Isomer Control in the Mononitration of Toluene

20 Isomer Control in the Mononitration of Toluene G. F. P. HARRIS

Downloaded by UNIV OF PITTSBURGH on May 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0022.ch020

Research Department, Imperial Chemical Industries Limited, Organics Division, Hexagon House, Blackley, Manchester 9, England

Introduction I t i s well known that nitration of aromatic molecules produces a number of isomeric products. According to literature data (1) the mononitration of toluene can yield products i n which the ratio of the orthonitrotoluene to paranitrotoluene varies between 1.41 and 1.62 depending on the nitrating agent and the reaction conditions used. These mononitrations are carried out industrially on a considerable scale to produce compounds which are required as intermediates for the manufacture of dyestuffs, pharmaceuticals etc. In most cases the para isomer i s required while the ortho and meta isomers are waste products. Thus with increasing costs of raw materials and environmental problems i n disposing of waste isomers, any increase i n the percentage of the desired isomer represents a considerable process improvement. A wide range of nitrating agents have been used i n laboratory scale experiments. Industrial scale nitrations, however, require a nitrating agent which is available i n quantity, reasonably cheap and relatively stable. The work described below i s an investigation of the isomer mixtures, one of which - sulphuric/nitric acid - has been i n industrial use for many years, and a second mixture - s u l p h u r i c / n i t r i c / phosphoric acid - which could be used for large scale n i t r a t i o n . Experimental A l l the nitrations were carried out on a laboratory scale i n a 700 ml glass reactor fitted with a s t i r r e r and cooling c o i l and mounted i n a thermostated bath. The cooled nitrating mixture was added slowly to the toluene with vigorous s t i r r i n g and the temperature of the reactor contents maintained at *f5 -2 C. After the addition of the nitrating mixture was complete, the reactor contents were stirred for a further one hour to complete the reaction. 300

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


20.

HARRIS

Mononitration

of

Toluene

301

The r e a c t i o n products were then cooled, washed with water and the organic l a y e r analysed by gas l i q u i d chromatography f o r unchanged s t a r t i n g m a t e r i a l , o r t h o , meta and para mononitro toluenes and f o r d i n i t r a t i o n products. I n the e a r l y stages o f the work a few experiments were c a r r i e d out with d i f f e r e n t r e a c t i o n temperatures and longer p e r i o d s o f s t i r r i n g a f t e r the completion o f a d d i t i o n . Increase i n the l e n g t h o f the p e r i o d o f s t i r r i n g had no e f f e c t on the composition o f the r e a c t i o n p r o d u c t s . Decrease i n r e a c t i o n temperature d i d tend to i n c r e a s e the amount o f para isomer produced, but i t a l s o decreased the r a t e o f r e a c t i o n , and any s i g n i f i c a n t i n c r e a s e i n the amount o f the para isomer could o n l y be obtained a t the expense o f a c o n s i d e r a b l e decrease i n the r a t e o f r e a c t i o n and consequently o f the output o f ρaranitrotoluene· I n view o f these experiments ^5 C was considered to be the optimum temperature f o r the n i t r a t i o n o f toluene and a l l f u r t h e r experiments were c a r r i e d out a t t h i s temperature. Results 1. S u l p h u r i c / n i t r i c a c i d mixtures. T h i s mixture has been used under v a r y i n g c o n d i t i o n s f o r i n d u s t r i a l n i t r a t i o n s f o r many years, and there i s a considerable amount o f l i t e r a t u r e data on the r e a c t i o n k i n e t i c s . Information on the r e l a t i o n s h i p between the composition o f the n i t r a t i n g mixture and the composition o f the r e a c t i o n product i s , however, s c a r c e and tends to be unsystematic. There are three v a r i a b l e s i n the composition o f the s u l p h u r i c / n i t r i c a c i d n i t r a t i n g mixture: ( i ) n i t r i c acid content, ( i i ) water content, ( i i i ) 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 . A few p r e l i m i n a r y experiments i n d i c a t e d that 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 d i d not a f f e c t the composition o f the r e a c t i o n product as long as there was s u f f i c i e n t o f i t to mononitrate a l l the toluene p r e s e n t . Hence a l l f u r t h e r experiments were c a r r i e d out with a s l i g h t excess o f n i t r i c a c i d - 1.0k mois HNO /mol o f t o l u e n e . A s e r i e s o f experiments was then c a r r i e d out^to i n v e s t i g a t e the e f f e c t o f the water and s u l p h u r i c a c i d concentrations i n the n i t r a t i n g mixture on the composition o f the r e a c t i o n products. The r e s u l t s o f these experiments are i l l u s t r a t e d by F i g s . 1 - 5 · F i g . 1 shows a p l o t o f r e a c t i o n product composition v s . mois HJSO^ ^ * t i £ mixture which contains too l i t t l e water Tor s a t i s f a c t o r y mononitration. E x c e s s i v e amounts o f d i n i t r a t i o n products are produced even at r e l a t i v e l y low HJSO^ concentrations, and consequently c o n s i d e r a b l e q u a n t i t i e s ox toluene remain u n n i t r a t e d . o r a

n

t r a

n


INDUSTRIAL AND LABORATORY NITRATIONS

302

1.04 Mois HN0 . „. , . .. rs m Mixture 0.63 Mois H2O 3


• = Toluene o=2 Nitro Toluene • = 3 Nitro Toluene • = 4 Nitro Toluene Δ = Dinitrotoluenes

Figure 1. Reaction product com position vs. sulphuric acid content of nitrating mixture

0.5

1.0

1.5 2.0 2.5 H2SO4 In Nitrating Mixture (Mois)

1.04 Mois 1.78 Mois

Reaction Products (WT %)

HNO3 H2O

60η

50

40

• = Toluene o=2 Nitro Toluene • = 3 Nitro Toluene • = 4 Nitro Toluene Δ = Dinitrotoluenes

30

20H

Figure 2. Reaction product com position vs. sulphuric acid content of nitrating mixture

1.1

1.2 1.3 1.4 H2SO4 In Nitrating Mixture (Mois)


20.

HARRIS

Mononitration

of

303

Toluene

F i g . 2 shows a more s a t i s f a c t o r y s i t u a t i o n where there i s more water (1.78 mois) i n the n i t r a t i n g mixture and r e a c t i o n products c o n t a i n i n g o n l y t r a c e s o f unchanged toluene and d i n i t r a t i o n products can be obtained. F i g . 3 shows the e f f e c t o f v a r i a t i o n i n water content i n a n i t r a t i n g mixture c o n t a i n i n g 1.17 mois s u l p h u r i c a c i d - the optimum H^SO^ c o n c e n t r a t i o n i n F i g . 2. In a l l these experiements the amounts o f ortho and p a r a n i t r o toluenes, unconverted toluene and d i n i t r a t i o n products v a r i e d with water o r 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 , but apart from extreme c o n d i t i o n s , ( F i g . l ) the amount o f metanitrotoluene remained constant a t about k%. F i g s , k and 5 show the e f f e c t o f v a r i a t i o n i n s u l p h u r i c a c i d and water content o f the n i t r a t i n g mixture on the o/p isomer r a t i o o f the mononitrotoluenes i n the r e a c t i o n products. O/p isomer r a t i o 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 content o f the mixture and i n c r e a s e s with i n c r e a s e i n water c o n c e n t r a t i o n . Examination o f F i g . 1 shows that some o f t h i s decrease i n o/p r a t i o 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 s due to the f a c t that the ο-nitrotoluene d i n i t r a t e s more r e a d i l y than the para isomer. Thus the i n c r e a s e i n the amount o f para isomer which can be produced by v a r y i n g the composition o f the n i t r a t i n g mixture i s l i m i t e d i f excessive amounts o f d i n i t r a t i o n products are t o be avoided. I n f a c t the optimum c o n d i t i o n s f o r mononitration are shown i n F i g . 2 - a n i t r a t i n g mixture c o n t a i n i n g 1.17 mois s u l p h u r i c a c i d and 1.78 mois water g i v i n g a r e a c t i o n product c o n t a i n i n g 36.5# ρ-nitrotoluene, 59·5$ ο-nitro toluene and *f.0# o f the meta isomer, o/p = 1.63·


t

2. N i t r a t i o n i n the presence o f phosphoric a c i d . L i t e r a t u r e data ( l ) i n d i c a t e d that n i t r a t i o n i n t h e p r e s e n c e o f phosphoric a c i d i n c r e a s e d the amount o f p - n i t r o t o l u e n e produced, hence a s e r i e s o f experiments was c a r r i e d out using n i t r a t i n g mixtures c o n t a i n i n g t h i s a c i d . Concentrated phosphoric a c i d i s a complex mixture o f the o r t h o , pyro and meta a c i d s (2), hence the phosphoric a c i d content o f the n i t r a t i n g mixture was expressed i n terms o f mois ^2^5* I n i t i a l experiments were c a r r i e d out u s i n g a n i t r a t i n g mixture o f j u s t phosphoric and n i t r i c a c i d s . Three s t r e n g t h s o f phosphoric a c i d were used, 65.2# P O (ÇX# HJO. ) which i s a v a i l a b l e commercially, and 72# Ρ 0 Γ99·2# Η PC ^) and 78% P 0 (107·5% **JP0^) which were prepared by adding e x t r a phosphorus pentoxide t o the commercial a c i d . I n each experiment 1.0^ mois 93% n i t r i c a c i d (per mol o f toluene) was added to a known amount o f phosphoric a c i d and the n i t r a t i o n c a r r i e d out as d e s c r i b e d above. The r e s u l t s o f these experiments are shown i n F i g s . 6-8 i n c l u s i v e , where the composition o f the r e a c t i o n products i s p l o t t e d against mois 2 ° 5 * n i t r a t i n g mixture f o r each o f the three phosphoric a c i d s used. p

f

2

P

nt

h

e


INDUSTRIAL A N D LABORATORY

304

NITRATIONS


1.04 Mois 1.17 Mois

Figure 3. Reaction product composition vs. water content of nitrating mixture

1.0 2.0 3.0 4.0 Water Content of Nitrating Mixture (Mois)

1

Figure 4. o/p isomer ratio vs. sulphuric acid content of nitrating mixture

HNO3 H2SO4

0

1 1.01 1.51 2.01

0.5

H2SO4

H

1

2.5 3.0 In Nitrating Mixture (Mois)


20.

HARRIS

Mononitration

of

305

Toluene

Examination o f these f i g u r e s shows that none o f the n i t r a t i n g mixtures gives s a t i s f a c t o r y n i t r a t i o n . The 65.2% P 2°5 i d g i v e s a very slow r e a c t i o n with excessive amounts o f unchanged toluene, while the 72# p 2 ° 5 * produces considerable amounts o f d i n i t r a t i o n products i f s u f f i c i e n t phosphoric a c i d i s added to ensure complete n i t r a t i o n o f the toluene. The 7&% P 2°5 P P i ^ produces excessive amounts o f d i n i t r a t i o n ^ e v e n a t low ^2^5 trations. T h i s was p a r t l y due to the v i s c o u s nature o f t h i s m a t e r i a l , which gave r i s e to d i f f i c u l t i e s i n mixing the reagents and to local overnitration. As i n the experiments with s u l p h u r i c / n i t r i c a c i d mixtures the amount o f metanitrotoluene was always about k%. F i g . 9 shows that f o r each o f the three phosphoric a c i d systems used o/p isomer r a t i o decreases with i n c r e a s e i n the amount o f phosphoric present i n the n i t r a t i n g mixture, with the more marked decrease o c c u r r i n g f o r the two s t r o n g e r phosphoric a c i d s (72 and 78% 5^ *amination Figs. 6 - 8 shows that t h i s decrease i n o/p isomer r a t i o i s due p a r t l y to an i n c r e a s e i n the amount o f p a r a n i t r o t o l u e n e produced and p a r t l y to the f a c t o r t h o n i t r o t o l u e n e d i n i t r a t e s more r e a d i l y than the para isomer. As mentioned above, no mixture o f phosphoric and n i t r i c a c i d s could be found to g i v e the d e s i r e d c o n d i t i o n s o f a product free from both unchanged toluene and d i n i t r o t o l u e n e s , but i t can be seen from F i g . 6 that the presence o f phosphoric a c i d i n the n i t r a t i n g mixture does reduce the o/p isomer r a t i o even when no d i n i t r a t i o n products are formed. The d i f f e r e n c e between the three p h o s p h o r i c / n i t r i c a c i d systems used i s the amount o f water p r e s e n t . Hence a second s e r i e s o f experiments was c a r r i e d out to i n v e s t i g a t e both the e f f e c t o f water and the a d d i t i o n o f s u l p h u r i c a c i d to the n i t r a t i n g mixture. The phosphoric a c i d s used before were employed i n these experiments and a s l i g h t excess o f n i t r i c a c i d - 1.0*+ mols/mol toluene was added i n each case. The concentration o f s u l p h u r i c a c i d was v a r i e d and the amount o f water present i n the n i t r a t i n g mixture was c o n t r o l l e d by v a r y i n g the s t r e n g t h o f the n i t r i c a c i d used and adding the s u l p h u r i c a c i d as e i t h e r 9&% 2 ° k S sulphuric acid. I t was found that the p h o s p n o r i c / n i t r i c / s u l p h u r i c a c i d mixtures gave s i m i l a r curves to those obtained p r e v i o u s l y , F i g s . 2, 8, with product compositions v a r y i n g with the amount o f s u l p h u r i c a c i d , phosphoric a c i d and water present i n the n i t r a t i n g mixture. An i n c r e a s e i n the amount o f phosphoric a c i d (as mois P 2°5^ present gave a decrease i n the o/p isomer r a t i o , while i n c r e a s e i n the amount o f water present had the opposite e f f e c t on isomer r a t i o . I f too l i t t l e water was present, however, excessive amounts o f d i n i t r a t i o n product were produced. Increase i n the amount o f s u l p h u r i c a c i d present a l s o decreased o/p isomer r a t i o , but excessive amounts l e d to o v e r n i t r a t i o n o f the toluene. a c

a c

n

o

s

n

o

r

c a c

d

d


c o n c e n

ρ

2

0

H

E

β

S

Ο

Γ

o

f

f u m i n



INDUSTRIAL A N D LABORATORY

Figure 5.

NITRATIONS

o/p isomer ratio vs. water content of nitrating mixture (H SQ content 1.17 Mois) 2

Reaction Product (WT %)

50η

40H

4

65.2% P2O5 Phosphoric Acid

• = Toluene o = 2 Nitro Toluene • = 3 Nitro Toluene • = 4 Nitro Toluene Δ = Dinitrotoluenes

30H

• 0A

A

r

1.0 2.0 3.0 4.0 Phosphoric Acid Content of Nitrating Mixture (Mois P 0 ) 2

Figure 6.

5

Reaction product composition vs. phosphoric acid content of nitrating mixture


20.

HARMS

Mononitration

of

307

Toluene

72% P2O5 Phosphoric Acid


30-

• = Toluene o = 2 Nitro Toluene • = 3 Nitro Toluene • = 4 Nitro Toluene Δ = Dinitrotoluenes

20

1.0 2.0 3.0 4.0 5.0 Phosphoric Acid Content of Nitrating Mixture (Mois P2O5)

Reaction Product (WT %) 50

Figure 7. Reaction product composition vs. phosphoric acid content of nitrating mixture

78% P2O5 Phosphoric Acid • = Toluene o = 2 Nitro Toluene • = 3 Nitro Toluene • = 4 Nitro Toluene Δ= Dinitrotoluenes

1.0 2.0 3.0 4.0 5.0 Phosphoric Acid Content of Nitrating Mixture (M0ISP2O5)

Figure 8. Reaction product composition vs. phosphoric acid content of nitrating mixture


INDUSTRIAL AND

308

LABORATORY

NITRATIONS

C e r t a i n n i t r a t i n g mixtures gave products which were f r e e from a p p r e c i a b l e q u a n t i t i e s o f both unchanged toluene and d i n i t r a t i o n products and a l s o gave more p - n i t r o t o l u e n e than obtained with the optimum s u l p h u r i c / n i t r i c a c i d mixture. The compositions o f these n i t r a t i n g mixtures are shown i n Table I , along with that f o r the best s u l p h u r i c / n i t r i c a c i d mixture. Table I Optimum N i t r a t i n g Mixtures


N i t r a t i n g Mixture Comp. Mois

Mois

Mole

"Λ

H0

P

0.2

9.1

2.5

0.95

9.9

1.17

10.0

1.17

2

2°

Reaction Product

ο-nitro p - n i t r o m-nitro Toluene o/p toluene toluene toluene and D i n i t r o toluenes

5

%

%

%

53.5

40.9

4.8

0.8

1.31

2.5

55.0

40.8

4.2

0

1.35

2.3

55.7

40.3

4.0

0

1.38

0

59.5

36.5

4.0

0.2

1.63

%

1.78

Composition

Discussion The above experimental r e s u l t s i n d i c a t e t h a t a d d i t i o n o f phosphoric a c i d to the n i t r a t i n g mixture does i n c r e a s e the amount o f para isomer produced ( F i g u r e 9)· Where a p p r e c i a b l e amounts o f d i n i t r a t i o n occur, however, the p o s i t i o n i s complicated by the f a c t that the o r t h o n i t r o t o l u e n e d i n i t r a t e s more r e a d i l y than the p a r a isomer and so the apparent e f f e c t o f phosphoric a c i d a d d i t i o n i s enhanced by conversion o f some o f the ortho isomer to d i n i t r o t o l u e n e s . Two other v a r i a b l e s a f f e c t the isomer r a t i o o f the product 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 and water content o f the n i t r a t i n g mixture. F i g . 10 shows a comparison o f the e f f e c t o f the three v a r i a b l e s - 2 ° 5 * ^2*®k water content o f the n i t r a t i n g mixture on isomer r a t i o o f the product f o r two systems c o n t a i n i n g phosphoric a c i d and one mixture o f s u l p h u r i c and n i t r i c acid. The r e g i o n where no a p p r e c i a b l e ( l e s s than 0.5%) d i n i t r a t i o n products were produced i s a l s o shown. The adoption o f a d i f f e r e n t convention to c a l c u l a t e the water content o f the n i t r a t i n g mixture (e.g. assuming t h a t the phosphoric a c i d i s a l l Η PO^ and not ° 5 ^ s h i f t the phosphoric a c i d l i n e s to the l e f t , but i t i s c l e a r t h a t a d d i t i o n o f phosphoric to the n i t r a t i n g mixture does reduce the o/p isomer r a t i o o f the product without producing unacceptable amounts o f d i n i t r a t i o n products. P

0 1 1 ( 1

p

c

o

u

l

d

2


20.

HARRIS

Mononitration

of Toluene

309


ο = 65.2% P2O5 Phosphoric Acid Δ=72.0% P2O5 Phosphoric Acid • = 78.0% P2O5 Phosphoric Acid

1.0 2.0 3.0 4.0 Phosphoric Acid Content of Nitrating Mixture (Mois P2O5)

o/p 1.8Isomer Ratio 1.7

NO 1.0 1.0 2.5 2.5

Figure 9. o/p isomer ratio vs. phosphoric acid content of ni trating mixture

P 0 Mois Mois Mois Mois 2

5

; P P P P

1.17 Mois H S 0 0 ;NO H S 0 0 ; 1.17 Mois H S 0 0 ; NO H S 0 0 ; 1.17 Mois H S 0 2

2

2

2

2

5

4

2

4

2

4

5

5

5

2

4

2

4

Dinitration products > 0.5%

4.0

10.0 11.0 12.0 5.0 6.0 7.0 8.0 9.0 Water Content of Nitrating Mixture (Mois H 0 ) 2

Figure 10.

o/p isomer ratio vs. water content of nitrating mixture



310

INDUSTRIAL AND

LABORATORY

NITRATIONS

T h i s r e d u c t i o n i n o/p isomer r a t i o i s l i m i t e d , however, to a minimum value o f about 1.3^· The presence o f s u l p h u r i c a c i d does tend to decrease the o/p isomer r a t i o s l i g h t l y , but i t s e f f e c t i s l i m i t e d and excessive amounts produce q u a n t i t i e s o f undesirable d i n i t r a t i o n products ( F i g . l ) . I t s main advantage i s to i n c r e a s e the amount o f conversion so that l a r g e q u a n t i t i e s o f u n n i t r a t e d toluene do not remain a t the end o f the r e a c t i o n . Considerable v a r i a t i o n i n the p r o p o r t i o n s o f the i s o m e r i c r e a c t i o n products from the mononitration o f toluene have been reported by a number o f workers, notably Tsang, P a u l and D i Giamio ( l ) f o r n i t r a t i o n i n the presence o f phosphoric a c i d , Olah and Kuhn (j>) f o r n i t r a t i o n by nitronium s a l t s , and Wright, T e i p e l and Thoennes (k) f o r n i t r a t i o n i n the presence o f sulphonic a c i d i o n exchange r e s i n s * A l l these workers a t t r i b u t e the v a r i a t i o n s i n o/p isomer r a t i o i n the n i t r a t i o n products to the e f f e c t o f s t e r i c hindrance. The change i n isomer r a t i o produced by the presence o f phosphoric a c i d i s probably due to t h i s e f f e c t . Thus as p o s t u l a t e d by Tsang et a l . ( l ) , i t i s l i k e l y that the a t t a c k i n g s p e c i e s i n the n i t r a t i o n r e a c t i o n i s not the normal nitronium i o n , but a bonded nitronium i o n formed from the polymeric phosphoric a c i d and n i t r i c a c i d . The s t e r i c bulk o f t h i s species together with the bulk o f the methyl group o f the toluene would favour s u b s t i t u t i o n i n the para p o s i t i o n r a t h e r than o r t h o . Fialkov and Tarasenko (£) found t h a t the e l e c t r i c a l c o n d u c t i v i t y and v i s c o s i t y data o f p h o s p h o r i c / n i t r i c a c i d mixtures i n d i c a t e d the two a c i d s d i d form a 1 : 1 complex, supporting the i d e a o f a complexed nitronium i o n . Chlorobenzene i s a l s o mononitrated on a l a r g e s c a l e i n i n d u s t r y and, as i n the case o f toluene, the p a r a n i t r o c h l o r o benzene i s the isomer which i s i n demand while the ortho isomer has few uses. U n l i k e toluene, n i t r a t i o n with a H^SO^/HNO mixture g i v e s a mixture with a preponderance o f the para isomer (63.8% para nitrochlorobenzene, 35·7% ortho o/p = Ο.56 and o n l y a t r a c e o f the meta isomer). However, any i n c r e a s e i n the amount o f the para isomer produced i s important, and so a few experiments on n i t r a t i o n with mixtures c o n t a i n i n g phosphoric a c i d were c a r r i e d out and the r e s u l t s are shown i n Table I I together with data f o r the n i t r a t i o n o f chlorobenzene with the optimum s u l p h u r i c / n i t r i c a c i d mixtures.


HARRIS

20.

Mononitration

of Toluene

311

Table I I Nitration of

Reaction Product

Mixlure^ioS p. Mois Mois Mois H SO. 2 4 2°5


P

Chlorobenzene

Composition

Chloro 4 N i t r o 5 N i t r o 2 N i t r o D i n i t r o chloro benzene chloro chloro chloro benzene benzene benzene benzene (%) {%)

o/p

0

10.6

2.5

21.9

46.7

0

31.4

0

0.67

0

3.1

1.0

7.1

55.5

0

37.5

0

0.68

0

7.5

2.5

0.7

56.6

0

42.7

0

0.75

0

6.3

2.5

0

58.7

0

41.3

0

0.70

1.11

7.6

2.5

0

57. 4

0

42.1

0.3

0.73

1.50

7.7

2.5

0

57.8

0

41.4

0.7

0.72

1.20

0.93 0

OA

63.8

0

35.7

0.1

0.56

In view o f the i n c r e a s e d amount o f para isomer produced by the n i t r a t i o n o f toluene i n the presence o f phosphoric a c i d , the r e s u l t s i n Table I I seemed s u r p r i s i n g . However, Sparks (6) found that n i t r a t i o n o f chlorobenzene with n i t r i c a c i d / a c e t i c anhydride mixtures gave a lower o/p isomer r a t i o than when n i t r i c / s u l p h u r i c a c i d was used, while toluene y i e l d e d e s s e n t i a l l y the same o/p r a t i o with each n i t r a t i n g mixture. Sparks considers that t h i s was due to the halogen s u b s t i t u e n t i n d u c i n g a p o s i t i v e charge a t the ortho p o s i t i o n ( r e l a t i v e to the para p o s i t i o n ) . I n t e r a c t i o n o f t h i s charge with the nitronium i o n i s enhanced i n s o l v e n t s o f low d i e l e c t r i c constant ( a c e t i c a c i d as opposed to s u l p h u r i c a c i d ) , hence ortho s u b s t i t u t i o n i s correspondingly decreased i n low d i e l e c t r i c constant s o l v e n t s ( a c e t i c anhydride). I t i s p o s s i b l e that a s i m i l a r mechanism a p p l i e s i n the above experiments with the p h o s p h o r i c / n i t r i c a c i d mixture having a higher d i e l e c t r i c constant than a mixture o f s u l p h u r i c and n i t r i c a c i d s .


312

INDUSTRIAL AND LABORATORY NITRATIONS

Literature Cited Tsang, Paul and Di Giamio, J . Org. Chem. (1964), 29, 3387. Bell, Ind. Eng. Chem., (1948), 40, (8), 1464. Olah and Kuhn, J.A.C.S. (1962), 84, 3684. Wright, Teipel and Thoennes, J . Org. Chem. (1965), 30, 1301. Fialkov and Tarasenko, Zh. Neorgan. Khim. (1966), 11(3), 619 Sparks, J . Org. Chem. (1966), 31, 2299.


(1) (2) (3) (4) (5) (6)


Isomer Control in the Mononitration of Toluene

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