Industrial and Laboratory Nitrations - ACS Publications

3 Selectivity in Aromatic Nitration *

R O B E R T G. COOMBES, J O H N G. G O L D I N G , L E S L I E W. R U S S E L L and G E O F F R E Y D. T O B I N

Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0022.ch003

Department of Chemistry, The City University, London E C 1 V 4PB, England

Some recent observations on the intramolecular and intermolecular selectivity of some nitrating agents are presented in summary form in this paper. Attention will be concentrated on nitration by nitric acid in aqueous sulphuric acid, and by way of introduction reference will be made both to our own earlier studies and also to those of other workers in this field. It will become apparent that a situation of increasing complexity is being gradually revealed. It is well established (1,2,3,4,5) that there exists a limiting rate of nitration of aromatic compounds which occurs at about the same threshold of aromatic reactivity in several nitration media. This limit was first observed (1) for nitration by nitric acid in aqueous sulphuric acid and in this medium the observed limit is in good agreement with a value calculated from diffusion theory for the rate of diffusion together or encounter of nitronium ions and substrate molecules. Only in aqueous sulphuric acid is i t possible to make the estimate of nitronium ion concentration which makes the calculation possible. It was therefore suggested that the rate-determining step for nitration changes at the limit to the one in which the encounter pair is formed (Figure 1). By analogy with literature discussions (6) of diffusioncontrolled processes i t was implied that there was no interaction within the encounter pair. If this were the case, as k 1 can be estimated (7) as about 10 -10 s" and k2 can in the limit approach 10 - 10 s' , i t would be possible for considerable intramolecular selectivity to remain between positions in a substrate which are sufficiently reactive to be nitrated at the limiting rate.(8) Another suggestion (9,10) has been that the existence of the limiting rate might be explained by what has been called the Hammond Effect. As one increases the reactivity along a series of suitable substrates the transition state will become 9

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In Industrial and Laboratory Nitrations; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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p r o g r e s s i v e l y e a r l i e r , bonding from the e l e c t r o p h i l e to the c a r b o n atom i n c r e a s i n g l y l e s s d e v e l o p e d and t h e a c t i v a t i o n e n e r g y l o w e r u n t i l d i f f e r e n c e s m i g h t have e f f e c t i v e l y d i s a p p e a r e d . I t has b e e n e n v i s a g e d t h a t t h e s e t r a n s i t i o n s t a t e s m i g h t be p o s i t i o n a l l y o r i e n t e d (10) and i n t h i s c i r c u m s t a n c e s e l e c t i v i t y between s u f f i c i e n t l y r e a c t i v e p o s i t i o n s should d i s a p p e a r a t the l i m i t i n g r a t e . T h i s w o u l d a l s o be t h e s i t u a t i o n i f t h e r e e x i s t e d separate encounter p a i r s f o r d i f f e r e n t p o s i t i o n s of s u b s t i t u t i o n i n a molecule,(11) I f these e a r l y t r a n s i t i o n s t a t e s are not p o s i t i o n a l l y o r i e n t e d t h e e x i s t e n c e o f a common d i s c r e t e i n t e r m e d i a t e , p r i o r t o f o r m a t i o n o f Wheland i n t e r m e d i a t e s , seems o b l i g a t o r y and t h e r e f o r e t h e s u g g e s t i o n w o u l d be o f a change i n r a t e - d e t e r m i n i n g step to t h a t of formation of t h i s i n t e r m e d i a t e . Recent e x p r e s s i o n s ( 1 0 , 12) o f t h e s e i d e a s s u g g e s t t h a t t h e i n t e r m e d i a t e has i n f a c t an o r i e n t e d π-complex t y p e o f structure. E n e r g y d i a g r a m s have b e e n d r a w n (12) s h o w i n g t h e p o s s i b l e Wheland i n t e r m e d i a t e s f o r a s u b s t i t u t i o n r e s u l t i n g f r o m one common π-complex s t r u c t u r e p r e s u m a b l y o f t h i s t y p e . For i t t o be n e c e s s a r y t o p o s t u l a t e a change o f r a t e - d e t e r m i n i n g stage t o one i n w h i c h an i n t e r m e d i a t e w h i c h i n v o l v e s a t t r a c t i v e i n t e r a c t i o n i s f o r m e d , an i n t r a m o l e c u l a r s e l e c t i v i t y g r e a t e r than t h a t p o s s i b l e f o r a n o n - i n t e r a c t i n g encounter p a i r would have t o be o b s e r v e d . (8) On o t h e r o c c a s i o n s (10) t h e o r i e n t e d π-complex t y p e o f s t r u c t u r e has b e e n drawn as a r e p r e s e n t a t i o n o f e a r l y t r a n s i t i o n s t a t e s of the p o s i t i o n a l l y o r i e n t e d type to w h i c h the p r e v i o u s arguments would a p p l y . The p r e s e n t e v i d e n c e on t h e s e p o i n t s comes f r o m p u b l i s h e d w o r k ( 8 ) o f M o o d i e , S c h o f i e l d and t h e i r g r o u p a t t h e U n i v e r s i t y of E x e t e r . They have s t u d i e d t h e n i t r a t i o n o f pseudocumene and have shown t h a t t h e r e i s a b o u t a 9:1 s e l e c t i v i t y o f a t t a c k on p o s i t i o n s 5 and 6 p r o v i d e d t h a t one makes t h e r e a s o n a b l e a s s u m p t i o n t h a t any i p s o - a t t a c k i s n o t f o l l o w e d by r e a r r a n g e m e n t under the chosen c o n d i t i o n s . Both p o s i t i o n s are r e a c t i v e enough t o a c h i e v e t h e l i m i t i n g r a t e and h a v e s i m i l a r s t e r i c requirements. T h i s d e d u c t i o n r u l e s out the e x p l a n a t i o n i n terms o f e a r l y t r a n s i t i o n s t a t e s o f t h e p o s i t i o n a l l y o r i e n t e d ^ t y p e and a l s o precludes the p o s s i b i l i t y of d e t e c t a b l e p o s i t i o n a l l y o r i e n t e d i n t e r m e d i a t e s p r i o r t o t h e Wheland i n t e r m e d i a t e s . It means t h a t t h e r e must be a common i n t e r m e d i a t e p r i o r t o Wheland i n t e r m e d i a t e f o r m a t i o n , but the e x t e n t of i n t r a m o l e c u l a r s e l e c t i v i t y i s c e r t a i n l y n o t s u f f i c i e n t t o make i t n e c e s s a r y t o p o s t u l a t e any a t t r a c t i v e i n t e r a c t i o n i n t h i s i n t e r m e d i a t e . Our i n i t i a l a p p r o a c h e s ( 1 1 , 13) t o t h e s e p r o b l e m s i n v o l v e d s u b s t r a t e s where i t seemed t h a t a change i n t h e rate-determining s t a g e o f r e a c t i o n m i g h t have o c c u r r e d o v e r a r a n g e o f c o n c e n t r a t i o n s o f aqueous s u l p h u r i c a c i d due t o t h e c h a n g e i n solvent v i s c o s i t y . F o r e x a m p l e , we s t u d i e d o j - x y l e n e (11) and o - d i e t h y l b e n z e n e (13) f o r w h i c h r a t e p r o f i l e s a r e shown i n



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F i g u r e 2* compared w i t h t h a t o f m e s i t y l e n e ( 1 , 1 1 ) . Mesitylene r e a c t s a t t h e l i m i t i n g r a t e and t h e p r o f i l e s f o r o_-xylene and o - d i e t h y l b e n z e n e become i n d i s t i n g u i s h a b l e f r o m t h a t o f m e s i t y l e n e above a b o u t 68 and 73% H2S0i* r e s p e c t i v e l y . C o n t r a r y t o g e n e r a l a s s u m p t i o n i t was c l e a r t h a t t h e r a t i o o f i s o m e r s from these a r o m a t i c s , w h i c h have s i m p l e a l k y l s u b s t i t u e n t s , changed c o n s i d e r a b l y w i t h a c i d i t y i n aqueous sulphuric acid. The m a j o r f a c t o r i n t h e s e c h a n g e s was e l e g a n t l y d e m o n s t r a t e d by P r o f e s s o r P.C. Myhre (14) who showed t h a t f o r o_-xylene c o n s i d e r a b l e i n i t i a l i p s o - a t t a c k o c c u r s a t a c a r b o n atom b e a r i n g a m e t h y l g r o u p l e a d i n g t o a Wheland i n t e r m e d i a t e which rearranges at h i g h a c i d i t i e s e v e n t u a l l y to form 3 - n i t r o - o x y l e n e and i s c a p t u r e d by a n u c l e o p h i l i c s p e c i e s a t l o w e r a c i d i t i e s t o p r o d u c e p r o d u c t s t h a t had e s c a p e d us i n o u r s t u d i e s (Figure 3). I t has t h e r e f o r e become n e c e s s a r y t o s t u d y n o t product nitrocompound r a t i o s but y i e l d s of the v a r i o u s nitroproducts. The n i t r a t i o n o f £-xylene has b e e n r e i n v e s t i g a t e d i n t h i s way by M o o d i e , S c h o f i e l d e t a l . (15) By e x t r a p o l a t i o n o f t h e i r r e s u l t s and c o m b i n a t i o n w i t h t h o s e o f P r o f e s s o r M y h r e , i t i s p o s s i b l e t o c a l c u l a t e t h e p r o p o r t i o n o f a t t a c k a t t h e i p s o and 3 - p o s i t i o n s and t o see t h a t t h e s e v a r y c o n s i d e r a b l y w i t h a c i d i t y as does t h e a t t a c k a t p o s i t i o n 4 w h i c h c a n be d i r e c t l y m e a s u r e d ( T a b l e I ) . These c h a n g e s h a v e b e e n a t t r i b u t e d t o a s t r o n g medium d e p e n d e n c e o f t h e r e l a t i v e r e a c t i v i t i e s o f t h e v a r i o u s s u b s t i t u t e d and u n s u b s t i t u t e d p o s i t i o n s i n t h e m o l e c u l e . (15) TABLE I % ATTACK AT

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The m a g n i t u d e o f t h e s e e f f e c t s however i s s u r p r i s i n g l y h i g h . F o r example one c a n compare t h e s e r e s u l t s w i t h c o r r e s p o n d i n g d a t a f o r t o l u e n e (15) ( T a b l e I ) . H e r e a s m a l l e r dependence o f i s o m e r y i e l d on a c i d i t y i s a p p a r e n t , and t h e % m e t a - s u b s t i t u t i o n i n toluene remains almost constant. * A l l r a t e and p r o d u c t d a t a q u o t e d i n t h i s p a p e r a r e f r o m r e a c t i o n s a t 25°C and a t v e r y l o w a r o m a t i c c o n c e n t r a t i o n , t y p i c a l l y 10" * m o l e l " , u n l e s s o t h e r w i s e s t a t e d . 1

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S o l v a t i o n e f f e c t s have been i n v o k e d i n the p a s t t o e x p l a i n anomalous r e s u l t s f r o m e l e c t r o p h i l i c s u b s t i t u t i o n r e a c t i o n s o f o - x y l e n e and some s i m i l a r compounds, (16) t h e e f f e c t s d e p e n d i n g on t h e p r e s e n c e o f two g r o u p s o r t h o t o one a n o t h e r . The a p p l i c a t i o n o f any r e l a t e d i d e a s r e c e i v e s l i t t l e s u p p o r t f r o m o u r s t u d y o f o _ - d i e t h y l b e n z e n e . (13) The r e s u l t s o f t h i s a r e shown i n F i g u r e 4. T h i s shows a p l o t o f t h e y i e l d o f e a c h n i t r o p r o d u c t a g a i n s t % HaSOi* and a l s o i n c o r p o r a t e d i s an i n d i c a t i o n o f t o t a l y i e l d of n i t r o p r o d u c t s recovered. A l l d a t a a r e b a s e d on t h e i n i t i a l q u a n t i t y o f a r o m a t i c compound. Q u a n t i t a t i v e y i e l d s o f t h e n i t r o p r o d u c t s a r e o b t a i n e d above a b o u t 70% HaSOi* and t h e d r o p i n y i e l d and i n t h e amount o f 3 - s u b s t i t u t i o n b e l o w t h a t a c i d i t y c a n be a t t r i b u t e d t o n u c l e o p h i l i c c a p t u r e o f t h e i p s o s u b s t i t u t e d wheland i n t e r m e d i a t e . The change i n amount o f 4 - s u b s t i t u t i o n i s however much l e s s t h a n t h e c o r r e s p o n d i n g c h a n g e f o r cv-xylene. F o r o - d i e t h y l b e n z e n e t h e c o m b i n a t i o n o f 3and i p s o - s u b s t i t u t i o n i s l e s s a t a l l a c i d i t i e s p r e s u m a b l y f o r s t e r i c reasons. α-Xylene t h e n b e h a v e s i n a way, e v e n when i p s o - s u b s t i t u t i o n i s a l l o w e d f o r , t h a t w o u l d be u n s u s p e c t e d f r o m a s t u d y o f t o l u e n e o r oj-die t h y l b e n z e n e . These i n i t i a l s t u d i e s of i n t r a m o l e c u l a r s e l e c t i v i t y o f a l k y l compounds w h i c h r e a c t a t o r n e a r t h e l i m i t i n g r a t e and t h e i n t e r e s t i n g d e v e l o p m e n t s t h a t e n s u e d p r o m p t e d us t o e x t e n d o u r s t u d i e s t o methoxy s u b s t i t u t e d a r o m a t i c s . (17) A n i s o l e i s n i t r a t e d a t a r a t e w h i c h i s shown i n F i g u r e 5. The p r o f i l e i s p a r a l l e l t o t h a t o f m e s i t y l e n e and t h e r a t e i s about h a l f t h a t f o r m e s i t y l e n e under s i m i l a r c o n d i t i o n s . The p r o d u c t d a t a a r e shown i n F i g u r e 6. The o c c u r r e n c e o f n i t r a t i o n v i a n i t r o s a t i o n w h i c h has b e e n shown t o h a v e c o m p l i c a t e d many p r e v i o u s s t u d i e s o f a n i s o l e was p r e c l u d e d . The r e s u l t s a r e remarkable; t h e % 4 - s u b s t i t u t i o n i n c r e a s e s o v e r t h e r a n g e up t o 82% H^SOi* and t h e % 2 - s u b s t i t u t i o n d e c r e a s e s c o r r e s p o n d i n g l y , < IO"" % o f 3 - n i t r o a n i s o l e i s f o r m e d i n t h e s e r e a c t i o n s . P r e s u m a b l y a s t r o n g medium e f f e c t on t h e s u b s t i t u e n t e f f e c t o f the methoxy group i s b e i n g o b s e r v e d . This i s consistent w i t h c h a n g e s o b s e r v e d (18) i n t h e u l t r a v i o l e t a b s o r p t i o n s p e c t r a o f a n i s o l e i n t h i s r e g i o n o f a c i d i t y w h i c h a r e most r e a s o n a b l y i n t e r p r e t e d i n t e r m s o f an i n t e r a c t i o n w i t h t h e medium. 3

The s i t u a t i o n i n above 82% U S0^ i s n o t c l e a r . The t o t a l y i e l d d r o p s and t h i s a p p e a r s t o be due t o t h e f o r m a t i o n o f l e s s 4 - n i t r o a n i s o l e t h a n c o u l d be e x p e c t e d by e x t r a p o l a t i o n . 2- and 4 - n i t r o a n i s o l e c a n be e x t r a c t e d q u a n t i t a t i v e l y f r o m s i m u l a t e d reaction solutions. Arguments w h i c h a s s o c i a t e isomer r a t i o changes f o r a n i s o l e w i t h a change o f e l e c t r o p h i l e must be s c r u t i n i z e d most c a r e f u l l y . One m i g h t i m a g i n e t h a t t h e s e c o m p r e h e n s i v e d a t a on a n i s o l e w o u l d e n a b l e us t o p r e d i c t t h e i n t r a m o l e c u l a r s e l e c t i v i t y o f n i t r a t i o n o f v e r a t r o l e ( 1 , 2 - d i m e t h o x y b e n z e n e ) w i t h some s u c c e s s . V e r a t r o l e i s n i t r a t e d a t t h e l i m i t i n g r a t e ( F i g u r e 5 ) . (19) The 2


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In Industrial and Laboratory Nitrations; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976. Figure 6. Nitration of anisole



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p r o f i l e l i e s between those of a n i s o l e and mesitylene and i s approximately p a r a l l e l to them* 3 - N i t r o v e r a t r o l e was not detected i n the products and was c e r t a i n l y l e s s than 0.5% of the y i e l d . N i t r a t i o n produced only 4 - n i t r o v e r a t r o l e i n 92-98% y i e l d s over the range 60-75% H2S0i*. N i t r a t i o n v i a n i t r o s a t i o n was precluded from these r e a c t i o n s . C l e a r l y the j u x t a p o s i t i o n of the methoxy groups has produced an e f f e c t f a r removed from that expected from c o n s i d e r a t i o n of t h e i r behaviour i n i s o l a t i o n . A study of the n i t r a t i o n of p-methylanisole (17) has a l s o provided anomalous r e s u l t s . The r a t e p r o f i l e f o r t h i s compound which i s a l s o n i t r a t e d at the l i m i t i n g r a t e i s shown i n F i g u r e 5. I t i s almost i d e n t i c a l with that of a n i s o l e . The reasons f o r the small spread of apparent r e a c t i v i t y of the group of compounds at the l i m i t i n g r a t e i n F i g u r e 5 i s not known. The product data are shown i n F i g u r e 7. Two products are formed i n constant p r o p o r t i o n and q u a n t i t a t i v e y i e l d over a wide range of a c i d i t y . These products are a l s o formed i n t h i s constant p r o p o r t i o n d u r i n g i n d i v i d u a l runs. The major product i s 2 - n i t r o - 4 - m e t h y l a n i s o l e and t h i s i s the product one would expect i n the absence of ipso - s u b s t i t u t i o n . The second product i s 2 - n i t r o - 4 methylphenol and t h i s i s not formed e i t h e r by demethylation of p-methylanisole before n i t r a t i o n or of 2 - n i t r o - 4 - m e t h y l a n i s o l e after nitration. The Wheland intermediate i n v o l v i n g i p s o attack at the methyl group must be formed and Figure 8 T s suggested as a p o s s i b l e route f o r r e a c t i o n . Rate-determining encounter p a i r formation i s followed by normal n i t r a t i o n or by formation of the i p s o - s u b s t i t u t e d Wheland intermediate. O v e r a l l demethylation to a cyclohexadienone intermediate followed by p r o t o n a t i o n and l o s s of n i t r o n i u m i o n gives p - c r e s o l . This w i l l be n i t r a t e d at the l i m i t i n g r a t e and so the n i t r o n i u m i o n w i l l not leave the solvent cage. I f , however, 2-nitro-4-methylphenol i s the r e s u l t of i p s o attack para to the methoxy group there i s no evidence f o r the change of the methoxy group s u b s t i t u e n t e f f e c t with a c i d i t y which was apparent with a n i s o l e i t s e l f . The p i c t u r e that emerges from these s t u d i e s so f a r then i s that the dominant f a c t o r s a f f e c t i n g the change i n y i e l d s from p a r t i c u l a r r e a c t i v e p o s i t i o n s i n a s u b s t r a t e are those a s s o c i a t e d with i p s o - s u b s t i t u t i o n and with medium e f f e c t s . The l a t t e r are o f t e n f o r an as yet unknown reason c o n s i d e r a b l y more important than one might expect. In the remaining s e c t i o n n i t r a t i o n (21) by d i n i t r o g e n t e t r o x i d e i n dry carbon t e t r a c h l o r i d e s o l u t i o n i s considered. T y p i c a l data are shown i n Table I I . There appear to be two d i s t i n c t types of r e a c t i o n with toluene. The f i r s t i s apparent at low concentrations of Ν 2 θ ^ (< about 0.2 mole 1 ) where product isomer d i s t r i b u t i o n s c h a r a c t e r i z e d by a high percentage of m-nitrotoluene were observed. At high Ν 0 ^ concentrations * A s i m i l a r phenomenon occurs with p-methylanisole ( 2 3 ) and a l s o p - c h l o r o a n i s o l e (20) on n i t r a t i o n i n a c e t i c anhydride. l

2




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COOMBES ET AL.

Selectivity

in Aromatic

Nitration

Figure 8.



84

TABLE I I INTRAMOLECULAR SELECTIVITY OF N 0 , I N C C I , AT 20° 2 4 4 nitrocompound % [N 0 ] [ArH] % yield £ Ξ £ o

2

4


Toluene 0.02

0.4

6

39

44

17

1.2

3.5

4

48

3

49

0.6

(a)

5

90

5

(a)

35

15

50

Ethylbenzene 0.2

t-Butylbenzene 0.2

(a)

0.6

0.5 - 5%

(y 1 m o l e l " " ) however t h e o p e r a t i o n o f a n e l e c t r o p h i l i c m e c h a n i s m w i t h l o w p r o p o r t i o n s o f m - n i t r o t o l u e n e was i n d i c a t e d . At i n t e r m e d i a t e c o n c e n t r a t i o n s r e a c t i o n s i n i t i a l l y y i e l d e d a h i g h p r o p o r t i o n o f m - n i t r o t o l u e n e b u t as t h e r e a c t i o n p r o c e e d e d t h e v a l u e s b e g a n t o r e s e m b l e an i s o m e r d i s t r i b u t i o n e x p e c t e d f o r e l e c t r o p h i l i c a t t a c k . R e a c t i o n s w e r e s l o w a t room t e m p e r a t u r e a t a l l c o n c e n t r a t i o n s and l o w c o n v e r s i o n s (y 5%) w e r e a c h i e v e d . O t h e r p r o d u c t s i n c l u d i n g b e n z a l d e h y d e , b e n z y l a l c o h o l and p h e n y l n i t r o m e t h a n e were formed, a l t h o u g h t h e r i n g nitrocompounds were n o t i n t e r m e d i a t e s i n t h e f o r m a t i o n o f t h e s e o r o t h e r products. U n u s u a l i s o m e r p r o p o r t i o n s a t l o w Ν2θι* c o n c e n t r a t i o n s a l s o o c c u r r e d w i t h o t h e r a l k y l benzenes (Table I I ) . Ethylbenzene gave a h i g h p r o p o r t i o n o f m e t a - n i t r a t i o n and J t - b u t y l b e n z e n e h i g h ortho-nitration. A most r e a s o n a b l e i n t e r p r e t a t i o n o f t h e s e r e s u l t s w o u l d be t h a t t h e r e a c t i v e e n t i t y i s some s p e c i e s , p e r h a p s r a d i c a l i n n a t u r e , w h i c h shows a v e r y d i f f e r e n t i n t r a m o l e c u l a r s e l e c t i v i t y f r o m t h a t o f t h e n i t r o n i u m i o n . The e f f i c a c y o f n i t r o g e n d i o x i d e i t s e l f appears r u l e d o u t , however, by t h e low c o n v e r s i o n s . Study o f t h e i n t e r m o l e c u l a r s e l e c t i v i t y o f t h e reagent by c o m p e t i t i o n (Table I I I ) i n d i c a t e s t h a t t h e r e a g e n t i s about as s e l e c t i v e as t h e n i t r o n i u m i o n between a l k y l b e n z e n e s . The r e l a t i v e r a t e s o b s e r v e d a r e compared w i t h two s y s t e m s (1,2,11,22) where t h e n i t r o n i u m i o n i s w e l l e s t a b l i s h e d as t h e e l e c t r o p h i l i c species. F o r t o l u e n e t h e u n u s u a l isomer r a t i o s were observed i n these competition runs. 1


3.

COOMBES ET AL.

Selectivity in Aromatic Nitration

85

TABLE III INTERMOLE CULAR SELECTIVITY HNO3 in HN0 in 57% HaSOif aq MeN0 135

400

N2ÛI+ in ccu 280

o-Xylene

90

139

100

Toluene

17

25

40

Benzene

1

1

1

0.06

0.03

0.2

3

3

a

2


Mesitylene

Chlorobenzene

b

a 250 b 20O We are therefore faced with an anomalous situation. A gross change of the reactive species from the nitronium ion is indicated by the results of study of intramolecular selectivity but this change would have gone unsuspected if reliance had been placed solely on the evidence of intermolecular selectivity. In conclusion we would express our gratitude to the S.R.C. for financial support (J.G.G. and L.W.R.) and to the PhysicoChemical Measurements Unit at Harwell. Dr. R.B. Moodie and Professor K. Schofield are thanked for their guidance in the past and for communication of results prior to publication. Literature Cited (1) Coombes, R.G., Moodie, R.B. and Schofield, K. J.Chem. Soc.(B), (1968), 800. (2) Hoggett, J.G., Moodie, R.B. and Schofield, K. J.Chem. Soc.(B), (1969), 1. (3) Coombes, R.G. J.Chem.Soc.(Β), (1969), 1256. (4) Hartshorn, S.R., Moodie, R.B., Schofield, K. and Thompson, M.J. J.Chem.Soc.(Β), (1971), 2447. (5) Hartshorn, S.R., Hoggett, J.G., Moodie, R.B., Schofield, K. and Thompson, M.J. J.Chem.Soc.(Β), (1971), 2461. (6) North, A.M. 'The Collision Theory of Chemical Reactions in Liquids , Methuen, London, 1964. (7) Amdur, I, and Hammes, G.G. 'Chemical Kinetics', p.63, McGraw-Hill, London, 1966. '



86


(8) Moodie, R.B., Schofield, K. and Weston, J.B. J.C.S.Chem. Comm., (1974), 382. (9) Hartshorn, S.R. and Schofield, K. 'Progress in Organic Chemistry', 8, p.278, Butterworth, London, 1973. (10) Olah, G.A., Kobayashi, S. and Tashiro, M. J.Amer.Chem.Soc. (1972), 94, 7448. (11) Coombes, R.G. and Russell, L.W. J.Chem.Soc.(Β), (1971), 2443. (12) Olah, G.A. Acc.Chem.Res., (1971), 4, 240. (13) Coombes, R.G. and Tobin, G.D. unpublished results. (14) Myhre, P.C. J.Amer.Chem.Soc., (1972), 94, 7921. (15) Barnett, J.W., Moodie, R.B., Schofield, K. and Weston, J.B. J.C.S. Perkin II, (1975), 648. (16) Ansell, H.V. and Taylor, R. J.Chem.Soc.(Β), (1968), 526. (17) Barnett, J.W., Coombes, R.G., Golding, J.G., Moodie, R.B., Schofield, K. and Tobin, G.D. to be published. (18) Arnett, E.M. and Wu, C.Y. J.Amer.Chem.Soc., (1960), 82, 5660; Svanholm, U. and Parker, V.D. J.C.S. Perkin II,(1972), 961. (19) Coombes, R.G. and Golding, J.G. unpublished results. (20) Perrin, C.L. and Skinner, G.A. J.Amer.Chem.Soc., (1971), 93, 3389. (21) Russell, L.W. Ph.D. Thesis, The City University, London, 1974. (22) Coombes, R.G., Crout, D.H.G., Hoggett, J.G., Moodie, R.B. and Schofield, K. J.Chem.Soc.(Β), (1970), 347. (23) Shabarov, Yu. S. and Mochalov, S.S. J. Org. Chem. (U.S.S.R.), (1973), 9, 2061.


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