Industrial and Laboratory Nitrations

at *f5 -2 C. After the addition of the nitrating mixture was complete, the reactor ..... (63.8% para nitrochlorobenzene, 35·7% ortho o/p = Ο.56 and ...
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20 Isomer Control in the Mononitration of Toluene G. F. P. HARRIS

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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.

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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

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

INDUSTRIAL AND LABORATORY NITRATIONS

302

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

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• = 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)

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

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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·

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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

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

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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)

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

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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

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s

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c o n c e n

ρ

2

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E

β

S

Ο

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o

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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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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

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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

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

20.

HARMS

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72% P2O5 Phosphoric Acid

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30-

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

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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

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

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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

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

Mois

Mole



H0

P

0.2

9.1

2.5

0.95

9.9

1.17

10.0

1.17

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

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

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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

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2

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2

4

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5

2

4

2

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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

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

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310

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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.

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

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

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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 .

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

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.

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

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