Oxidation of Organic Compounds by Sulfur Dioxide under Pressure

5 Oxidation of Organic Compounds by Sulfur Dioxide under Pressure

Downloaded by UNIV OF MISSOURI COLUMBIA on October 28, 2017 | http://pubs.acs.org Publication Date: January 1, 1965 | doi: 10.1021/ba-1965-0051.ch005

A. J . SHIPMAN Imperial Chemical Industries, Ltd., Mond Division, Runcorn, Cheshire,

England

Alkyl substituted aromatic compounds are oxidized to the corresponding carboxylic acids by sulfur dioxide under pressure in the absence of catalysts or solvents. For example, toluene is oxidized to benzoic acid, xylenes to phthalic acids, and pseudocumene to trimellitic acid. The overall reaction may be represented by the equation: RCH + 1½ SO --> RCOOH + 1½ S + H O, sulfur and water being by-products. In the presence of excess sulfur dioxide, yields of 80-90% carboxylic acid may be achieved. The effects of reaction variables upon the reaction rate and yield have been studied. The reaction is catalyzed by compounds of bromine and iodine and inhibited by metallic copper and silver. Several other novel reactions of sulfur dioxide have been observed. 3

2

2

Q u r i n g a n i n v e s t i g a t i o n i n t o the reactions of s u l f u r d i o x i d e w i t h some s i m p l e o r g a n i c c o m p o u n d s u n d e r pressure, several n o v e l effects w e r e f o u n d w h i c h , so f a r as is k n o w n , h a v e n o t b e e n r e p o r t e d p r e v i o u s l y . T h e most i m p o r t a n t is t h e o x i d a t i o n of a l k y l s u b s t i t u t e d a r o m a t i c c o m p o u n d s to t h e c o r r e s p o n d i n g c a r b o x y l i c a c i d b y s u l f u r d i o x i d e i n the absence of b o t h a solvent o r a catalyst. T h e r e a c t i o n m a y b e i l l u s t r a t e d b y t h e o x i d a t i o n o f toluene to b e n z o i c a c i d at temperatures a r o u n d 250°—300° C . a n d a pressure of 300 a t m : CeH CK + 1 7 S0 -» C H COOH + V/ S + H 0 t h e c o p r o d u c t s b e i n g s u l f u r a n d w a t e r ; t h e h e a t of r e a c t i o n is — 5 0 k c a l . / m o l e a n d t h e c h a n g e i n free e n e r g y is — 3 4 k c a l . / m o l e . I n t h e presence of excess s u l f u r d i o x i d e , t h e c o n v e r s i o n of t h e h y d r o c a r b o n is c o m p l e t e , a n d y i e l d s of 9 0 % h a v e b e e n a c h i e v e d . T h e reactions of s u l f u r d i o x i d e w i t h a w i d e r a n g e of o r g a n i c c o m p o u n d s h a v e b e e n s t u d i e d q u a l i 5

2

s

2

6

5

2

52 Fields; Selective Oxidation Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1965.

2

S.

SHIPMAN

Organic Compounds by Sulfur Dioxide

53

t a t i v e l y , a n d the oxidations of toluene, xylenes, a n d p s e u d o c u m e n e

to

benzoic, phthalic, a n d trimellitic acids, respectively, have been studied quantitatively i n m u c h more detail.

T h e p u r p o s e of this p a p e r is b r i e f l y

to r e v i e w the e x p e r i m e n t a l results d e r i v e d f r o m these studies.

Previous Work


T h e earliest o x i d a t i o n effected b y a s u l f u r c o m p o u n d w a s c a r r i e d o u t · b y W i l l g e r o d t i n 1887. H e s h o w e d t h a t a r y l m e t h y l ketones are c o n v e r t e d to a r y l acetamides b y aqueous a m m o n i u m p o l y s u l f i d e (26). Ο II Ar—G—GH

Aqueous 3

Ο || > Ar—GH —G—NH 2

(NH4) Sx200 -300° C.

2

o

2

T h e W i l l g e r o d t r e a c t i o n has b e e n w i d e l y s t u d i e d a n d w a s r e v i e w e d i n 1946 b y C a r m a c k a n d S p i e l m a n (4)

who, i n addition, indicated that early

attempts to o x i d i z e a s a t u r a t e d h y d r o c a r b o n side c h a i n o n a n a r o m a t i c n u c l e u s b y aqueous p o l y s u l f i d e h a d f a i l e d . C a l i f o r n i a R e s e a r c h C o r p . (19)

S u b s e q u e n t l y , T o l a n d at the

a n d N a y l o r at d u P o n t ( 8 )

found that

u n d e r m o r e v i g o r o u s c o n d i t i o n s , alkanes a n d a r a l k y l h y d r o c a r b o n s o x i d i z e d to a c i d d e r i v a t i v e s b y a w i d e v a r i e t y of s u l f u r c o m p o u n d s aqueous

solution.

F o r e x a m p l e , T o l a n d h e a t e d the h y d r o c a r b o n

are in and

s u l f u r c o m p o u n d together at 300° C . i n the presence of a l a r g e excess of w a t e r at pressures a r o u n d 170 a t m .

H e u s e d s u l f u r d i o x i d e alone

(20)

or i n c o n j u n c t i o n w i t h h y d r o g e n sulfide, or a n a l k a l i or a m m o n i u m sulfite, or a n a l k a l i s u l f i d e / s u l f a t e m i x t u r e (21).

I n other processes, e l e m e n t a l

s u l f u r has b e e n u s e d i n w a t e r either alone or w i t h a base ( 22, 23, 24, 25 ). G o o d y i e l d s are d e s c r i b e d f o r the p r e p a r a t i o n of b e n z o i c a n d p h t h a l i c acids.

It is l i k e l y t h a t t h e o x i d a n t i n these reactions is w a t e r - s o l u b l e

i n o r g a n i c polysulfides (10,11 ) p r o d u c e d b y b a s e - c a t a l y z e d d i s p r o p o r t i o n a t i o n reactions of s u l f u r or s u l f u r anions. T h e m e c h a n i s m of t h e r e a c t i o n is r e v i e w e d b y P r y o r ( 9 ) w h o c o n c l u d e s t h a t the effective species is the p o l y s u l f e n y l r a d i c a l a n d suggests the f o l l o w i n g s c h e m e : Y S 2

a

+

b

,

» YS

k l

a

+

YS

Y

b

=

H , N a , A r C H , etc. 2

kt YS (or YS ) + a

ArCH

b

Ar G H . + 2

[Ar G H

2

S Y] +

a

2

a

fast

YS

a

[Ar G H ( S Y ) ] 2

YS

+

^—> slow

3

fast a

fast

ArCH -

a

> [Ar G H

a

YS

YS H +

2

2

S Y]

> [Ar G H

a

2

> [Ar G H I φ

(S Y) ] a

2

2

(S.Y),] Rapid Hydrolysis

Ar G0 H 2

Fields; Selective Oxidation Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1965.

SELECTIVE OXIDATION PROCESSES

54

S t r i c k l a n d a n d B e l l of E a s t m a n K o d a k ( 1 7 ) h a v e o x i d i z e d t o l u e n e a n d xylenes w i t h s u l f u r d i o x i d e o v e r v a n a d i u m p e n t o x i d e catalysts a t a t m o s p h e r i c pressure.

T o l u e n e , at 410° C . w i t h a contact t i m e of five seconds,

gave a c o n v e r s i o n of 2 8 % a n d 7 5 % y i e l d of b e n z o i c a c i d w h i l e a l o w e r t e m p e r a t u r e gave p r e d o m i n a n t l y b e n z a l d e h y d e .

This method

is n o t

a p p l i c a b l e to t h e p r e p a r a t i o n of p h t h a l i c a c i d s ; v e r y l o w y i e l d s (less t h a n 5 % ) w e r e o b t a i n e d p r e s u m a b l y b e c a u s e these acids a r e n o t sufficiently


v o l a t i l e to leave the fixed b e d r e a c t i o n zone.

Experimental F o r t h e p r e l i m i n a r y q u a l i t a t i v e e x p e r i m e n t s , h i g h pressure reactors w i t h a n o m i n a l c a p a c i t y o f 100 m l . w e r e u s e d w h i c h w e r e l i n e d w i t h , o r m a d e of stainless steel a n d s t i r r e d b y m a g n e t i c a l l y o p e r a t e d r e c i p r o c a t i n g stirrers. H e a t i n g w a s effected b y a n e x t e r n a l , c o n t r o l l e d e l e c t r i c f u r n a c e . W o r k i n g w i t h s u c h s m a l l q u a n t i t i e s of reagents t o r e d u c e t h e h a z a r d , p r o d u c t r e c o v e r y w a s p o o r , a n d i t w a s n o t possible to d e r i v e m e a n i n g f u l reaction yields. I n m o r e d e t a i l e d q u a n t i t a t i v e studies of specific reactions, reactors of 1-liter a n d 2 5 - l i t e r c a p a c i t y w e r e u s e d . T h e e x p e r i m e n t a l a r r a n g e m e n t w a s s i m i l a r t o the s m a l l reactors except that t h e 2 5 - l i t e r reactor w a s fitted w i t h a n i n t e r n a l c o o l i n g c o i l to assist i n t h e c o n t r o l o f t h e e x o t h e r m i c reaction. T h e e x p e r i m e n t a l p r o c e d u r e most c o m m o n l y u s e d w a s to c h a r g e a k n o w n w e i g h t of t h e o r g a n i c c o m p o u n d to t h e reactor, assemble i t i n t o the f u r n a c e i n a b l a s t - p r o o f reactor b a y , heat to r e a c t i o n t e m p e r a t u r e , a n d pressurize w i t h s u l f u r d i o x i d e . A s t h e r e a c t i o n p r o c e e d e d , t h e pressure f e l l o w i n g to s u l f u r dioxide's b e i n g c o n s u m e d , a n d m o r e w a s a d d e d i n t e r m i t t e n t l y f r o m a compressor t o m a i n t a i n t h e r e a c t i o n pressure. A t t h e e n d of t h e r e a c t i o n , t h e p r o d u c t s c o u l d either b e r e m o v e d f r o m t h e reactor b y b l o w i n g off" v i a a d i p p i p e a t a r o u n d 200° C . o r b y c o o l i n g t h e reactor, p e r h a p s to — 8 0 ° C , a n d d i s m a n t l i n g . T h e best p r o d u c t r e c o v e r y w a s o b t a i n e d b y the latter p r o c e d u r e . F o r those c a r b o x y l i c acids w h i c h a r e s o l u b l e i n w a t e r , a s i m p l e w a t e r e x t r a c t i o n w a s sufficient t o g i v e p r i m a r y s e p a r a t i o n f r o m t h e s u l f u r c o p r o d u c t . M e t h o d s w e r e d e v e l o p e d to r e d u c e r e s i d u a l s u l f u r c o n t a m i n a t i o n i n b e n z o i c a c i d a n d t r i m e l l i t i c a n h y d r i d e , f o r e x a m p l e , d o w n to t h e o r d e r of parts p e r m i l l i o n . F o r w a t e r - i n s o l u b l e a c i d s , i t is necessary to a v o i d s t r o n g a l k a l i s , s u c h as c a u s t i c soda, w h i c h dissolve s u l f u r ; e x t r a c t i o n w i t h s o d i u m acetate s o l u t i o n w a s f o u n d to b e a p p l i c a b l e to t e r e p h t h a l i c acid. Example 1. Preparation of Benzoic A c i d from Toluene. I n t o a l i t e r r e a c t o r w e r e p l a c e d 260 grams of t o l u e n e , h e a t e d to 300° C . a n d p r e s s u r i z e d t o 300 a t m . b y a d d i n g a p p r o x i m a t e l y 590 g r a m s o f s u l f u r d i o x i d e . T h e reaction was completed i n t w o a n d a half hours, during w h i c h a f u r t h e r 150 grams s u l f u r d i o x i d e w e r e a d d e d to m a i n t a i n r e a c t i o n pressure. T h e reactor w a s c o o l e d , excess s u l f u r d i o x i d e v e n t e d , a n d after d r y i n g t h e p r o d u c t w e i g h e d 469 grams. A f t e r e x t r a c t i o n w i t h w a t e r , 306 grams of b e n z o i c a c i d w e r e p r e c i p i t a t e d c o r r e s p o n d i n g to a y i e l d o f 9 1 % c a l c u lated o n toluene.



5.

SHIPMAN


55

Example 2. Preparation of Trimellitic A c i d from Pseudocumene. I n a s i m i l a r e x p e r i m e n t , 263 grams of p s e u d o c u m e n e a n d 700 g r a m s o f s u l f u r d i o x i d e w e r e h e a t e d together at 275° C . a n d 300 a t m . D u r i n g a r e a c t i o n t i m e of 11 h o u r s , a f u r t h e r 2 0 0 g r a m s o f s u l f u r d i o x i d e w a s a d d e d . 773 grams of d r i e d p r o d u c t w a s o b t a i n e d w h i c h , o n e x t r a c t i o n w i t h w a t e r , gave 394 grams o f t r i m e l l i t i c a c i d of a p p r o x i m a t e l y 9 4 % p u r i t y , c o r r e s p o n d i n g to a y i e l d of 8 1 % c a l c u l a t e d o n p s e u d o c u m e n e . S u l f u r d i o x i d e i s , i n g e n e r a l , a g o o d solvent f o r o r g a n i c c o m p o u n d s ; toluene, x y l e n e , a n d p s e u d o c u m e n e are m i s c i b l e i n a l l p r o p o r t i o n s w i t h s u l f u r d i o x i d e . T h e r e a c t i o n t e m p e r a t u r e s u s e d f o r t h e o x i d a t i o n reactions are a b o v e the c r i t i c a l temperatures o f t h e b i n a r y phase, a n d t h e i n i t i a l r e a c t i o n a t least is i n t h e v a p o r phase. A t t h e e n d o f t h e r e a c t i o n , t h e c a r b o x y l i c a c i d is s o l u b l e i n s u l f u r d i o x i d e , a n d s u l f u r is i n s o l u b l e . I f t h e p r o d u c t of a r e a c t i o n s u c h as E x a m p l e 1 w e r e t a k e n o u t of t h e reactor c a r e f u l l y , i t w a s f o u n d t o consist o f t w o l a y e r s , t h e l o w e r b e i n g m a i n l y s u l f u r , containing about 5 - 1 0 % benzoic acid a n d the upper layer containing a b o u t 8 5 % a c i d . I n t h e p u r e state t h e m u t u a l s o l u b i l i t i e s i n t h e m o l t e n systems b e n z o i c a c i d / s u l f u r , t r i m e l l i t i c a n h y d r i d e / s u l f u r a r e a r o u n d 1 % only.

Results E x p l o r a t o r y R e a c t i o n s . I n g e n e r a l , o n l y t h e m o r e o b v i o u s results of these s c o u t i n g experiments h a v e b e e n n o t e d . I n most cases, n o a t t e m p t has b e e n m a d e t o o p t i m i z e t h e y i e l d o r s t u d y t h e r e a c t i o n o v e r a w i d e range of c o n d i t i o n s . T a b l e I s u m m a r i z e s t h e results of reactions o f s u l f u r d i o x i d e w i t h m a n y different o r g a n i c c o m p o u n d s . T h e o x i d a t i o n o f t h e m e t h y l g r o u p s u b s t i t u t e d i n a n a r o m a t i c r i n g gives a h i g h y i e l d of t h e c o r r e s p o n d i n g c a r b o x y l i c a c i d i n t h e presence o f excess s u l f u r d i o x i d e (12). I f the m e t h y l g r o u p is f u r t h e r s u b s t i t u t e d to e t h y l o r i s o p r o p y l (e.g., i n c u m e n e ) , the y i e l d of b e n z o i c a c i d is v e r y r e d u c e d . A s t h e n u m b e r of m e t h y l g r o u p s s u b s t i t u t e d i n t h e b e n z e n e r i n g is i n c r e a s e d , t h e y i e l d of t h e c a r b o x y l i c a c i d decreases as s h o w n i n T a b l e I I . T h e rate o f o x i d a t i o n r e a c t i o n is n o t affected b y t h e a d d i t i o n o f freer a d i c a l initiators o r i n h i b i t o r s , o x y g e n i n s m a l l a m o u n t s , s u l f u r o r i n o r g a n i c c o m p o u n d s of s u l f u r . T h e r e a c t i o n is c a t a l y z e d b y b r o m i n e a n d i o d i n e a n d t h e i r c o m p o u n d s (13), those c o n t a i n i n g b r o m i n e b e i n g t h e m o r e effective. T h i s w i l l b e d i s c u s s e d i n m o r e d e t a i l i n this p a p e r . B e n z a l d e h y d e is o x i d i z e d to b e n z o i c a c i d i n g o o d y i e l d . T h e r a t e of this r e a c t i o n is a b o u t 10 times as great as t h e o x i d a t i o n o f t o l u e n e u n d e r c o m p a r a b l e c o n d i t i o n s . T h i s suggests that i f t h e r e a c t i o n p r o c e e d s v i a t h e a l d e h y d e , t h e i n i t i a l step is rate d e t e r m i n i n g . I f t h e o x i d a t i o n of a n a r o m a t i c h y d r o c a r b o n is s t o p p e d b e f o r e c o m p l e t i o n , s m a l l a m o u n t s o n l y of aldehydes m a y be f o u n d i n the product. T h e chlorotoluenes a r e o x i d i z e d to t h e a c i d s , b u t n i t r o t o l u e n e s are decomposed even under m i l d conditions, Y i e l d s of n a p h t h a l e n e c a r -



56 boxylic acids were generally rather l o w .

T h e activated methylene group

i n d i p h e n y l m e t h a n e w a s o x i d i z e d to a ketone.

Phenol and naphthols d i d

not react u p to 300° C , b u t a n i l i n e d e c o m p o s e d w i t h explosive v i o l e n c e w i t h s u l f u r d i o x i d e at 200° C . Table I.

Reactions of Organic Compounds with Sulfur Dioxide Temp., ° C.

Pressure, atm.

Toluene

250-400

50-2500

Ethylbenzene Gumene o-Xylene Tw-Xylene /^-Xylene

275 300 275-300 300 250-320

1000 300 1000 300 100-500

Mesitylene Pseudocumene Durene a-Methylnaphthalene 2,6- a n d 2 , 7 - D i m e t h y l naphthalenes Acenaphthene Tetrahydronaphthalene Decahydronaphthalene Dibenzyl Stilbene Styrene Diphenylmethane Methylcyclohexane Gyclohexane Gyclohexene Benzene Naphthalene Anthracene o-, m-, / > - C h l o r o b e n z a l dehyde ο-, m-, />-Chlorotoluene />-Dichlorotoluene Benzaldehyde G i n n a m i c acid Benzyl alcohol Phenol a-Naphthol Aniline Gyclohexanol Nitrobenzaldehyde Nitrotoluene 2-Nitro-/>-xylene Pyridine

275 250-310 250 300 280

1000 100-500 300 1000 300

300 250 250 300 275 300 300 300 257 250 450 500 500 275

300 300 300 400 1000 1000 1000 300 300 900 900 300 300 300

275

300

275 300 300 250 300 275 200 200 200 200 200 400

300 300 1000 1000 300 1000 1000 300 300 300 300 300

a-Picolene

300

300

Ethylene Propylene Butene Decene E t h y l e n e oxide M e t h y l - b u t y l alcohols E t h y l e n e glycol Acetic acid

250 250 250 250 250 250 250 300

300 300 300 300 500 300 300 300


Compound

Product Benzoic acid 9 0 % yield, dibenzyl, polybenzylidenes Benzoic acid—moderate yield Benzoic a c i d — l o w yield o-Toluic acid, phthalic acid m - T o l u i c a c i d , isophthalic a c i d />-Toluic a c i d , terephthalic a c i d 9 0 % y i e l d polyxylyenes Trimesic acid Trimellitic acid 8 0 % yield Pyromellitic acid, low yield α-Naphthoic acid N a p h t h a l e n e m o n o - a n d d i c a r b o x y l i c acids Naphthalic acid Naphthalene Naphthalene Benzoic acid Benzoic acid Benzoic acid Benzophenone Benzoic acid D i p h e n y l , sulfide, etc. D i p h e n y l , sulfide, etc. D i p h e n y l , sulfide, etc. N o reaction N o reaction G h l o r o b e n z o i c acids C h l o r o b e n z o i c acids j&-Dichlorobenzoic a c i d Benzoic acid, good yield Benzoic acid Benzoic acid N o reaction N o reaction D e c o m p o s i t i o n w i t h explosive violence A d i p i c acid, low yield Decomposition Decomposition Decomposition Addition compound pyridine—sulfur dioxide A d d i t i o n c o m p o u n d picolene—sulfur dioxide Polysulfone, sulfolane Polysulfone, m e t h y l sulfolane Decomposition Decomposition E t h y l e n e sulfite E t h e r s , g o o d yields 1.4 D i o x a n N o reaction


5.

SHIPMAN

57

Organic Compounds by Sulfur Dioxide Table II

Hydrocarbon

Product

Yield

Temp., °C.

Pressure, atm.

Time, hrs.

Toluene /^-Xylene Pseudocumene Durene

Benzoic acid Terephthalic acid Trimellitic acid Pyromellitic acid

>90% 90% 80% low

300 280 270 250

300 300 300 300

3 6 8

U n s u b s t i t u t e d a r o m a t i c c o m p o u n d s are c o m p a r a t i v e l y u n r e a c t i v e to sulfur dioxide.

B e n z e n e r e a c t e d a b o v e 450° C . to g i v e d i p h e n y l , d i p h e n y l


sulfide etc., a n d n a p h t h a l e n e a n d a n t h r a c e n e d i d n o t react u p to 500° C . Neither p y r i d i n e nor picolines undergo oxidation or coupling reactions.

U n d e r m i l d e r conditions, a p y r i d i n e / s u l f u r dioxide addition c o m -

p o u n d is f o r m e d , a n d u n d e r m o r e v i g o r o u s c o n d i t i o n s extensive c a r b o n i z a t i o n takes p l a c e .

I t is p o s s i b l e that t h e strong a t t r a c t i o n b e t w e e n t h e

aromatic nitrogen atom a n d sulfur dioxide precludes the possibility of attack at a n o t h e r p a r t of t h e m o l e c u l e .

A similar result was obtained

with pyridine hydrochloride. Alicyclic compounds compounds.

are, i n general, dehydrogenated

to a r o m a t i c

T h i s is a w e l l k n o w n r e a c t i o n of s u l f u r b u t h a s n o t p r e v i -

ously been described for sulfur dioxide.

Cyclohexane a n d cyclohexene

g a v e d i p h e n y l a n d d i p h e n y l sulfides, s i m i l a r to results d e s c r i b e d p r e v i ously

( 5 ) ; methylcyclohexane

decalin gave naphthalene.

gave some b e n z o i c

acid.

Tetralin and

C y c l o h e x a n o l g a v e a s m a l l y i e l d of a d i p i c

a c i d , this b e i n g t h e o n l y case of those e x a m i n e d w h e r e a n a l i p h a t i c a c i d was derived from a sulfur dioxide oxidation reaction. F o r t h e o x i d a t i o n of a n a r a l k y l c o m p o u n d to t h e c a r b o x y l i c a c i d to p r o c e e d to c o m p l e t i o n , i t is necessary to m a i n t a i n a n excess of s u l f u r d i o x i d e present.

I f t h e h y d r o c a r b o n is present i n excess o v e r s u l f u r d i o x i d e ,

o x i d a t i v e c o u p l i n g t h r o u g h t h e m e t h y l groups takes p l a c e . d i b e n z y l was formed from toluene a n d compounds

F o r example,

of h i g h molecular

w e i g h t ( u p to ~ 6 0 0 ) w h i c h w e r e p r o b a b l y p o l y b e n z y l i d i n e s c o n t a i n i n g sulfur links:

T h e r e a c t i o n of xylenes w i t h s u l f u r d i o x i d e g a v e p o l y x y l y l e n e s . example, from p-xylene l o w molecular weight polymers were w h i c h c o n t a i n e d a m o l a r r a t i o of a b o u t 6 : 1 x y l e n e to s u l f u r .

For

formed

These are

w e l l k n o w n reactions o f s u l f u r , b u t t h e y h a v e n o t p r e v i o u s l y b e e n d e scribed for sulfur dioxide. O t h e r n o v e l reactions of s u l f u r d i o x i d e h a v e b e e n

found which,

w h i l e n o t o x i d a t i o n reactions, are o f interest.



58 ETHYLENE.

I t is w e l l k n o w n that e t h y l e n e w i l l c o p o l y m e r i z e i n excess

s u l f u r d i o x i d e i n t h e p r e s e n c e of a free r a d i c a l i n i t i a t o r to f o r m e t h y l e n e polysulfone

(—CH —CH —S0 ) 2

2

2

n

(-Z6).

I f a n excess of e t h y l e n e

over

s u l f u r d i o x i d e t h e n is u s e d w i t h a free r a d i c a l i n i t i a t o r , a s m a l l y i e l d of s u l f o l a n e ( t e t r a m e t h y l e n e s u l f o n e ) is o b t a i n e d (14). i n a similar manner.

Propylene behaves

A t t e m p t s to o x i d i z e p r o p y l e n e a n d i s o b u t e n e

to

acrolein a n d acrylic a c i d derivatives were unsuccessful i n b o t h b a t c h a n d c o n t i n u o u s flow r e a c t i o n s . O L E F I N OXIDES.

S u l f u r d i o x i d e w i l l react w i t h olefin oxides i n t h e


presence of a base s u c h as p y r i d i n e to f o r m c y c l i c sulfites—e.g., e t h y l e n e sulfite—in g o o d y i e l d

(15).

ALIPHATIC ALCOHOLS.

E t h e r s are f o r m e d i n g o o d y i e l d b y t h e r e a c -

tions of the l o w e r a l i p h a t i c alcohols w i t h s u l f u r d i o x i d e p r e s u m a b l y b y a r e a c t i o n analogous to t h a t w i t h s u l f u r i c a c i d .

E t h y l e n e g l y c o l is c o n v e r t e d

to 1.4 d i o x a n . T o s u m m a r i z e , i t has b e e n f o u n d t h a t s u l f u r d i o x i d e m a y r e a c t i n t h e following ways w i t h organic compounds: 1. 2. 3. 4. 5. 6.

T o o x i d i z e a n a l k y l s u b s t i t u t e d a r o m a t i c c o m p o u n d to t h e c o r r e sponding carboxylic acid. T o b r i n g a b o u t o x i d a t i v e c o u p l i n g of a l k y l s u b s t i t u t e d a r o m a t i c c o m pounds a n d benzene. T o dehydrogenate or aromatize alicyclic compounds. T o f o r m c y c l i c sulfones w i t h olefins. T o f o r m c y c l i c sulfites w i t h olefin oxides. T o f o r m ethers f r o m a l i p h a t i c a l c o h o l s . O x i d a t i o n of A l k y l Substituted Benzenes.

T h e reactions of

most

c o m m e r c i a l interest w e r e those l e a d i n g to the s i m p l e r a r o m a t i c a c i d s . to

benzoic,

p h t h a l i c , a n d t r i m e l l i t i c a c i d s h a v e b e e n s t u d i e d i n some d e t a i l .

The

oxidations

of

toluene,

xylenes,

and

pseudocumene

T h e ob-

ject of this w o r k w a s to d e t e r m i n e the effect of t h e r e a c t i o n v a r i a b l e s u p o n the rate a n d p r o d u c t y i e l d a n d h e n c e find t h e o p t i m u m c o n d i t i o n s . s e c t i o n s u m m a r i z e s the d e t a i l e d results o b t a i n e d .

This

K i n e t i c a l l y , t h e three

reactions a r e s i m i l a r ; w h e r e a p p a r e n t differences w e r e f o u n d , t h e y c o u l d b e r e l a t e d to t h e different heats of reaction—about methyl group.

T h e o x i d a t i o n of p s e u d o c u m e n e

50 k c a l . / m o l e

was studied i n

per most

d e t a i l u p to a r e a c t o r size of 25 liters, t o l u e n e a n d xylenes b e i n g s t u d i e d u p to a r e a c t o r s i z e of 1 l i t e r . A

t y p i c a l r e a c t i o n w a s c a r r i e d o u t as f o l l o w s .

T h e reactor

was

c h a r g e d w i t h reactants a n d b r o u g h t to t h e r e q u i r e d t e m p e r a t u r e a n d p r e s sure as q u i c k l y as p o s s i b l e .

W h e n t h e r e a c t i o n b e g a n , t h e pressure f e l l

o w i n g to c o n s u m p t i o n of s u l f u r d i o x i d e , the pressure w a s a l l o w e d to f a l l , say 5 - 1 0 % , t h e pressure d r o p m e a s u r e d a n d t h e n r e s t o r e d b y a d d i n g m o r e sulfur dioxide from a compressor

(Figure 1 ( A ) ) .

I n this w a y a n i n t e -

g r a t e d pressure d r o p vs. t i m e c u r v e c a n b e p l o t t e d ( F i g u r e 1 ( B ) ).


For

5.

SHIPMAN

5 to £

300 275 250225L


59 (A)

[2 5 cc

F

(B)

100

INDUCTION PERIOD

\ \


200

Q-

o ο

300

400

OC ZD

to

£ 500

REACTION TIME

Ou

600,

0-5

_L 1-0

1-5 2-0 TIME t HOURS

2-5

30

Figure 1. (A) Pressure curve; (B) Pressure drop curve for typical SO reaction

3-5

pseudocumene-

B

c o m p a r i s o n , e i t h e r t h e slope o f t h e ( n e a r l y ) l i n e a r p o r t i o n o f t h e c u r v e dP/dt

d i v i d e d b y the t o t a l pressure d r o p Δ Ρ , o r t h e t i m e f o r h a l f t h e

pressure d r o p t o o c c u r , t h e t i m e o f h a l f - c h a n g e f measures o f t h e r e a c t i o n rate.

l / f

, h a v e b e e n u s e d as

I n some e x p e r i m e n t s , p a r t i c u l a r l y i n t h e

smallest reactor, a n i n d u c t i o n p e r i o d w a s sometimes o b s e r v e d b u t this r a r e l y h a p p e n e d i n t h e l a r g e r reactors.

T h e "tail" o n the curve was

t h o u g h t to b e c a u s e d b y the final stages o f the reaction's t a k i n g p l a c e i n a m o r e viscous m e d i u m . Effect of R e a c t i o n V a r i a b l e s u p o n t h e R a t e .

TEMPERATURE.

The

a c t i v a t i o n energies f o r the o x i d a t i o n o f t o l u e n e , xylenes, a n d p s e u d o c u m e n e are a l l w i t h i n the r a n g e 42 db 2 k c a l . / m o l e (see F i g u r e 2, f o r e x a m p l e ) . CONCENTRATION.

T h e o v e r a l l r e a c t i o n rate is a p p r o x i m a t e l y p r o p o r

t i o n a l t o t h e s q u a r e o f t h e h y d r o c a r b o n c o n c e n t r a t i o n (see F i g u r e 3, f o r example). with

i n c r e a s i n g pressure.

S o m e results f o r a fixed w e i g h t o f p s e u d o c u m e n e

are s u m m a r i z e d i n

PRESSURE.

T h e r e a c t i o n rate decreases

Table III. I n these reactions t h e pressure is g e n e r a t e d b y c o m p r e s s i n g s u l f u r d i o x i d e i n t o t h e reactor, a n d a n increase i n pressure c o r r e s p o n d s t o a r e d u c t i o n i n t h e m o l e f r a c t i o n of t h e h y d r o c a r b o n .

It has already been




60

Ι·60

170

±

y

| f t

3

1-80

Figure 2. Arrhenius plots for oxidation of pseudocumene

Figure 3. Effect of pseudocumene concentration on reaction rate


1-90

5.

SHIPMAN


61

Table III Ratio of Reaction Rates Pressure, atm.

Pseudocumene, mole fraction

Reaction rate 7/h/2 Χ 70

200 300 400

0.170 0.157 0.148

2.27 1.51 1.25

2

Observed

Concentration Effect (Calculated)

Net Pressure Effect

1.0 0.85 0.76

1.0 0.79 0.72

1.0 0.67 0.55

stated t h a t a t a g i v e n pressure t h e r e a c t i o n r a t e is p r o p o r t i o n a l to t h e


s q u a r e of t h e h y d r o c a r b o n m o l e f r a c t i o n , a n d a c o r r e c t i o n m u s t b e m a d e f o r this.

I n T a b l e I I I , t h e ratios o f t h e o b s e r v e d r e a c t i o n rates m a y b e

compared

w i t h those e x p e c t e d

from

variation i n concentration.

The

o b s e r v e d decrease i n rate is greater t h a n c o u l d b e a c c o u n t e d f o r b y a concentration

change,

i n d i c a t i n g t h a t there

is a n e t r e t a r d i n g

effect

c a u s e d b y pressure. Experiments have been carried out using

PURITY OF REACTANTS.

c o m m e r c i a l a n d s p e c i a l l y p u r i f i e d samples of e a c h r e a c t a n t , b u t n e i t h e r the r e a c t i o n rate n o r t h e p r o d u c t v a r i e d s i g n i f i c a n t l y w i t h t h e p u r i t i e s o f the reactants. ADDITIVES.

T h e r e a c t i o n w a s n o t affected b y t h e p r e s e n c e o f a f e w

atmospheres ( s a y 5 a t m . i n 300 a t m . ) of o x y g e n o r a i r .

I n the search for

p o s s i b l e catalysts, m a n y c o m p o u n d s w e r e f o u n d to b e w i t h o u t effect, f o r e x a m p l e , free r a d i c a l i n i t i a t o r s a n d i n h i b i t o r s , m e t a l salts (sulfates, s u l fites, sulfides o f a l k a l i a n d t r a n s i t i o n m e t a l s ) , m e t a l oxides, a n d c h l o r i n e compounds. T h e r e a c t i o n is c o m p l e t e l y i n h i b i t e d b y t h e p r e s e n c e of m e t a l l i c s i l v e r or c o p p e r .

F o r e x a m p l e , t h e r e a c t i o n w o u l d n o t p r o c e e d i n a 100 m l .

reactor w h i c h w a s e i t h e r l i n e d w i t h s i l v e r o r h a d a s i l v e r stirrer.

When

s m a l l pieces of s i l v e r w e r e a d d e d to a 1 l i t e r r e a c t o r a n d a r e a c t i o n c a r r i e d o u t , there w a s i n i t i a l l y a v e r y l o n g i n d u c t i o n p e r i o d after w h i c h the r e a c t i o n p r o c e e d e d at t h e rate a p p r o p r i a t e to t h e c o n d i t i o n s S i l v e r sulfide w a s r e c o v e r e d behave similarly.

from the product.

used.

C o p p e r w a s f o u n d to

I t appears t h a t these metals i n h i b i t t h e r e a c t i o n , b u t

the m e t a l sulfides d o not. T h e r e a c t i o n is c a t a l y z e d b y c o m p o u n d s o f b r o m i n e a n d i o d i n e , a s m a l l catalyst c o n c e n t r a t i o n g i v i n g a l a r g e increase i n rate ( F i g u r e 4 ) . C a t a l y s i s b y 1% H B r ( c a l c u l a t e d c n t h e h y d r o c a r b o n )

has b e e n s h o w n t o

l o w e r t h e a c t i v a t i o n e n e r g y f r o m 4 2 k c a l . / m o l e to a b o u t 2 1 k c a l . / m o l e , a n d t h e increase i n rate is p r o p o r t i o n a l to l o g [ H B r ] ( F i g u r e 5 ) .

Both

o r g a n i c a n d i n o r g a n i c c o m p o u n d s of b r o m i n e a n d i o d i n e h a v e b e e n f o u n d to c a t a l y z e t h e r e a c t i o n , t h e effect of b r o m i n e c o m p o u n d s b e i n g g e n e r a l l y greater t h a n t h a t o f i o d i n e .

I t is w e l l k n o w n t h a t the b r o m i n e a t o m is a b l e

to abstract a h y d r o g e n a t o m f r o m a m e t h y l g r o u p s u b s t i t u t e d o n t o a n aromatic ring:

R

C

H

8

+

B

r

>f]RCH - + HBr ?



62


oc

Ol 270

I

I

I

I

I

L_

280

290 300 310 320 330 TEMPERATURE °C Figure 4. Effect of additives upon reaction rate (80 ml.)

S u c h a mechanism c o u l d account for the l o w e r i n g of the activation energy of t h e o v e r a l l o x i d a t i o n r e a c t i o n b y catalysis of d i e i n i t i a l step.

Chlorine

c o m p o u n d s a r e ineffective as catalysts b e c a u s e of t h e h i g h r e d o x p o t e n t i a l r e q u i r e d to f o r m c h l o r i n e atoms. SOLVENTS.

I t w a s t h o u g h t that i n t r o d u c i n g a solvent t o a c t as a n i n e r t

diluent m i g h t h e l p the reaction b y r e d u c i n g the viscosity i n the latter stages a n d h e n c e , a i d m i x i n g . zene, acetic a c i d , a n d w a t e r .

Experiments were carried out w i t h benT h e q u a l i t y of the product was not signifi-

c a n t l y i m p r o v e d , a n d thus solvents w e r e n o t g e n e r a l l y u s e d .

W i t h water

a n d a c e t i c a c i d , v e r y h e a v y c o r r o s i o n of t h e stainless steel r e a c t o r took place.

I n t h e absence of a d d i t i v e s , t h e r e a c t i o n m i x t u r e attacks stainless

steel ( 1 8 . 1 0 . M o . T i . )

a t a r a t e o f a b o u t 0.1 i n c h p e r y e a r u n d e r n o r m a l Table Volume

Radius

of Reactor V, liters

of Reactor, cm.

Surface Area A Volume cm.-

0.08

1.3

1.55

1.20

4.5

0.56

9.5

0.72»

1

Ratio of A/V 2.8 1.3

25.0

W i t h

cooling coil.

Uncooled

(estimated).


5.

SHIPMAN

63


reaction conditions.

A d d i t i o n of hydrogen

bromide

considerably i n

creases c o r r o s i o n . REACTOR SIZE.

T h i s t o p i c is r e l e v a n t t o t h e g e n e r a l p r o b l e m of s c a l

i n g u p t h e size of t h e reactor.

T a b l e I V s u m m a r i z e s s o m e results.

T h e t h r e e reactors w e r e r o u g h l y s i m i l a r g e o m e t r i c a l l y ; t h e o n e of 25-liter c a p a c i t y w a s fitted w i t h a n i n t e r n a l c o o l i n g c o i l . agitation was comparable i n each.

T h e degree of

A l l h a d r e c i p r o c a t i n g stirrers w h i c h

w e r e s l o w a n d r a t h e r inefficient, a n d i t is a s s u m e d t h a t t h e effect o f t h e stirrer o n the t e m p e r a t u r e d i s t r i b u t i o n i n e a c h vessel is s i m i l a r .


T h e m a x i m u m o p e r a t i n g t e m p e r a t u r e is t h a t at w h i c h i t w o u l d just b e p o s s i b l e t o c a r r y o u t a r e a c t i o n at a c o n t r o l l e d r a t e f o r a m a x i m u m c h a r g e

TEMPERATURE

275 ° C

PRESSURE

300 ATM.

TWO DIFFERENT PSEUDOCUMENE CONCENTRATIONS USED.

Θ

ft

Ν >ο

'

Θ

ι

1

I

20

-10

LOG 10 (% HBr)

Figure 5. Effect of HBr catalyst concentration

on reaction rate

IV Maximum Operating Temperature, °C. 310

Time of HalfChange 285° C. min. 260

280

113

290» 250 »

122

Ratio of Reaction Rates 2.3 1.1

Time of HalfChange 285° C, min.

Ratio of Reaction Rates

115 70 91


1.7 1.3

64


of p s e u d o c u m e n e .

T h e t e m p e r a t u r e m e a s u r e d corresponds a p p r o x i m a t e l y

to t h a t of t h e reactor w a l l , T . T h e r e is a c l e a r r e l a t i o n s h i p b e t w e e n this 0

t e m p e r a t u r e a n d t h e reactor r a d i u s as s h o w n i n F i g u r e 6. mum

A t the m a x i

o p e r a t i n g t e m p e r a t u r e , t h e t e m p e r a t u r e d i s t r i b u t i o n across t h e r e

actor is just o n t h e p o i n t of b e c o m i n g u n s t a b l e b e c a u s e t h e rate o f heat g e n e r a t i o n p e r u n i t v o l u m e b y t h e r e a c t i o n is greater t h a n t h e rate at 320

- Λ ·


310

Η—100 ml VESSEL

300 ο ο

290 280

5 — 1 LITER VESSEL

270 260

y/

250

25 U T E R I " VESSEL

240 230 1

1

1

1

1

I

I

I

I

I

ι

10 9 8 7 6 5 4 3 2 1 0 1 RADIUS OF VESSEL

ι

ι

ι

ι

ι

ι

ι

2 3 4 5 6 7 (cm)

ι

8 9 10

Figure 6. Temperature profile for 25-liter reactor with profiles for the smaller reactors indicated at maximum operating temperature w h i c h h e a t c a n b e lost b y c o n d u c t i o n to the vessel w a l l .

T h e results g i v e n

i n T a b l e I V s h o w a r e l a t i o n s h i p b e t w e e n t h e r e a c t i o n rate a n d t h e surface a r e a t o v o l u m e r a t i o f o r t h e vessel.

T h e h e a t o f r e a c t i o n causes t h e

t e m p e r a t u r e w i t h i n t h e r e a c t i o n to b e greater t h a n at t h e r e a c t o r w a l l .

A

t e m p e r a t u r e d i s t r i b u t i o n is e s t a b l i s h e d across t h e reactor s i m i l a r to that s h o w n d i a g r a m m a t i c a l l y i n F i g u r e 7.

T h e o b s e r v e d r e a c t i o n rate i n e a c h

case w i l l c o r r e s p o n d to a t e m p e r a t u r e w h i c h is a n average o f T

l9

m u m temperature a n d T the measured temperature. 0

the maxi

T h e rates w i l l f o r m

a series s u c h t h a t : rate i n 100 m l < rate i n 1 l i t e r < rate 25 liters u n c o o l e d T h e effect o f c o o l i n g t h e 2 5 - l i t e r reactor w i t h a n i n t e r n a l c o i l is effectively to r e d u c e T as s h o w n b y t h e b r o k e n c u r v e i n F i g u r e 7. 0

T h i s is w h y t h e

m e a s u r e d rates i n the 1-liter a n d c o o l e d 25-liter reactors are s i m i l a r . I n this context, i t is of interest to c o m p a r e t h e m a x i m u m o p e r a t i n g t e m p e r a t u r e s of s i m i l a r charges of t o l u e n e , p - x y l e n e , a n d p s e u d o c u m e n e



5.

SHIPMAN


65

Figure 7. Increase in reactor internal temperature with increase in reactor radius; T constant 0

i n the 1-liter reactors.

T h e y w e r e 335°, 310°, a n d 280° C , r e s p e c t i v e l y , i n

the o r d e r e x p e c t e d f r o m t h e i r i n c r e a s i n g heats o f r e a c t i o n . M u c h of t h e b e h a v i o r o f these r e a c t i o n systems m a y b e e x p l a i n e d q u a l i t a t i v e l y o n t h e basis of t h e F r a n k - K a m e n e t s k i i s t a t i o n a r y t h e o r y of t h e r m a l e x p l o s i o n ( 6 ) , p a r t i c u l a r l y t h e effects of size o f reactor, s t i r r i n g , pressure, solvents, etc. u p o n r e a c t i o n rate. PRODUCT YrELD.

T h e y i e l d from the sulfur dioxide oxidation reaction

seems p r i n c i p a l l y to d e p e n d u p o n t h e average r e a c t i o n rate p e r u n i t v o l ume.

F i g u r e 8 shows t h e v a r i a t i o n of y i e l d o f t r i m e l l i t i c a c i d w i t h r e a c -

t i o n rate f o r p s e u d o c u m e n e i n a 1-liter reactor.

F r o m this i t c a n b e seen

that at rates b e l o w 10 X 10~ g m . m o l e s . / m l . t h e y i e l d m a y e x c e e d 8 0 % 6

w h i l e at a rate h i g h e r t h a n 30 X 10~ g m . m o l e s . / m l . / m i n . , t h e y i e l d m a y 6

f a l l to 5 0 % .

M a x i m u m y i e l d s i n excess of 9 0 % h a v e b e e n o b t a i n e d f o r t h e

o x i d a t i o n of t o l u e n e a n d x y l e n e . T h e y i e l d m a y b e l i m i t e d b y the formation of intermediate p o u n d s , tars, a n d d e g r a d a t i o n to c a r b o n .

com-

E v e n i n t h e presence o f excess

s u l f u r d i o x i d e , w h e n t h e r e a c t i o n is a p p a r e n t l y t a k e n t o c o m p l e t i o n , a s m a l l p e r c e n t o f i n t e r m e d i a t e acids m a y b e present i n t h e p r o d u c t .

If the

r e a c t i o n is s t o p p e d b e f o r e c o m p l e t i o n , t h e n s m a l l q u a n t i t i e s o f a l d e h y d e s m a y also b e f o u n d .

T h e major by-product from p-xylene was p-toluic a c i d

and from pseudocumene was 4-methylphthalic acid.




66

30120'

0

aKJ

1

1

5

10

REACTION RATE

1

"

1

15

20

9m. molt ml" min. χ ιο· 1

25

30

-1

Figure 8. Variation of crude yield with reaction rate in 1-liter reactor D e g r a d a t i o n w a s n o t a serious p r o b l e m i n the 1-liter reactor, b u t i t w a s a m o r e serious p r o b l e m f r o m p s e u d o c u m e n e l i m i t i n g the y i e l d to a b o u t 7 5 % .

i n the 2 5 - l i t e r r e a c t o r

F o r m a t i o n of c a r b o n i n the p r o d u c t w a s

a c c o m p a n i e d b y the f o r m a t i o n o f c a r b o n d i o x i d e i n the gas phase.

Small

a m o u n t s o f t a r r y m a t e r i a l s w e r e sometimes f o u n d i n the p r o d u c t s .

From

t h e i r i n f r a r e d s p e c t r a a n d analyses, i t seems l i k e l y t h a t t h e y w e r e p o l y m e r i c , h a v i n g b e e n f o r m e d b y o x i d a t i v e c o u p l i n g reactions as d e s c r i b e d earlier.

Discussion T h e p r i m a r y objective of this w o r k w a s to d e t e r m i n e t h e o p t i m u m c o n d i t i o n s f o r those reactions w h i c h m i g h t b e c o m m e r c i a l l y v i a b l e for m a n u f a c t u r i n g s i m p l e b e n z e n e c a r b o x y l i c acids.

N o serious a t t e m p t has

b e e n m a d e to e l u c i d a t e t h e m e c h a n i s m of the r e a c t i o n .

Because initially

the reactants are o n l y s u l f u r d i o x i d e a n d the h y d r o c a r b o n , i t is u n l i k e l y that t h e r e a c t i o n m e c h a n i s m is i d e n t i c a l to that for o x i d a t i o n b y a l k a l i polysulfides i n the presence of a l a r g e excess of w a t e r w h i c h w a s o u t l i n e d i n the i n t r o d u c t i o n . T o s u m m a r i z e the e x p e r i m e n t a l observations: 1. I n the presence of excess s u l f u r d i o x i d e the o x i d a t i o n m a y p r o c e e d s t o i c h i o m e t r i c a l l y a c c o r d i n g to E q u a t i o n 1.


s.

SHIPMAN

Organic Compounds by Sulfur Dioxide R C H

+

8

S0

lVi

— R ·G O O H

2

+

IV2S +

H

2

0

67 (1)

2. I f t h e r e a c t i o n is s t o p p e d b e f o r e c o m p l e t i o n , a l d e h y d e s a n d h y d r o g e n sulfide are f o u n d i n significant q u a n t i t i e s suggesting that t h e r e a c t i o n p r o c e e d s v i a several steps s u c h a s : R C H

+

3

S0

R-GHO R C H

+

3

H S


2

R C H O

2

VîSOt

+

S0

+

V2SO2

+

H

0 +

R ·G O O H

— R ·G O O H

2

2

+

+

IV2S +

H

S

(2)

S

(3)

H S

(4)

0

(5)

2

2

T h e rate of o x i d a t i o n of a n a l d e h y d e — R e a c t i o n 3—has b e e n s h o w n to b e r a p i d c o m p a r e d w i t h t h e o v e r a l l R e a c t i o n 1, suggesting t h a t t h e o x i d a t i o n of m e t h y l to a l d e h y d e is t h e r a t e - c o n t r o l l i n g step. R e a c t i o n 5 is k n o w n to b e fast. 3. I n t h e presence of excess h y d r o c a r b o n , o x i d a t i v e c o u p l i n g r e a c tions m a y occur. 2RGH

+

3

S0

2

—> R G H 2 G H 2 R or R G H 2 S G H 2 R

(6)

4. T h e r e a c t i o n appears to b e h o m o g e n e o u s a n d n o t affected b y surface catalysis. 5. T h e r e a c t i o n rate is p r o p o r t i o n a l to t h e s q u a r e o f t h e h y d r o c a r bon concentration. 6. T h e r e a c t i o n rate decreases w i t h i n c r e a s i n g pressure. 7. T h e r e a c t i o n is c a t a l y z e d b y b r o m i n e , i o d i n e , a n d c o m p o u n d s of these elements. 8. T h e r e a c t i o n is i n h i b i t e d b y s i l v e r a n d c o p p e r metals. 9. T h e r e a c t i o n is n e i t h e r c a t a l y z e d b y s t a n d a r d f r e e - r a d i c a l i n i t i ators n o r i n h i b i t e d b y s t a n d a r d free r a d i c a l i n h i b i t o r s ( t h o u g h i t is p o s s i b l e that t h e catalysts a n d i n h i b i t o r s u s e d m i g h t h a v e b e e n i n a c t i v a t e d b y the r e a c t i o n c o n d i t i o n s u s e d ) . 10. A t t e m p t s to o x i d i z e olefins s u c h as e t h y l e n e , p r o p y l e n e , butènes, etc. w i t h s u l f u r d i o x i d e so f a r h a v e b e e n unsuccessful. A n y r e a c t i o n o t h e r t h a n a n a d d i t i o n r e a c t i o n , p r o c e e d e d to c a r b o n i z a t i o n . N o m o r e t h a n traces of o x i d a t i o n p r o d u c t s h a v e b e e n f o u n d . I t is suggested that one possible r e a c t i o n m e c h a n i s m f o r this t y p e o f r e a c t i o n is a free r a d i c a l c h a i n r e a c t i o n b a s e d o n t h e f o l l o w i n g s c h e m e w h i c h is analogous to t h e a c c e p t e d m e c h a n i s m f o r the o x i d a t i o n of h y d r o carbon b y oxygen Initiation

(2,3) RGH

(RGH

Propagation

— RCH -

3

2

3

+

χ

+

H -

RCH -

(RGH3 +

B r - -> R G H S0

+

RCH S0 -

+

or R C H S 0 -

+

2

2

2

2

+

2

RCH 2

(7)

2

Hx)

(8)

+ HBr)

2

(9)

RGH S0 2

RGH

RCH S0 H

3

RGH

(10)

2

2

3

2

+

RGH S0 GH R 2

2

2

RGH 2

+

H

Termination


(11) (12) (13)


68 Subsequent reactions

2

RCH S0 CH R -> 2 R G H O + H S

(15)

RGHO + V2SO1 — RCOOH + S

(16)

2


(14)

RCH2SO2H — RCOOH + H S

2

2

2

It is known that hydrogen abstraction reactions are catalyzed b y bromine (Reaction 9 ) . Indeed, this forms the basis of the bromidecatalyzed air oxidation process ( 7 ) , and the benzyl radical so formed is stable ( 1 8 ) . The probability of forming benzyl radicals by thermal dis sociation (Reaction 7) at around 250-300° C . is very low, and it is more likely that i n the absence of a halogen, a sulfur species designated b y χ i n Equation 8 is responsible. Absorption of χ b y copper or silver could cause the inhibition of the reaction. It is unlikely that elemental sulfur is the initiator because the addition of sulfur to the reactants d i d not affect the reaction rate. It may be noted that the initiation step may involve a net increase i n volume and may be retarded by an increase i n pressure. Sulfur dioxide w i l l copolymerize with olefins i n the presence of free radical initiators to form 1:1 copolymers (16). F o r example, ethylene polysulfone ( — C H — C H — S 0 — ) may be prepared under conditions similar to those used for aromatic oxidations. A t higher temperatures, the polysulfones decompose to the olefins and sulfur dioxide. The reaction between styrene and sulfur dioxide proceeds differently ( I ) . T h e primary product is a 1:1 complex C6H5CH2CH2SO2" which then copolymerizes with another styrene molecule and not sulfur dioxide. T h e re sulting polysulfone contains a 2:1 molecular ratio of styrene:sulfur d i oxide. O n the basis of this, the addition of a benzyl radical to sulfur dioxide and the subsequent reaction with the hydrocarbon is postulated (Reactions 10,11,12). The subsequent rearrangement of the sulfinic acid or diaryl sulfone may occur to give oxidation products w i t h a net increase i n v o l u m e Reactions 14 and 15—reactions which again might be retarded b y an i n crease of pressure. O n the basis of experimental evidence so far available, it is not possible to propose such a reaction mechanism as anything more than a tentative hypothesis. In a full analysis, the contributions to the reaction of all the species present at various stages such as sulfur, hydrogen sulfide, organic intermediates, and less stable compounds would a l l have to be taken into account. 2

2

2

n

Literature Cited (1) Barb, W . G., Proc. Roy. Soc. A.212, 66, 177 (1953). (2) Bateman, L . , Quart. Rev. 8, 147 (1954). (3) Bolland, J. L . , Quart. Rev. 3, 1 (1949). (4) Carmack, M., Spielman, Μ. Α., "Organic Reactions," edited by R. Adams. Vol. III, John Wiley & Sons Inc., New York, 1946.



5.

SHIPMAN


69

(5) Danford, J. D., Bender, M. J., Ind. Eng. Chem. 46, 1701 (1954). (6) Frank-Kamenetskii, D . Α., "Diffusion and Heat Transfer in Chemical Kinetics," Princeton University Press, Princeton, 1955. (7) Mid-Century Corp., B.P. 807,091 (1959). (8) Naylor, Μ. Α., U.S.P. 2,610,980 (1952); U.S.P. 2,640,077 (1953). (9) Pryor, W . Α., "Mechanism of Sulphur Reactions," McGraw-Hill, New York, 1962. (10) Pryor, W . Α., J. Am. Chem. Soc. 80, 6481 (1958). (11) Pryor, W . Α., Ferstandig, L. L . , J. Am. Chem. Soc. 82, 283 (1960). (12) Shipman, A . J., B.P. 926,019. (13) Shipman, A . J., B.P. 952,524. (14) Shipman, A . J., B.P. 956,624. (15) Shipman, A . J., B.P. 898,630. (16) Snow, R. D., Frey, F . E., Ind. Eng. Chem. 30, 176 (1938). (17) Strickland, T. H . , Bell, Α., Chem. & Eng. News 1959, Sept. 21, p. 36; Ind. Eng. Chem. 53, 7 (1961); U.S.P. 2,821,552 (1958); U.S.P. 2,982,879 (1960). (18) Szwarc, M., J. Chem. Phys. 16, 128 (1948); Chem. Rev. 47, 75 (1950). (19) Toland, W . G., U.S.P. 2,531,172 (1950); U.S.P. 2,587,666 (1952). (20) Toland, W . G., U.S.P. 2,900,412 (1959). (21) Toland, W . G., J. Am. Chem. Soc. 82, 1911 (1960). (22) Toland, W . G., U.S.P. 2,903,480 (1959). (23) Toland, W . G., Hagmann, D . L., Wilkes, J. B., Brutschy, F . J., J. Am. Chem. Soc. 80, 5423 (1958). (24) Toland, W . G., U.S.P. 2,722,548 (1955); U.S.P. 2,734,079; U.S.P. 2,762,839. (25) Toland, W . G., J. Org. Chem. 26, 2929 (1961). (26) Willgerodt, C., Ber. 20, 2467 (1887); 21, 534 (1888). RECEIVED October 15, 1964.


Oxidation of Organic Compounds by Sulfur Dioxide under Pressure

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