, M . C . A G R A W A L , a n d B . PRASAD, J . d i e m .

S o c . [ L o n d o n ] ( B ) 1 9 7 1 , 1 7 1 2 . 8 B . SINGH, A . S INGH, a n d M . SINGH, R e s . B u l l . E a s t P u n -

jab Univ., N 30, 55 [1953]. 9 A . BERKA a n d J . Z Y K A , C e s k o s l o v . F a r m a c . 5 , 3 3 5 [ 1 9 5 6 ] .

1 0 B . CARLI a n d R . AIROLDI , A n n . C h i m . A p p l i c a t a 2 7 , 5 6 [ 1 9 3 7 ] .

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 conden-sation reactions of aldehydes may also be respon-sible 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 form-aldehyde and benzaldehyde which cannot enolize it was observed that chloramine-T does not oxidize formaldehyde while that of benzaldehyde is ex-ceedingly 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 pro-ceeds mainly through a rate determining reaction between chloramine-T and the enol anion of the aldehyde.

The authors thank Council of Scientific and Indu-strial Research, New Delhi for a Senior Research Fel-lowship to MCA.

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1 2 K . B O T T G E R a n d W . B O T T G E R , Z . a n a l y t . C h e m . 7 0 , 2 2 5 [ 1 9 2 7 ] .

13 A. I. VOGEL, Elementary Practical Organic Chemistry, Part III, Longmans, 1958, p. 734.

1 4 A . BERKA, J . VULTERIN, a n d J . Z Y K A , N e w e r R e d o x Titrants, Pergamon Press Ltd., London 1965, p. 43.

1 5 A . Y . D R U M M O N D a n d W . A . W A T E R S , J . c h e m . S o c . [ L o n -don] 1953, 440.

1 6 P . T . SPEAKMAN a n d W . A . W A T E R S , J . c h e m . S o c . [ L o n -don] 1955, 40.

1 7 R . DIETZEL a n d K . TAUFEL, A p o t h e k e r - Z t g . 3 4 , 9 8 9 [ 1 9 2 9 ] ,

1 8 T . HIGUCHI a n d A . HUSSAIN, J . d i e m . S o c . [ L o n d o n ] B 1967, 546.