Side Reactions During Aromatic Nitration - ACS Symposium Series

8 Side Reactions During Aromatic Nitration C. H A N S O N ,

T.

KAGHAZCHI

*

and M. W. T. P R A T T

Schools of Chemical Engineering, University of Bradford, Bradford England, and Institute of Chemical and Petrochemical Engineering, Tehran Polytechnic, Tehran, Iran

Downloaded by UNIV OF TEXAS EL PASO on November 8, 2014 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0022.ch008

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Aromatic nitration has long been an important process in industrial chemistry for nitro-aromatic products are widely used as explosives, solvents, pharmaceuticals and intermediates in the manufacture of synthetic dyestuffs and other chemicals. The nitrating agent most commonly used industrially for aromatic mononitrations is an aqueous mixture of nitric and sulphuric acids, typically 15 mole %HNO ,30 mole %H SO and 55 mole % H O. The organic substrate, together with the nitro product, forms a separate phase from the aqueous mixed acid and so mass transfer is involved simultaneously with chemical reaction. The reaction mixture should be well stirred, therefore, to minimise the mass transfer resistances, although these may never be completely overcome. The mixed acid generally gives the wanted nitro product more quickly, at a lower temperature and with less loss through higher oxidation reactions than when nitric acid alone is used. Aromatic nitration also has been of great importance in the development o f the theory o f organic chemistry and has r e c e i v e d 3

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much a t t e n t i o n as an example o f 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 . As a r e s u l t o f t h i s , the involvement o f the n i t r o n i u m i o n , Ν0>2 * as the n i t r a t i n g agent adding t o the aromatic r i n g has been w e l l e s t a b l i s h e d , although the g r e a t m a j o r i t y o f t h e o r e t i c a l s t u d i e s o f the k i n e t i c s and mechanism have been c a r r i e d out w i t h systems which a r e homogeneous and e i t h e r anhydrous o r c o n t a i n a r e l a t i v e l y low c o n c e n t r a t i o n o f water. Tliis i s i n c o n t r a s t with the two-phase aqueous systems used so widely i n commercial aromatic n i t r a t i o n . The e f f e c t on the r e a c t i o n o f water, which i s known t o decrease n i t r o n i u m i o n c o n c e n t r a t i o n , i s d i s c u s s e d i n another paper c o n t r i b u t e d t o t h i s symposium by some o f the p r e s e n t authors, but the g e n e r a l l y accepted view a t present i s t h a t the n i t r o n i u m i o n mechanism s t i l l a p p l i e s t o aromatic n i t r a t i o n i n the mixed a c i d system. I t i s q u i t e c l e a r , however, t h a t s i d e r e a c t i o n s do occur, even d u r i n g mononitration r e a c t i o n s . I n the mononitration o f toluene, f o r example, the pure n i t r o products, toluene and spent n i t r a t i n g a c i d , a r e p a l e +

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i n c o l o u r and y e t the organic and aqueous phases of the r e a c t i o n mixture are sometimes h i g h l y coloured, from strong yellow to dark r e d . Product y i e l d s are u s u a l l y l e s s than 98% and the occurrence o f p h e n o l i c and other i m p u r i t i e s has been reported i n the l i t e r a t u r e . Side r e a c t i o n s are important f o r s e v e r a l reasons:Economic c o n s i d e r a t i o n s . By-product formation represents l o s s o f r e a c t a n t s or n i t r o product. I t i s a l s o l i k e l y to r e s u l t i n i n c r e a s e d c o s t s f o r the s e p a r a t i o n and p u r i f i c a t i o n of the main product, such as i n c r e a s e d c a p i t a l c o s t s f o r d i s t i l l a t i o n and washing stages, together with h i g h e r operating costs f o r steam and treatment chemicals. Environmental c o n s i d e r a t i o n s . By-products o f t e n appear as p o l l u t a n t s discharged from the manufacturing p l a n t as c o n s t i t u e n t s o f gaseous, l i q u i d or s o l i d e f f l u e n t s . The p o l l u t i o n o f the environment by chemical e f f l u e n t s i s o f i n c r e a s i n g concern and f u r t h e r l e g a l c o n s t r a i n t s and l i m i t s are l i k e l y to be imposed. P h e n o l i c i m p u r i t i e s , commonly found as by-products of aromatic n i t r a t i o n , are p a r t i c u l a r l y harmful i n the damage they cause i n water courses because o f t h e i r h i g h toxicity. Safety f a c t o r s . By-product formation may i n v o l v e a s a f e t y hazard: f o r example, the presence of even s m a l l q u a n t i t i e s of chemically unstable i m p u r i t i e s , such as h i g h l y n i t r a t e d compounds, may c a t a l y s e e x p l o s i v e decompositions. For a l l these reasons, avoidance or r e d u c t i o n of by-product formation would be o f c l e a r b e n e f i t , but as previous knowledge o f s i d e r e a c t i o n s during aromatic n i t r a t i o n r e p o r t e d i n the l i t e r a t u r e i s not comprehensive, t h i s f u r t h e r i n v e s t i g a t i o n has been made. The aims were t o c h a r a c t e r i s e the by-products formed during the mononitration o f toluene and some o t h e r aromatic compounds, and t o measure the r a t e and extent of p r o d u c t i o n of by-products as a f u n c t i o n o f r e a c t i o n c o n d i t i o n s so t h a t the e f f e c t of s i d e r e a c t i o n s may be minimised. P a r t i c u l a r a t t e n t i o n has been g i v e n t o the formation of n i t r o u s a c i d and organic a c i d by-products as these appear to be the major i m p u r i t i e s d u r i n g toluene mononitration. At present, i t i s normal p r a c t i c e t o remove the p h e n o l i c by-products from aromatic n i t r o products by washing the product s u c c e s s i v e l y with water and aqueous c a u s t i c soda. The a l k a l i n e washings are commonly a c i d i f i e d t o f r e e the bulk of the phenols, which may be separated f o r d i s p o s a l , f o r example, by b u r n i n g . A p r o p o r t i o n o f them, however, remains d i s s o l v e d i n the aqueous e f f l u e n t . An a l t e r n a t i v e process i s suggested here f o r removal of the a c i d i c organic by-products which avoids continuous consumption o f c a u s t i c soda and a c i d . T h i s c o u l d have economic advantages through lower o p e r a t i n g costs as w e l l as a r e d u c t i o n i n the phenol c o n c e n t r a t i o n of the p l a n t e f f l u e n t .


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L i t e r a t u r e Survey During the l a s t n i n e t y years, s t u d i e s o f aromatic n i t r a t i o n s reported i n the l i t e r a t u r e have revealed t h a t s m a l l q u a n t i t i e s o f hydroxy by-products a r e generated i n these r e a c t i o n s , t h e i r nature and q u a n t i t y being dependent c h i e f l y upon the p a r t i c u l a r type o f aromatic compound and the n i t r a t i n g system. N o e l t i n g and F o r e l ( l ) i n 1885 reported t h a t , i n the mononitration o f toluene, n i t r o c r e s o l s are formed and i f they are not removed by an a l k a l i wash, they undergo f u r t h e r n i t r a t i o n t o t r i n i t r o c r e s o l o r o x i d a t i o n t o o x a l i c a c i d . I n 1889, N o e l t i n g and Pick(2) i s o l a t e d d i n i t r o - o - x y l e n o l i n the p r e p a r a t i o n o f n i t r o - x y l e n e and i n 1891 Armstrong and R o s s i t e r p ) n o t i c e d t h a t p h e n o l i c compounds were formed d u r i n g the n i t r a t i o n o f benzene and toluene. F o l l o w i n g these i n i t i a l r e p o r t s , s i m i l a r p h e n o l i c by-products were found d u r i n g many aromatic n i t r a t i o n s . I n 1933, Berkenheim and Coster(4) suggested t h a t the mechanism o f formation o f hydroxy by-products could i n v o l v e a rearrangement o f complex n i t r i t e e t h e r s . T i t o v and Baryshnikova(5) found i n 1936 t h a t , i n the n i t r a t i o n o f toluene by d i n i t r o g e n d i o x i d e i n the presence o f s u l p h u r i c and phosphoric a c i d s , about 2% o f 2,6 d i n i t r o - p - c r e s o l i s produced. They f i r s t suggested a mechanism i n v o l v i n g r e a c t i o n o f n i t r o u s a c i d w i t h toluene t o give a diazonium hydroxide, which loses n i t r o g e n t o y i e l d a phenol. L a t e r , T i t o v ( 6 ) proposed a d i f f e r e n t r e a c t i o n scheme i n which N0 r e a c t s with the aromatic t o form a n i t r o s o compound which rearranges t o a n i t r o p h e n o l . This mechanism w i l l be discussed l a t e r . Bennett and Youle(7,8,9) a l s o found p h e n o l i c i m p u r i t i e s amounting t o about 2% o f the y i e l d i n the n i t r a t i o n o f s e v e r a l d i f f e r e n t aromatic compounds. They observed t h a t the hydroxyl group enters the aromatic molecule according t o the normal o r i e n t a t i o n r u l e f o r the s u b s t i t u e n t o r i g i n a l l y present and t h a t the amount o f by-products i s g r e a t e r f o r meta-directing substituents. +

The most comprehensive c o n t r i b u t i o n t o knowledge o f s i d e r e a c t i o n s during aromatic n i t r a t i o n up t o the present time has been made by Dadak and co-workers i n a s e r i e s o f four papers(1013). During toluene n i t r a t i o n , they detected the formation o f n i t r o c r e s o l s , ρ-nitrophenol, 4,6 d i n i t r o c r e s o l , 2,4 d i n i t r o p h e n o l and 3 n i t r o - 4 hydroxy-benzoic a c i d . They reported some q u a n t i t a t i v e analyses f o r t h i s and other n i t r a t i o n s , although they d i d not i n v e s t i g a t e a wide range o f r e a c t i o n c o n d i t i o n s . They suggested t h a t the by-products o r i g i n a t e d from the o x i d a t i o n o f toluene t o c r e s o l s , followed by mono-, d i - o r t r i n i t r a t i o n , the presumed r e a c t i o n schemes being shown i n F i g . l , i n which the compounds shown i n square brackets were not i s o l a t e d . No explanation f o r the mechanism o f i n t r o d u c t i o n o f the hydroxyl group was given and no a t t e n t i o n was given t o n i t r o u s a c i d formation. The gaps and i n c o n s i s t e n c i e s i n previous i n v e s t i g a t i o n s


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Figure 1.

Reaction schemes suggested by Dadak et al.


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reported i n the l i t e r a t u r e , a more complete account of which i s given elsewhere(14), c l e a r l y leave room f o r f u r t h e r work on s i d e r e a c t i o n s d u r i n g aromatic n i t r a t i o n .


Experimental Results and D i s c u s s i o n The experimental programme was d i v i d e d i n t o the f o l l o w i n g aspects, which are reported s e p a r a t e l y : (1) formation of n i t r o u s a c i d d u r i n g the heterogeneous n i t r a t i o n of toluene. (2) formation of organic a c i d by-products and t h e i r q u a n t i t a t i v e r e l a t i o n with n i t r o u s a c i d . (3) e f f e c t of s u l p h u r i c a c i d c o n c e n t r a t i o n upon organic a c i d by-product formation. (4) by-product formation from other s u b s t r a t e s . (5) e f f e c t o f a d d i t i o n o f urea and n i t r i t e i o n on by-product formation. Further experimental work concerned with the development of an a l t e r n a t i v e process f o r the removal o f p h e n o l i c by-products from n i t r o products i s d i s c u s s e d i n a f i n a l s e c t i o n . Formation o f N i t r o u s A c i d during the Heterogeous N i t r a t i o n of Toluene. Known volumes o f the aromatic hydrocarbon were mechanically shaken at a constant speed with twice the volume o f n i t r a t i n g a c i d (30 mole % s u l p h u r i c a c i d , 15 mole % n i t r i c a c i d , 55 mole % water) i n a stoppered f l a s k immersed i n a thermostat at 25.0° + 0.1°C. A f t e r a p e r i o d o f time, the contents o f the f l a s k were poured i n t o a s e p a r a t i n g funnel and the unwashed organic and aqueous phases analysed f o r n i t r o u s a c i d by a c o l o r i m e t r i c method u s i n g s u l p h a n i l i c a c i d and α-naphthylamine. The aqueous and washed organic phases were then analysed f o r mono-nitrotoluene (MNT) by UV spectrophotometry u s i n g the absorption peak at 350 nm. The n i t r o u s a c i d content of the f r e s h mixed a c i d used was l e s s than 10~ moles/1. As Fig.2 shows, the n i t r o u s a c i d c o n c e n t r a t i o n ( t o t a l moles n i t r o u s a c i d d i v i d e d by t o t a l volume of both phases) i n c r e a s e s w i t h time, approximately i n p r o p o r t i o n to the c o n c e n t r a t i o n o f MNT formed. The r a t e o f formation decreases towards the end of the r e a c t i o n between toluene and n i t r i c a c i d . The d i s t r i b u t i o n c o e f f i c i e n t f o r n i t r o u s a c i d between the two phases, [HNC^Jorg./ [HN02]aq., decreases from 0.48 towards the s t a r t of r e a c t i o n to 0.28 near the end. To determine whether n i t r o u s a c i d i s formed only from the toluene present, the experiment was repeated with MNT i n s t e a d o f toluene as the aromatic compound present i n i t i a l l y . N i t r o u s a c i d was produced a t approximately the same r a t e as from toluene ( F i g . 3 ) . This i n d i c a t e s t h a t the decrease i n r a t e o f n i t r o u s a c i d formation towards the end of the main r e a c t i o n i s because the n i t r i c a c i d has been n e a r l y used up, r a t h e r than being due to the replacement of toluene by MNT. 4



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Figure 2.

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2 0 0 250 3 0 0

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Production of MNT b- HN0 during the nitration of toluene with mixed acid 2

Figure 3. Production of HN0 during the reaction of MNT with mixed acid 2



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The e f f e c t of change i n r e a c t i o n c o n d i t i o n s upon the r a t e of formation of n i t r o u s a c i d was measured i n a s e r i e s of experiments A t o F. The r e s u l t s are shown i n F i g . 4 . In experiment A, i n d u s t r i a l mixed a c i d i s used a t 25°C with an organic/aqueous phase r a t i o o f 0.5. I n B, the c o n c e n t r a t i o n o f n i t r i c a c i d i s reduced from 15 to 3 mole %. The concentration o f s u l p h u r i c a c i d i s lower i n C:20 mole % i n s t e a d o f the normal 30%, the n i t r i c a c i d being the u s u a l 15 mole %. The temperature i s i n c r e a s e d t o 35°C i n D, with normal mixed a c i d , and i n Ε the phase r a t i o i s i n c r e a s e d t o 1.0. F i n a l l y , i n F, n i t r i c a c i d (70%) i s used alone, without s u l p h u r i c a c i d . On the b a s i s of these experiments, the r a t e o f formation o f n i t r o u s a c i d i s seen t o be very dependent upon s u l p h u r i c a c i d c o n c e n t r a t i o n . N i t r o u s a c i d p r o d u c t i o n decreases with i n c r e a s e i n s u l p h u r i c a c i d c o n c e n t r a t i o n i n the mixed a c i d , although MNT p r o d u c t i o n i s enhanced by i n c r e a s e i n s u l p h u r i c a c i d content. In C, f o r example, the n i t r o u s a c i d represents 2 mole % o f the MNT. The p l o t of the c o n c e n t r a t i o n o f MNT versus n i t r o u s a c i d i s a s t r a i g h t l i n e f o r each experiment (Fig.5) which i n d i c a t e s t h a t n i t r o u s a c i d and MNT are produced i n constant p r o p o r t i o n . The r a t i o s [HNO2]/[MNT], although constant d u r i n g each experiment, vary with r e a c t i o n c o n d i t i o n s as f o l l o w s : (1) the r a t i o increases with i n c r e a s e i n n i t r i c a c i d concentra t i o n (at constant s u l p h u r i c a c i d concentration) (2) i t decreases with i n c r e a s e i n s u l p h u r i c a c i d concentration (at constant n i t r i c acid) (3) the r a t i o increases with i n c r e a s e i n r e a c t i o n temperature (4) i t increases with i n c r e a s e i n volume r a t i o of organic t o aqueous phase from 1:2 t o 1:1 (5) i n the absence o f s u l p h u r i c a c i d , l a r g e q u a n t i t i e s o f n i t r o u s a c i d are formed and the r a t i o [HN02l/[MNT] i s p a r t i c u l a r l y h i g h . To e s t a b l i s h whether n i t r o u s a c i d i s produced mainly from attack on toluene or n i t r o t o l u e n e , about 20 ml of the aromatic compound was shaken with an equal volume o f d i l u t e (6N) n i t r i c a c i d i n a stoppered f l a s k a t 25°C f o r 16 hours, a f t e r which the n i t r o u s a c i d formed was measured. N i t r i c a c i d o f t h i s c o n c e n t r a t i o n i s not s t r o n g enough to n i t r a t e toluene under these c o n d i t i o n s , no t r a c e o f MNT being d e t e c t a b l e by UV a n a l y s i s of the organic l a y e r a f t e r 24 hours. The i n i t i a l c o n c e n t r a t i o n o f n i t r o u s a c i d i n the 6N n i t r i c a c i d used was 1.5 χ 10~5 moles/1. This i n c r e a s e d when shaken with the aromatic compound to 8.15 χ 10-5 moles/1 i n the case o f toluene and t o 2.85 χ 10"* moles/1 f o r MNT. These r e s u l t s do not represent the t o t a l n i t r o u s a c i d formed d u r i n g the r e a c t i o n , as some o f i t would decompose d u r i n g the 16 hour p e r i o d . They do confirm, however, t h a t n i t r o u s a c i d may be formed from both toluene and MNT even by the a c t i o n o f d i l u t e n i t r i c a c i d . The k i n e t i c s o f r e a c t i o n between d i l u t e n i t r i c a c i d , up t o 8N s t r e n g t h , and MNT to produce n i t r o u s a c i d were followed over a two week time i n t e r v a l i n homogeneous a c e t i c a c i d s o l u t i o n . With both excess 4


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Production of MNT 6- HN0

2

under various reaction conditions


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141


MNT and excess n i t r i c a c i d , the c o n c e n t r a t i o n o f n i t r o u s a c i d reaches a maximum and then f a l l s as the r a t e o f n i t r o u s a c i d decomposition exceeds i t s formation r a t e . D e t a i l e d r e s u l t s o f t h i s work are given elsewhere(14). Formation of Organic A c i d By-products and t h e i r Q u a n t i t a t i v e R e l a t i o n with N i t r o u s A c i d . I f the raw MNT product c o n t a i n i n g by-products i s d i s s o l v e d i n a s o l v e n t such as carbon t e t r a c h l o r i d e or chloroform and subjected to i n f r a - r e d a n a l y s i s , the absorption peaks correspond t o MNT and d i s s o l v e d n i t r i c a c i d ; peaks due t o organic by-products are not g e n e r a l l y seen, as they are below the d e t e c t i o n l i m i t under these c o n d i t i o n s . The presence o f p h e n o l i c compounds may be confirmed i f they are concentrated by e x t r a c t i o n i n t o aqueous a l k a l i , which i s then a c i d i f i e d t o l i b e r a t e f r e e acids f o r r e - e x t r a c t i o n by a s m a l l q u a n t i t y o f o r g a n i c s o l v e n t . Another confirmatory t e s t i n v o l v e s the KBr pressed d i s c technique, i n which a t h i n f i l m of the organic phase product i s examined. The l a r g e excess o f MNT compared with the by-products makes t h i s method u n s u i t a b l e f o r q u a n t i t a t i v e a n a l y s i s . I t d i d i n d i c a t e , however, t h a t n e u t r a l or b a s i c organic i m p u r i t i e s were not present i n l a r g e amounts as no s i g n i f i c a n t a b s o r p t i o n peaks which could correspond t o such compounds were found, although peaks corresponding to phenols were i d e n t i f i e d . The general technique used f o r e x t r a c t i o n of by-products was, t h e r e f o r e , t h a t o f a l k a l i e x t r a c t i o n . The organic product phase was washed three times with 2N sodium hydroxide t o e x t r a c t the water-soluble s a l t s o f the organic a c i d s . H y d r o c h l o r i c a c i d was added t o l i b e r a t e the organic compounds f o r e x t r a c t i o n by ether. F o l l o w i n g evaporation o f the ether, the r e s i d u a l by-products were weighed. In some cases t r a c e s o f common s a l t , which i s s l i g h t l y s o l u b l e i n ether, contaminated the o r g a n i c by-products; a r e c r y s t a l l i s a t i o n from chloroform was then performed. A p o r t i o n of the organic by-products was d i s s o l v e d i n carbon t e t r a c h l o r i d e o r chloroform (spectroscopic grade) f o r i n f r a - r e d a n a l y s i s . The a b s o r p t i o n peaks corresponded to nitrophenols, n i t r o c r e s o l s , dinitrophenols, d i n i t r o c r e s o l s , traces of t r i n i t r o compounds and nitro-hydroxy c a r b o x y l i c a c i d s . The presence o f i n d i v i d u a l compounds was confirmed by comparison with the a b s o r p t i o n spectrum o f the pure compounds and by t h i n l a y e r chromotography. Q u a n t i t a t i v e analyses o f the concentrations o f s e v e r a l components o f the by-products were made. I f a component d i d not have an a b s o r p t i o n band f r e e from i n t e r f e r e n c e by another c o n s t i t u e n t o f the mixture, q u a n t i t a t i v e a n a l y s i s c o u l d s t i l l be achieved by measurement of the e x t i n c t i o n c o e f f i c i e n t s o f the i n t e r f e r i n g components a t s e v e r a l d i f f e r e n t f r e q u e n c i e s , together with the absorbance o f the mixture a t these f r e q u e n c i e s . C h a r a c t e r i s t i c a b s o r p t i o n bands used are shown i n Table I .


142

INDUSTRIAL AND LABORATORY

NITRATIONS

TABLE I C h a r a c t e r i s t i c IR Absorption Bands 1

Wavenumber(cm )

Compound


2, 6 d i n i t r o p - c r e s o l 4, 6 d i n i t r o o - c r e s o l 2, 4 d i n i t r o p h e n o l p-nitrophenol mono-nitroc r e s o l s 3 n i t r o 4 hydroxy benzoic a c i d

875 950 and 1140 and 1110 and 860 1730

1090 1110 860

The r e s u l t s o f q u a n t i t a t i v e a n a l y s i s o f the by-products from n i t r a t i o n o f toluene by standard mixed a c i d are given i n Table I I . The major c o n s t i t u e n t i s 2, 6 d i n i t r o - p - c r e s o l , r e p r e s e n t i n g almost 80% o f the a c i d i c organic by-products. The s i x components l i s t e d represent 99% o f the t o t a l and t h e r e f o r e represent a l l major c o n s t i t u e n t s of the mixture, f o r which the weighted mean molecular weight i s 198. TABLE I I Composition o f Organic A c i d Compound

By-Products % by weight

2, 6 d i n i t r o p - c r e s o l 79.6 4, 6 d i n i t r o o - c r e s o l 10.3 2, 4 d i n i t r o p h e n o l 1.4 p - n i t r o p h e n o l and mono-nitrocresol 4.3 3 n i t r o 4 hydroxy benzoic a c i d 3.3 TOTAL 98.9 Mean Molecular Weight 198 An i n t e r e s t i n g r e l a t i o n between the amounts o f n i t r o u s a c i d and organic a c i d by-products became apparent. The concentrations o f the organic by-products formed from the n i t r a t i o n o f toluene under various r e a c t i o n c o n d i t i o n s are p l o t t e d i n Fig.6 a g a i n s t the concentrations of n i t r o u s a c i d produced i n the same experiment. The r e l a t i o n s h i p i s approximately l i n e a r , showing t h a t n i t r o u s a c i d and organic acids are formed i n a constant p r o p o r t i o n . From the b e s t s t r a i g h t l i n e , i t i s seen t h a t 6 χ 10~ moles/1 o f n i t r o u s a c i d are produced f o r 12 g/1 o f o r g a n i c by-products. From the mean molecular weight o f the mixture o f by-products, 12 g/1 i s e q u i v a l e n t to 6.0 χ 10~ moles/1. W i t h i n experimental e r r o r , one mole o f organic by-product i s formed f o r each mole o f n i t r o u s a c i d . The mechanisms o f formation o f these two by-products are t h e r e f o r e l i k e l y t o be directly linked. During' these experiments, a constant h i g h degree o f a g i t a t i o n was used so t h a t r e a c t i o n times were a l l r e l a t i v e l y 2

2


8.

HANSON E T AL.

Side Reactions

During

Aromatic

Nitration


15

Figure 6.

The relationship between HN0

2

and organic by-products formed


143


144

INDUSTRIAL AND

LABORATORY

NITRATIONS

s h o r t and any decomposition o f n i t r o u s a c i d would be minimal. For comparison, the formation o f n i t r o u s a c i d and organic by-products during n i t r a t i o n under homogeneous c o n d i t i o n s was determined, a c e t i c a c i d b e i n g used as s o l v e n t . The r e a c t i o n mixture contained 0.3 moles/1 toluene and 10.06 moles/1 n i t r i c a c i d with v a r y i n g concentrations o f s u l p h u r i c a c i d . Reaction temperature was 25°C. Again, a one t o one molar r a t i o was found between n i t r o u s a c i d and organic a c i d by-products over a time up to one hour d u r i n g which about 98% conversion o f toluene to MNT was obtained. As f u r t h e r time elapsed, the c o n c e n t r a t i o n of o r g a n i c by-products decreased but n i t r o u s a c i d continued t o i n c r e a s e . This c o u l d be because an excess of n i t r i c a c i d was used which, a f t e r completion o f mononitration o f toluene, could r e a c t f u r t h e r with the organic by-products causing t h e i r degeneration and producing f u r t h e r n i t r o u s a c i d . Increase i n s u l p h u r i c a c i d c o n c e n t r a t i o n , as f o r heterogeneous r e a c t i o n , increased the r a t e o f p r o d u c t i o n o f MNT and decreased the p r o p o r t i o n o f by-products t o MNT. E f f e c t o f S u l p h u r i c A c i d Concentration upon Organic A c i d By-Product Formation. I t i s known t h a t i n c r e a s e i n s u l p h u r i c a c i d c o n c e n t r a t i o n i n c r e a s e s the r a t e o f aromatic n i t r a t i o n but i t s e f f e c t on by-product formation had to be found. Three s e r i e s of experiments were made a t 35°C, i n each of which n i t r i c a c i d c o n c e n t r a t i o n was kept constant at 15 mole %, w h i l e s u l p h u r i c a c i d concentrations were r e s p e c t i v e l y 2 5 , 30 and 35 mole %. The organic/aqueous phase r a t i o was 2/3 i n a l l cases. P l o t s o f the v a r i a t i o n with time of organic a c i d by-product c o n c e n t r a t i o n are given i n F i g . 7 , and of MNT c o n c e n t r a t i o n i n F i g . 8 . C l e a r l y , i n c r e a s e i n the s u l p h u r i c a c i d p r o p o r t i o n i n mixed n i t r a t i n g a c i d increases the r a t e of formation o f MNT, as expected, but i t decreases the formation o f organic by-products. This r e s u l t i s o f s i g n i f i c a n c e f o r the o p e r a t i o n o f commercial p l a n t s f o r aromatic n i t r a t i o n . I t i n d i c a t e s t h a t h i g h e r y i e l d s o f MNT may be achieved i f the p r o p o r t i o n o f s u l p h u r i c a c i d i s i n c r e a s e d towards the l i m i t o f about 37 mole % s u l p h u r i c a c i d , 15 mole % n i t r i c a c i d and 48 mole % water, a t which the onset of d i n i t r a t i o n i s j u s t d e t e c t e d . Increase i n s u l p h u r i c a c i d c o n c e n t r a t i o n from the normal 30 mole % to 35 mole % decreases organic by-product formation by over 2 5 % . By-Product Formation from other S u b s t r a t e s . The formation of n i t r o u s a c i d and organic a c i d by-products during the heterogeneous n i t r a t i o n of other s u b s t r a t e s was compared with the corresponding r e s u l t s d u r i n g toluene mononitration. Besides toluene, the hydrocarbons s t u d i e d were benzene, ethylbenzene, ο-, m- and p-xylenes and mesitylene. In a l l cases, the normal mixed n i t r a t i n g a c i d composition was used with organic/aqueous phase r a t i o o f 2/3 and r e a c t i o n temperature o f 35°C. Fig.9 shows that the r a t e of formation of the mononitro products does not f o l l o w



Figure 7. 2

The effect of H SO> concentration on the rate of production of

time (min) by-products


mole %

NITRATIONS


INDUSTRIAL A N D LABORATORY

•|S3|OUJ

3uan|o;oj}!N

JO

UOi:pj}U3DUCO



8.

HANSON E T A L .

Side Reactions

During

Aromatic

Nitration

147

the o r d i n a r y r u l e of r e a c t i v i t y of aromatic hydrocarbons i n homogeneous s o l u t i o n (benzene < ethylbenzene £ toluene < o-, mand p-xylene < mesitylene) f o r the r a t e s o f n i t r a t i o n o f m- xylene and mesitylene are below t h a t o f toluene i n t h i s twophase n i t r a t i o n , as has been noted b e f o r e ( 1 5 ) . The corresponding production of organic a c i d by-products i s given i n F i g . 1 0 , which shows t h a t , o f the aromatic hydrocarbons i n v e s t i g a t e d , o- and pxylene are most prone towards by-product formation. Benzene forms n e g l i g i b l e q u a n t i t i e s o f by-products; m-xylene and mesitylene, which have h i g h r e a c t i v i t y towards n i t r a t i o n , are next lowest, below toluene and ethylbenzene. For a l l the aromatics, the r a t i o of organic a c i d by-products t o n i t r o aromatic product i s almost constant with r e s p e c t t o time. The average y i e l d o f organic a c i d i c by-products compared with the n i t r o aromatics i s l i s t e d i n Table I I I . TABLE I I I Organic By-Product Y i e l d from Various Hydrocarbons Hydrocarbon

Weight % organic by-products compared with mononitro compound

benzene toluene ethylbenzene o-xylene p-xylene m-xylene mesitylene

0.018 1.72 1.63 6.19 3.13 0.77 1.31

Formation of n i t r o u s a c i d d u r i n g the n i t r a t i o n s (Fig.11) shows the same r e l a t i v e v a r i a t i o n as t h a t of the o r g a n i c by-products. E f f e c t of A d d i t i o n o f Urea and N i t r i t e Ion on By-Product Formation. The e f f e c t of a d d i t i o n of 0.1 mole/1 o f urea and, s e p a r a t e l y , 0.1 mole/1 of sodium n i t r i t e on toluene n i t r a t i o n was examined but, w i t h i n experimental e r r o r , no d i f f e r e n c e was detected i n the q u a n t i t i e s o f organic a c i d by-products formed. The i m p l i c a t i o n s o f these observations i n d e c i d i n g the l i k e l y mechanism o f by-product formation are d i s c u s s e d i n the next section. Mechanism o f By-Product

Formation

The extensive work o f Dadak and co-workers(10-13), i n the course of which many by-products o f aromatic n i t r a t i o n were i d e n t i f i e d , d i d not i n c l u d e any w e l l - e s t a b l i s h e d e x p l a n a t i o n o f the mechanism by which p h e n o l i c intermediates are produced and



148

INDUSTRIAL A N D L A B O R A T O R Y NITRATIONS

Figure

9. Nitration of various aromatic hydrocarbons by mixed acid at35°C

Figure 10.

Formation of by-products during the nitration of various aromatic compounds


8.

HANSON E T A L .

Side Reactions

During

Aromatic

149

Nitration

d i d not d i s c u s s the involvement o f n i t r o u s a c i d . Two r e a c t i o n mechanisms are prominent among those which have been suggested p r e v i o u s l y . One of the schemes put forward by Titov(6J p o s t u l a t e s r e a c t i o n between the aromatic compound and the nitrosonium i o n , NO . A n i t r o s o compound i s f i r s t formed which undergoes rearrangement to a phenol and may be n i t r a t e d to a mono-, d i - or t r i - n i t r o p h e n o l : +


NO

NOH

NOH

θ

H

NHOH

OH

OH

I f t h i s mechanism were accepted, then a d d i t i o n o f urea, which removes any n i t r o u s a c i d o r N0 present i n the n i t r a t i n g mixture, should prevent or reduce the formation of by-products. On the other hand, the a d d i t i o n o f n i t r i t e i o n should promote by-product formation:+

HN0 + H

+

-*

2

H

N 2

0

2

+

H

+

2°

N

0

+

I t was noted t h a t a d d i t i o n of urea or sodium n i t r a t e has no e f f e c t on the formation o f organic by-products d u r i n g toluene n i t r a t i o n . This evidence implies t h a t t h i s mechanism does not p l a y a s i g n i f i c a n t p a r t i n by-product formation. A second mechanism, f o r which urea or n i t r i t e i o n would be p r e d i c t e d t o have no e f f e c t on p h e n o l i c by-product formation, has been suggested by Bennett(16,17) and a l s o T i t o v ( 1 8 ) . This supposes t h a t a nitronium i o n becomes attached to the aromatic r i n g through one o f i t s oxygen atoms i n s t e a d of the normal n i t r o g e n atom, t o form an a r y l n i t r i t e . This then decomposes t o a phenol and a n i t r o s y l i o n : Ar-H

+

0 = Ν = 0

ArO-N = 0

+

H

+

-*

ArO-N = 0

->

ArOH

+

+ N0

H

+

+

A mononitro-phenol o r c r e s o l may then n i t r a t e f u r t h e r to d i n i t r o or h i g h e r compounds. The N0 i o n u l t i m a t e l y becomes n i t r o u s a c i d ( i n the absence of urea) e i t h e r by:+

N0

+

+

N0 "

N 0

3

2

4

+

H0

HN0

2

3

+

HN0

2

or ArOH

+

N0

+

-*

Ar°" — • N N O HN0

3

Ar°" ^N0

2

+

HNO_ 2

This i s i n accord w i t h the o b s e r v a t i o n t h a t 1 mole o f n i t r o u s a c i d i s formed f o r each mole o f organic a c i d by-products.


INDUSTRIAL AND LABORATORY NITRATIONS


150

Figure 11. Production of nitrous acid during the nitration of various aromatic compounds

Ο

20

40

60

Mono-nit rotoluene

Feed : MNT+ by-products^

80 lOO time (min.)

I20

140

I60

Sodium phosphate solution

Liquid — Liquid Contactor

separated aq. phase

by-products + solvent

Liquid — Liquid Contactor

Figure 12. Flow-sheet for extrac tion of phenols by dissociation ex traction

Solvent

Sodium phosphate solution


8.

HANSON E T A L .

Side Reactions

During

Aromatic

Nitration

151

However, there i s no confirmatory evidence t h a t the nitronium i o n may r e a c t with an aromatic r i n g i n t h i s manner. An a l t e r n a t i v e and more l i k e l y e x p l a n a t i o n , f o r which we are g r a t e f u l to Dr. R.G. Coomb es(19) who f i r s t drew i t to our a t t e n t i o n , i s t h a t an additionr-elimination type o f mechanism a p p l i e s , s i m i l a r t o t h a t proposed by Blackstock e t al(20) i n the a u t o x y l a t i o n o f o-xylene by n i t r i c a c i d i n a c e t i c anhydride. In t h i s r e a c t i o n , intermediates p o s i t i v e l y i d e n t i f i e d were the c i s and trans isomers o f : CH3 ^ 2 -CH-> Downloaded by UNIV OF TEXAS EL PASO on November 8, 2014 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0022.ch008

N 0

Η Λ ΟOAc ί which decompose t o 4-acetoxy-o-xylene. Many s i m i l a r adducts are now known. T h i s type o f i p s o - n i t r a t i o n mechanism c o u l d be r e s p o n s i b l e f o r p h e n o l i c by-products during aromatic n i t r a t i o n ; r e a c t i o n by nitronium i o n a t the aromatic carbon to which the methyl group i s attached b e i n g f o l l o w e d by attack by water and then by the e l i m i n a t i o n of n i t r o u s a c i d : CH

3

N0

2

(i)

CH

H

3

N0

2

OH

Under homogeneous c o n d i t i o n s , t h i s i s b e l i e v e d t o occur(21): a t high a c i d i t y , (I) rearranges g i v i n g o - n i t r o toluene and a t low a c i d i t i e s the p - c r e s o l i s detected as:-

Removal of P h e n o l i c By-Products To f r e e the aromatic n i t r o products from the p h e n o l i c i m p u r i t i e s , two a l t e r n a t i v e procedures may be compared w i t h the e x t r a c t i o n process commonly used i n d u s t r i a l l y a t p r e s e n t . In t h i s , the organic product l a y e r a f t e r n i t r a t i o n i s f i r s t washed with water t o remove f r e e a c i d s and then washed with d i l u t e sodium hydroxide (usually 1 t o 4N, 15% e x c e s s ) . The separated aqueous phase i s n e u t r a l i s e d by excess mineral a c i d t o l i b e r a t e the phenols, 60% to 80% of which separate and may be removed. The aqueous phase i s s t i l l s a t u r a t e d with phenols and i s an obnoxious e f f l u e n t . The process also i n v o l v e s the continuous consumption of c a u s t i c soda and s t r o n g a c i d , with consequent high operating c o s t s .



152

INDUSTRIAL AND

LABORATORY

NITRATIONS

As a f i r s t a l t e r n a t i v e , the aqueous wash l i q u o r , a f t e r a c i d i f i c a t i o n as before, may be t r e a t e d with an organic s o l v e n t immiscible with water. This d i s s o l v e s the phenols and, a f t e r s e p a r a t i o n , the s o l v e n t may be d i s t i l l e d and r e c y c l e d . This type of e x t r a c t i o n process f o r phenols i s w e l l known but to i l l u s t r a t e i t s p o s s i b i l i t i e s , Table IV l i s t s measurements o f the a b i l i t y of 50 ml samples of various organic s o l v e n t s to remove p h e n o l i c compounds from 100 ml o f the aqueous phase obtained by a l k a l i n e e x t r a c t i o n o f MNT product, followed by a c i d i f i c a t i o n i n the usual way. The aqueous phase contained 0.4 g of phenols. The s o l v e n t with the h i g h e s t a f f i n i t y f o r the phenols i s MIBK, which removes 98% of the i m p u r i t i e s . Two contact stages with such a s o l v e n t should reduce the phenol content of the aqueous phase to an acceptable l e v e l . However, t h i s process would s t i l l i n v o l v e the continuous consumption o f s t r o n g base and a c i d p l u s some l o s s o f MIBK by s o l u t i o n . TABLE IV E f f i c i e n c y of Organic Solvents i n Removing Phenolic N i t r a t i o n By-Products Wt. of Phenols (g) % extracted ( t o t a l Extraction present:- 0.4 g)

Solvent

d i e t h y l ether chloroform methyl i s o b u t y l ketone (MIBK) cyclohexane carbon t e t r a c h l o r i d e dichloroethane benzene toluene

92 70 98 57 79 78 88 82

0.369 0.281 0.393 0.229 0.319 0.311 0.353 0.328

A more novel process was considered, based upon the process of d i s s o c i a t i o n e x t r a c t i o n u s i n g weakly a c i d i c o r b a s i c reagents which has been developed by some of the present authors(22,23). For the e x t r a c t i o n of p h e n o l i c by-products, the method i s based upon contact between the organic phase c o n t a i n i n g the a c i d i c phenols and an aqueous phase c o n t a i n i n g a weakly b a s i c reagent, such as a phosphate s a l t . There should be s u f f i c i e n t a f f i n i t y f o r r e a c t i o n t o r e s u l t i n e s s e n t i a l l y a l l the phenols being e x t r a c t e d i n t o the aqueous phase as t h e i r d i s s o c i a t e d s a l t s , i . e . the e q u i l i b r i a i n r e a c t i o n s (1) and (2) should be towards the r i g h t hand s i d e : -

Ο

Γ

ArOH

+

3P0„

±

ArO

-

ArOH

+

HPO^~

^

ArO~

+

2HP0„

(1)

+

H P0~

(2)

2



8.

HANSON E T A L .

Side Reactions

During

Aromatic

153

Nitration

The aqueous phase, a f t e r s e p a r a t i o n from the p u r i f i e d o r g a n i c product, i s then contacted with an organic s o l v e n t having a high a f f i n i t y f o r u n d i s s o c i a t e d phenols. The e x t r a c t i o n r e a c t i o n s (1) and (2) are then reversed i n the aqueous phase, regenerating the o r i g i n a l phosphate s a l t e x t r a c t a n t , which may be r e c y c l e d , w h i l e the l i b e r a t e d phenols are e x t r a c t e d i n t o the o r g a n i c phase. A f t e r s e p a r a t i o n , the o r g a n i c s o l v e n t phase i s d i s t i l l e d t o recover the phenols and p u r i f i e d organic s o l v e n t f o r r e c y c l e . The s e p a r a t i o n i s achieved, t h e r e f o r e , without the continuous consumption o f chemicals and the o p e r a t i n g costs should be reduced. S e v e r a l counter-current c o n t a c t stages w i l l be necessary to achieve both an e f f i c i e n t e x t r a c t i o n o f the phenols from the n i t r o product and a l s o t h e i r recovery from the aqueous e x t r a c t phase i n t o the o r g a n i c s o l v e n t . The p r i n c i p l e s o f the flow sheet are i l l u s t r a t e d i n Fig.12. The p h e n o l i c by-products of toluene n i t r a t i o n have p K values a t 25°C i n the range 4 t o 8, which may be compared w i t h the three p K values f o r o-phosphoric a c i d o f 2.12, 7.20 and 12.30. I f t r i s o d i u m phosphate i s used as the s e p a r a t i n g reagent, the e x t r a c t i o n o f phenols i s e s s e n t i a l l y complete but the e q u i l i b r i u m of r e a c t i o n (1) l i e s f a r over to the r i g h t hand s i d e , the phenols being much s t r o n g e r a c i d s than HP0^~. The r e a c t i o n i s t h e r e f o r e d i f f i c u l t t o reverse on contact with an organic s o l v e n t . A more balanced r e a c t i o n i s obtained with disodium hydrogen phosphate. However, H2PO4 i s a s t r o n g e r a c i d (pK = 7.20) than some o f the more weakly a c i d phenols, which w i l l not be e x t r a c t e d by t h i s reagent. In f a c t , the percentage e x t r a c t i o n o f phenols when t h i s s a l t was used alone was never g r e a t e r than 65%. Therefore, a mixture o f the two phosphate s a l t s was used c o n t a i n i n g the minimum p r o p o r t i o n o f sodium o-phosphate. A s u i t a b l e mixture, g i v i n g v i r t u a l l y complete e x t r a c t i o n o f the p h e n o l i c by-products, was an aqueous s o l u t i o n c o n t a i n i n g 64.2 g/1 of Na2HP0 .2H 0 with 21.9 g/1 of Na3P04.12H 0. Benzene and MIBK were found t o be e f f i c i e n t s o l v e n t s f o r the back e x t r a c t i o n o f the phenols. D i s t r i b u t i o n of the phenols, present i n v a r y i n g amounts, between t h i s aqueous phosphate s o l u t i o n and benzene or MIBK i s shown i n Table V and the number o f stages r e q u i r e d f o r a c o u n t e r - c u r r e n t back e x t r a c t i o n o f phenols may be obtained by a McCabe-Thiele type c a l c u l a t i o n . I t i s not necessary, of course, t o recover 100% o f the phenols from the aqueous phosphate stream b e f o r e i t may be r e c y c l e d . a

a

a

4

2

2

I n i t i a l economic comparison of the d i s s o c i a t i o n - e x t r a c t i o n type process with the chemical-consuming process o f c a u s t i c wash, a c i d i f i c a t i o n , and s o l v e n t e x t r a c t i o n i n d i c a t e s t h a t the d i s s o c i a t i o n e x t r a c t i o n process, which has lower o p e r a t i n g costs but may have h i g h e r c a p i t a l c o s t f o r contactors and a s s o c i a t e d equipment, may be l e s s c o s t l y o v e r a l l . A process o p t i m i s a t i o n and more d e t a i l e d economic assessment would have t o be made, however, b e f o r e a d e f i n i t e d e c i s i o n c o u l d be made between the two p r o c e s s e s .


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TABLE V D i s t r i b u t i o n of Phenolic By-Products between Aqueous Phosphate and Organic Solvent


By-products i n aqueous phase g/1 6.84 5.44 4.43 3.58 2.86 2.32 1.40

By-products i n MIBK g/1 1.98 1.38 1.02 0.85 0.65 0.54 0.28

By-products i n aqueous phase g/1 6.10 5.27 4.67 3.20 1.96

By-products i n Benzene g/1 1.18 0.83 0.60 0.36 0.15

Conclusions The main conclusions of t h i s i n v e s t i g a t i o n a r e : 1. The p r i n c i p a l by-products formed during the mixed-acid mononitration of toluene are n i t r o u s a c i d and n i t r a t e d p h e n o l i c compounds, of which 2 6 d i n i t r o p - c r e s o l c o n s t i t u t e s about 80%. 2. N i t r o u s a c i d and the organic acids by-product are formed i n equimolar q u a n t i t i e s , w i t h i n experimental e r r o r . 3. The r a t i o of by-products to mono-nitrotoluene i s reduced i f the s u l p h u r i c a c i d c o n c e n t r a t i o n i n mixed a c i d i s a h i g h as p o s s i b l e although s t i l l below the c o n c e n t r a t i o n at which d i - n i t r a t i o n commences. Increase i n s u l p h u r i c a c i d c o n c e n t r a t i o n from the normal 30 mole % t o 35 mole % decreases organic by-products by over 25%. 4. N i t r o - p h e n o l i c compounds and n i t r o u s a c i d are s i m i l a r l y formed during the n i t r a t i o n of other aromatic hydrocarbons. Of those s t u d i e d , the order from h i g h e s t to lowest i n p r o p o r t i o n of by-products to mono-nitro product was:- o-xylene, p-xylene, toluene, e t h y l benzene, mesitylene, m-xylene, benzene. 5. For removal of the n i t r o - p h e n o l i c by-products, a process of d i s s o c i a t i o n e x t r a c t i o n using a mixture of phosphate s a l t s i s v i a b l e and could be conomically p r e f e r a b l e to simple s o l v e n t e x t r a c t i o n by an organic s o l v e n t . f

Literature Cited 1. 2. 3. 4. 5. 6.

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8. HANSON ET AL.

7. 8. 9. 10. 11. 12. 13. 14.


15. 16. 17. 18. 19. 20. 21. 22. 23.

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Bennett,G.M. and Youle,P.V.,J.Chem.Soc.(1938) 887 Bennett,G.M. and Youle,P.V.,Nature (1938) 142,356 Bennett,G.M. and Youle,Ρ.V.,J.Chem.Soc. (1938) 1816 Dadak,V. and Seitl,J.,J.Chem.Prumsyl.(1959) 88,276 Dadak,V.,Seitl,J. and Smejkal,K.,Chem.Prumsyl.(1962) 12,69 Dadak,V. and Seitl,J.,Chem.Prumsyl.(1962) 12,352 Dadak,V.,Smejkal,K. and Balasova,E.,Chem.Prumsyl.(1963)13,241 Sohrabi,T.(née Kaghazchi),PhD Thesis, University of Bradford (1972) Olah,G.A.,Kuhn,S.J.,Flood,S.H. and Evans,J.C.,J.Amer.Chem. Soc.(1962) 84,3687 Bennett,G. and Grove,J.F.,J.Chem.Soc.(1945) 378 Bennett,G.M.,J.Proc.Roy.Inst.Chem.(1943) 107 Titov,A.I.,Zhur.Obshchei.Khim.(J.Gen.Chem.) (1952) 22,1329 Coombes,R.G., Department of Chemistry, The City University, London, private communication Blackstock,D.J. et al, Tet.Letters,(1970) 2793 Barnett,J.W.,Moodie,R.R.,Schofield,K. and Weston,J.Β., J.C.S. Perkin II (in the press) Anwar,M.M.,Hanson,C. and Pratt,M.W.Τ.,Proc.ISEC(1971) 911, Academic Press (London) Anwar,M.M.,Cook,S.T.M.,Hanson,C. and Pratt,M.W.T., Proc. ISEC (1974),Vol.1, 895, SCI(London)


Side Reactions During Aromatic Nitration - ACS Symposium Series

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