1
Preparative and Mechanistic Aspects o f
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Electrophilic Nitration GEORGE A. OLAH Case Western Reserve University, Cleveland, Ohio 44106
Electrophilic nitration of aromatics is the most studied, and supposedly best understood of organic reactions (1). Culminating their work, Ingold, Hughes, and their associates in 1950 published simultaneously a series of papers (2) which rightfully ever since is considered the most fundamental study in our understanding of electrophilic nitration reactions. Thus, not unexpectedly, for years following these publications the general view prevailed, that all the significant work in nitration was done, and little additional knowledge was to be gained. Chemists' interest consequently rapidly faded in the field, and by the end of the 1950's from a scientific point of view the study of nitration looked like as one of the most unattractive and inactive areas. In a way, no greater compliment of the achievements of Ingold and his group could have been made than the generally accepted view, that their work indeed, completed what there was worthwhile to study in nitration. Any further research, indeed, seemed to be directed only to fill in the few still uncompleted details for reviews in text books and scientific monographs. In 1975, a quarter of a century after the appearance of the Ingold papers, the field of electrophilic nitration, as testified by this broad scope symposium, is again a field of substantial interest and activity. This is the case both as far as the more practical, preparative and technical aspects of nitration are concerned, and equally so concerning the mechanistic and basic aspects of the field. It is, of course, an old cliché to say that each generation of chemists feels that their work is the last word in their field, and nothing new ever will be added. I for one, however, strongly feel that chemistry is certainly not a closed-end chapter, and we are just starting to realize and explore in many areas how limited our knowledge s t i l l is. The renewed interest in nitration clearly testifies to this point. With my group, we have had the good fortune to be able to contribute something in the last two decases to rekindle interest in nitration reactions. It is thus a great pleasure and privilege to be 1
Albright and Hanson; Industrial and Laboratory Nitrations ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
INDUSTRIAL AND LABORATORY NITRATIONS
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2
able a t t h i s Symposium, t o b r i e f l y summarize some of our own r e search and i t s more recent aspects, as w e l l as t o t r y at the end o f my l e c t u r e t o i n d i c a t e some of the areas where I t h i n k c o n t i n ued work would promise s i g n i f i c a n t new advances and r e s u l t s . It c e r t a i n l y would be most f a s c i n a t i n g to foresee what a future symposium on n i t r a t i o n would present i n another quarter o f a century, h e r a l d i n g i n the new millennium. But, many of us may not be around any more f o r the occasion. However, maybe some of the younger generation w i l l look up at the time the p u b l i s h e d volume o f t h i s Meeting, and g l a n c i n g over what we were i n t e r e s t e d i n , studying and p r o j e c t i n g , compare i t with the o b v i o u s l y v a s t l y more fundamental knowledge acquired by the year 2000. For the record then l e t ' s t r y to give you a b r i e f account o f what I have to present i n 1975 with some comparison of where our knowledge stood i n 1950. I hope t h a t the symposium of 2000 w i l l look back a t our present l i m i t e d e f f o r t s with some degree o f acknowledgment and conclude t h a t our present generation c o n t r i b u t e d to the continued progress of a c l e a r l y f a s c i n a t i n g and very p r a c t i c a l field. P r e p a r a t i v e Aspects N i t r a t i o n with Nitronium S a l t s (Olah-Kuhn N i t r a t i o n ) . The Ingold group (2) c l a r i f i e d the nature of the s a l t obtained by Hantsch (3) from n i t r i c and p e r c h l o r i c a c i d s as a mixture of n i t ronium p e r c h l o r a t e , and hydronium p e r c h l o r a t e . They subsequently prepared, and s t u d i e d (by Raman spectroscopy) pure n i t r o n i u m perc h l o r a t e (2). T h i s was a s i g n i f i c a n t step, because even i f i n a separate system, but i t d i r e c t l y proved the e x i s t e n c e of the n i t r o n i u m ion suggested by t h e i r k i n e t i c s t u d i e s . Nitronium perc h l o r a t e , however, has r e c e i v e d l i t t l e subsequent i n t e r e s t . I once was t o l d by the l a t e P r o f e s s o r Ingold that he had a s m a l l sample o f the compound i n a s e a l e d v i a l on h i s desk which one morning s h a t t e r e d . T h i s was very much the end of i n t e r e s t at U n i v e r s i t y College i n what was considered an unstable, dangerous compound. My own research work on e l e c t r o p h i l i c n i t r a t i o n , which s t a r ted i n Hungary i n the e a r l y 1950's, showed, however, that any d i f f i c u l t y connected with n i t r o n i u m p e r c h l o r a t e was not at a l l the f a u l t of the c a t i o n , but of the r a t h e r u n s u i t a b l e p e r c h l o r a t e anion. As i t i s w e l l recognized now, p e r c h l o r a t e s a l t s g e n e r a l l y are r a t h e r unstable and s u s c e p t i b l e t o e x p l o s i v e behavior because of t h e i r a b i l i t y to form c o v a l e n t e s t e r s . In the case of n i t r o n ium p e r c h l o r a t e the i s o l a t e d c r y s t a l l i n e n i t r o n i u m s a l t can form, i n e q u i l i b r i u m , s m a l l amounts o f the covalent n i t r y l p e r c h l o r a t e e s t e r which then could be s u s c e p t i b l e t o e x p l o s i v e decomposition. +
N0 OCL0 ^=i NO C10 " 2
3
2
4
To prepare n i t r o n i u m s a l t s as s t a b l e n i t r a t i n g reagents thus
Albright and Hanson; Industrial and Laboratory Nitrations ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
1.
OLAH
Electrophilic
3
Nitrations
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n e c e s s i t a t e d t h e u s e o f c o u n t e r i o n s , w h i c h c o u l d g i v e no u n s t a b l e e s t e r s . We r e p o r t e d i n 1956 w i t h S t e v e Kuhn t h e s i m p l e p r e p a r a t i o n o f n i t r o n i u m t e t r a f l u o r o b o r a t e , and i t s a p p l i c a t i o n as a n i t r a t i n g agent N i t r o n i u m t e t r a f l u o r o b o r a t e i s most c o n v e n i e n t l y p r e p a r e d by a d d i n g a n h y d r o u s HF t o n i t r i c a c i d i n a s o l v e n t s u c h a s n i t r o methane, methylene c h l o r i d e e t c , and t h e n s a t u r a t i n g t h e s o l u t i o n w i t h BF^ HN0
3
+ HF + 2 B F
3
= N O ^ B F ^ " + BF^ · H 0 2
A n e a r l y q u a n t i t i v e y i e l d o f t h e s t a b l e n i t r o n i u m s a l t can be obtained. An u n d e r g r a d u a t e i n a n a f t e r n o o n ' s w o r k , c a n p r e p a r e u p t o a pound o f t h e s a l t w h i c h i s now a l s o c o m m e r c i a l l y a v a i l a b l e . To my b e s t k n o w l e d g e i n i t s t w o d e c a d e s o f u s e , t h e r e was n e v e r any d i f f i c u l t y c o n n e c t e d w i t h i t s s t a b i l i t y . Nitronium t e t r a f l u o r o b o r a t e , and s u b s e q u e n t l y o t h e r c o m p l e x f l u o r i d e s a l t s * s u c h as t h e h e x a f l u o r o p h o s p h a t e , h a v e g a i n e d w i d e s p r e a d a p p l i c a t i o n a s most r e a c t i v e n i t r a t i n g a g e n t s . N i t r o n i u m s a l t s a r e c o l o r l e s s , c r y s t a l l i n e , v e r y s t a b l e comp o u n d s ; N0 BF1 "" decomposes a t a t m o s p h e r i c p r e s s u r e o n l y a b o v e 170°, w i t h o u t s u b l i m i n g , i n t o i t s c o m p o n e n t s : N 0 F + BF3. The h e x a f l u o r o a n t i m o n a t e s a l t s a r e e v e n more s t a b l e . T h e h i g h e r t h e r m a l s t a b i l i t y may b e p a r t i a l l y a l s o a c o n s e q u e n c e o f t h e higher b o i l i n g p o i n t s o f t h e corresponding Lewis a c i d f l u o r i d e s compared t o b o r o n t r i f l u o r i d e . N i t r o n i u m s a l t s c a n b e s t o r e d a t room t e m p e r a t u r e i n d e f i n i t e l y w i t h o u t d e c o m p o s i t i o n . R e f r i g e r a t i o n i s u n n e c e s s a r y and n o other s p e c i a l precautions a r e required. A l l nitronium salts a r e , h o w e v e r , v e r y h y g r o s c o p i c a n d must b e s t o r e d a n d h a n d l e d w i t h precautions t o avoid moisture. N i t r o n i u m t e t r a f l u o r o b o r a t e and r e l a t e d n i t r o n i u m s a l t s a r e extremely a c t i v e n i t r a t i n g agents f o r aromatics +
2
+
2
ArH
+
+ N0 BF^" 2
* ArNO^ + HF + B F
3
The n i t r a t i o n s a r e c a r r i e d o u t u n d e r a n h y d r o u s c o n d i t i o n s . T h i s i s o f s p e c i a l a d v a n t a g e i n d e a l i n g w i t h compounds w h i c h u n d e r u s u a l s t r o n g a c i d i c n i t r a t i o n c o n d i t i o n s may u n d e r g o h y d r o l y s i s o r o x i d a t i o n . A r y l n i t r i l e s , f o r example, a r e e a s i l y hydrol y z e d u n d e r n i t r a t i o n c o n d i t i o n s a n d no d i r e c t d i n i t r a t i o n , r e q u i r i n g f o r c e f u l c o n d i t i o n s , was p r e v i o u s l y p o s s i b l e . The n i t r o n ium f l u o r o b o r a t e m e t h o d e n a b l e u s t o c a r r y o u t mono a n d d i n i t r a tion o f a r y l n i t r i l e s w i t h h i g h y i e l d s w i t h o u t any h y d r o l y s i s o f t h e -CN g r o u p . Results of preparative n i t r a t i o n s of arenes, haloarenes, n i t r o a r e n e s , a r y l c a r b o x y l i c a c i d e s t e r s and h a l i d e s and a r y l n i t r i l e s a r e s u m m a r i z e d i n T a b l e s 1-V. G e n e r a l l y a t h r e e f o l d e x c e s s o f t h e a r o m a t i c was n i t r a t e d a t 20° i f n o t o t h e r w i s e s t a t e d . Yields relate t o isolated nitro
Albright and Hanson; Industrial and Laboratory Nitrations ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
4
INDUSTRIAL AND LABORATORY NITRATIONS
TABLE I
NITRATION OF ARENES WITH N 0
Product
Substrate
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2
Benzene Toluene o-Xylcnc m-Xylene /(-Xylene Mesitylene Ethylbenzene n-Propylbenzene i-Propylbenzene ti-Butylbcnzene j*c-Butylbenzene f-Butylbenzene Naphthalene Anthracene
Nitrobenzene Nitrotoluenes Nitroxylenes Nitroxylenes Ν itro-/(-xylene Nitromesitylene Nitroethylbenzenes Nitro-n-propylbenzencs Nitro-i-propylbenzenes Nitro-n-butylbcnzenes Nitro-j«c-butylbenzenes Nitro f-butylbenzcnes Nitronaphthalenes 9-Nitroanthracene
+
BF«
_
Yield of Reac isolated tion mononitro time, product, min. % 93 10 95 10 91 10 10 90 93 10 89 10 10 93 10 91 10 93 90 10 92 10 88 10 79 25 25 85
• All nitrations were carried out in tetramethylene sulfone solutions at temperatures between 0 and 4-5°. T A B L E II NITRATION OF HALOARENES AND HALOARALKANES WITH N 0 Substrate
Fluorobenzene Chlorobenzene Bromobenzene Iodobenzene Benzotri fluoride />-Fluorobenzotrifluoride o- Dichloroben zen e tw-Dichlorobenzene £-Dichlorobenzene 0-Difluorobenzene w-Difluorobenzene />-Difluorobenzene a-Fluoronaphthalene /3-Fluoronaphthalene Benzyl chloride /S-Fluoroethylbenzene /5-Chloroethylben zene /3-Bromoethylbenzene
2
+
BF ~ 4
Product
Reaction temp., ° C .
Reaction time, min.
Yield of mononitro prod., %
0,/>-Fluoronitrobenzenes o,/>-Chloronitrobenzenes 0,/>-Brornonitrobenzenes 0,/>-Iodonitroben zen es m-Nitrobenzotrifluoride 3-Nitro-4-fluoro-benzotrifluoride Nitro-o-dichlorobenzenes Nitro-w-dichlorobenzenes Nitro-£-dichlorobenzene Nitro-o-difluorobenzenes Nitro-wi-difluorobenzenes Nitro-/>-difluorobenzene Nitro-α-fluoronaphthalen es Nitro-/3-fluoronaphthalenes Nitrobenzyl chlorides Nitro-0-fluoroethylbenzenes Niti o-/S-chloroethy lben zen es Nitro-/S-bromoethylbenzenes
5 10 10 10 30 50 50 50 50 50 50 50 30 30 0 0 10 10
10 10 10 10 20 20 20 20 20 20 20 20 20 20 10 10 15 15
90 92 87 90 20 85 70 74 80 82 79 85 75 79 52 69 82 78
T A B L E III NITRATION OF NITROARENES AND NITROHALOARENES WITH N 0 B F « • Reaction Temp., ° C . Substrate Product 2
Nitrobenzene a-Nitronaphthalene £-Fluoron itroben zen e o-Fluoronitrobenzene 2,4-Dinitrofiuorobenzene p- Ni troch loroben zen e 0-Nitrochlorobenzene 2,4- Din i trochloroben zene
m-Dinitrobenzene Dinitronaphthalenes 2,4-Dinitrofluorobenzene 2,4-Dinitrofluorobenzene Picryl fluoride 2,4-Dinitrochlorobenzene 2,4- Dinitrochlorobenzene Picryl chloride
25 25 30 30 120 30 30 100
+
a
Time
»
20 min. 20 min. 30 min. 30 min. 12 hr. 30 hr. 20 hr. 10 hr.
Yield of nitro prod., %
81 85 78 84 40 75 77 80
t e t r a m e t h y l e n e sulfone -was used as the solvent i n a l l cases except f o r 2 , U - d i n i t r o f l u o r o - and 2,U-dinitrochlorobenzene when 100$ HgSO^ was used.
Albright and Hanson; Industrial and Laboratory Nitrations ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
1.
OLAH
Electrophilic
5
Nitrations
TABLE I V NITRATION OF ARYLCARBOXYLIC ACID ESTERS AND HALIDES WITH N 0
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Substrate
+
BF 4
E
> Reaction » Temp., C . Time, min.
Product
Methyl benzoate Ethyl benzoate Propyl benzoate w-Nitroethyl benzoate Benzoyl fluoride Benzoyl chloride
2
e
w-Nitromethyl benzoate fw-Nitromethyl benzoate «ι-Nitropropyl benzoate 3,5-Dinitroethyl benzoate w-Nitrobenzoyl fluoride m-Nitrobenzoyl chloride 6
30 30 30 85-90 50 50
20 20 20 120 30 30
Yield of mononitro prod., %
88 79 82 60 69 70
A l l n i t r a t i o n s were c a r r i e d o u t i n t e t r a m e t h y l e n e s u l f o n e s o l utions. ^Halogen exchange t o a c i d f l u o r i d e t a k e s p l a c e w i t h b y p r o d u c t HF. TABLE V NITRATION OF ARYL AND A R A L K Y L NITRILES WITH Substrate
Product
Benzonitrile
