Some Aspects of Aromatic Nitration in Aqueous Systems - ACS

15 Some Aspects of Aromatic Nitration in Aqueous Systems C.

HANSON

and M. W. T. PRATT

School of Chemical Engineering, University of Bradford, Bradford, England

Downloaded by UNIV OF NEW SOUTH WALES on April 15, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0022.ch015

M. SOHRABI Institute of Chemical and Petrochemical Engineering, Tehran Polytechnic, Tehran, Iran

Aromatic nitration has been extensively studied and has played an important rôle in the development of theoretical organic chemistry. Most of the early published work, excellent reviews of which are available(1,2,3), has been concerned with nitration as a homogeneous liquid-phase reaction, usually with little or no water present. However, aromatic nitration is an important reaction in the chemical industry where it is commonly achieved by the use of an aqueous 'nitrating acid' mixture of 15 mole % nitric acid, 30 mole % sulphuric acid and 55 mole % water. The reaction is normally heterogeneous, the organic substrate and nitro product forming a separate phase from the aqueous acid. Published information about nitration in such aqueous systems has been relatively scant, although some attention has been given to them recently. Ingold and his co-workers(4) proposed and firmly established a scheme now generally accepted for the mechanism of aromatic nitration with nitric acid or mixed nitric and sulphuric acids as nitrating agents, typically, in homogeneous s o l u t i o n i n an organic s o l v e n t . I n t h i s the nitronium i o n , NO+2, i s t h e a c t i v e intermediate which r e a c t s w i t h the aromatic compound, ArH:HN0

+ H

3

H*N0

3

+

^±

H

2

N 0

3

(fast)

NO* + H 0 2

(1)

(slow)

(2)

(slow)

(3)

(fast)

(4)

+

+ H N0 + ArH ^ ± A ^ x r r ^ '2 ^ "NO' 9

ArNO„ + H

+

They a l s o i n v e s t i g a t e d the e f f e c t o f water on the r a t e and order o f the homogeneous r e a c t i o n system. F o r r e a c t i v e aromatic s u b s t r a t e s , the r e a c t i o n i n the absence o f water i s zero order i n 225

Albright and Hanson; Industrial and Laboratory Nitrations ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


226

INDUSTRIAL A N D LABORATORY

NITRATIONS

aromatic concentration. This i s because reaction (3) i s much faster than the reverse of reaction (2). Nitronium ions are therefore consumed by reaction with the aromatic substrate as fast as they are formed. The rate controlling step i s the production of nitronium ions by reaction (2), which i s independent of the concentration and nature of the reactive substrate. Ingold and co-workers (15) found that water addition decreased the rate of reaction and ultimately caused a change from zero to f i r s t order kinetics i n aromatic concentration. This was attributed to the effect of added water i n increasing the rate of the reverse of reaction (2). The rate of reaction of nitronium ions with water becomes comparable with or greater than the rate of their forward reaction with aromatic substrate, so that the reactivity of the substrate becomes important and the rate of formation of nitro product involves both the concentration of substrate and nitronium ion. Similar observations were also made by Hoggett, Moodie and Schofield(6) during their studies of the relative reactivities of aromatic compounds. The effect of water i n depressing nitronium ion concentration was shown directly by ChédinÇ7) from the Raman spectra of mixtures of n i t r i c acid, sulphuric acid and water, the Raman absorption band at 1400 cm"" being attributed to the nitronium ion. Chédin s data were used to construct the triangular diagram, Figure 1, i n which nitronium ion concentration i s shown as a function of composition of the aqueous acid mixture. The diagram confirms the decrease i n nitronium ion concentration with increase i n the proportion of water. The limit of detectability of nitronium ion l i e s close to the line of Hetherington and Masson(8) representing the limiting acid strength for dinitration to occur and the composition of the usual 'mixed a c i d used for industrial aromatic mononitrations l i e s i n the region where nitronium ion i s not detectable. Also shown on the diagram i s the band obtained by Butt(9) representing the acid composition limits to the right of which detectable heterogeneous mononitration occurs within 5 hours at 20°C. Butt found that, for a l l three substrates, benzene, toluene and chlorobenzene, the band shown represented the nitration l i m i t , allowing for experimental error of +_ 2 mole %. Although the relative reactivities of these substrates to aromatic nitration are very different, the detection limit of nitro product under the given experimental conditions i s reached within this narrow band of acid compositions, which represents acid mixtures with an aqueous proportion (approximately 90 mole %) far greater than those at which nitronium ions are spectroscopically detectable. The experimental work described here was designed chiefly to provide more information, relevant to the aqueous nitration systems used industrially, concerning the effect of water on change i n reaction order for a number of substrates and the 1

f

1


15.

HANSON ET AL.

Aromatic

Nitration

in Aqueous

Systems

227

l i m i t i n g a c i d compositions a t which t h i s o c c u r s . A study was a l s o made of the r e l a t i v e r e a c t i v i t i e s o f aromatic s u b s t r a t e s i n homogeneous aqueous systems without the presence o f organic solvents.


E f f e c t of Water A d d i t i o n upon Homogeneous N i t r a t i o n by N i t r i c Acid The e f f e c t of water a d d i t i o n on the n i t r a t i o n of toluene and some more r e a c t i v e aromatic hydrocarbons i n homogeneous s o l u t i o n i n a c e t o n i t r i l e o r a c e t i c a c i d was s t u d i e d a t 20°C. Anhydrous n i t r i c a c i d was used at f i r s t , no s u l p h u r i c a c i d being present, and then experiments were repeated w i t h p r o g r e s s i v e l y higher i n i t i a l c o n c e n t r a t i o n s o f water. N i t r i c a c i d was always i n l a r g e excess over the aromatic s u b s t r a t e c o n c e n t r a t i o n . For some experiments, fuming n i t r i c a c i d was used. Three d i f f e r e n t procedures were used t o f o l l o w the r e a c t i o n k i n e t i c s :For slow r e a c t i o n s with a h a l f l i f e o f more than 30 minutes, r e a c t i o n was c a r r i e d out i n 100 ml stoppered f l a s k s , immersed i n a thermostat with temperature c o n t r o l l e d t o +_ 0.1°C. Each f l a s k i n i t i a l l y contained n i t r a t i n g a c i d o f known composition. A f t e r thermal e q u i l i b r i u m had been e s t a b l i s h e d , a measured volume o f aromatic s u b s t r a t e , p r e v i o u s l y d i s s o l v e d i n organic s o l v e n t and immersed i n the same thermostat, was added to the n i t r a t i n g mixture. Samples were taken from the r e a c t i o n v e s s e l a t i n t e r v a l s , d i l u t e d with s o l v e n t , and the absorbance of a s u i t a b l e peak i n the U.V. o r v i s i b l e s p e c t r a of the n i t r o products was measured by a Beckman double-beam spectrophotometer. For f a s t e r r e a c t i o n s with a h a l f l i f e o f l e s s than 20 to 30 minutes, the procedure was the same, u n t i l a f t e r a d d i t i o n o f the s u b s t r a t e to the n i t r i c a c i d s o l u t i o n , f o l l o w i n g which the mixture was r a p i d l y shaken to achieve a homogeneous s o l u t i o n . P a r t was t r a n s f e r r e d to a 1cm s i l i c a c e l l and the r e a c t i o n followed a t an a p p r o p r i a t e wavelength i n the spectrophotometer, of which the sample compartment was maintained a t a c o n t r o l l e d temperature by the c i r c u l a t i o n through surrounding c o i l s o f water from a thermostat. N i t r a t i o n by fuming n i t r i c a c i d presented some d i f f i c u l t i e s , as the presence o f f r e e n i t r o u s fumes causes some absorbance which overlaps w i t h that of the 350nm peak o f n i t r o t o l u e n e . A c o r r e c t i o n was a p p l i e d t o the measured absorbance to allow f o r t h i s , based upon the known s p e c i f i c absorbance o f fuming n i t r i c a c i d a t the wavelength used. Zeroth-order r e a c t i o n s were u s u a l l y followed to completion and s t r a i g h t l i n e p l o t s o f absorbance a g a i n s t time obtained up to a t l e a s t 90% o f the r e a c t i o n . F i r s t o r mixed-order r e a c t i o n s were g e n e r a l l y followed to about t h r e e - q u a r t e r s completion and rate constants were determined from the p l o t of l o g (A - A )


INDUSTRIAL A N D LABORATORY

228

TABLE

NITRATIONS

I

E f f e c t o f water on the r a t e and order of the n i t r a t i o n o f aromatic hydrocarbons by n i t r i c a c i d i n organic s o l v e n t s a t 20°C


Reaction conditions

[Substrate] mole 1

Nitration of mesitylene by 7.41 mole l * * HN0 (a) i n acetonitrile

1

3

Nitration of m-xylene by 7.41 mole l ^ HN0 (b) i n acetonitrile

1

3

Nitration of toluene by 8.895 mole l " " HN0 (a) i n acetonitrile 3

Nitration of toluene by 7.41 mole l " " HN0 (a) i n acetic acid 3

1

1

[H 0] mole l " " 2

1

Order

0.0099 0.0171 0.0178 0.0198 0.0250 0.0171 0.0171 0.0171 0.0171 0.0171 0.0171

0 0 0 0 0 0.125 0.452 0.986 1.64 1.97 2.30

0 0 0 0 0 0 Mixed Mixed Mixed Mixed 1

0.0110 0.0304 0.0332 0.0304 0.0304 0.0304 0.0304 0.0304 0.0304

0 0 0 0.130 0.986 1.64 1.97 2.15 2.30

0 0 0 0 Mixed Mixed Mixed Mixed 1

0.0156 0.0217 0.0434 0.0434 0.0434 0.0434 0.0434 0.0434 0.0434

0 0 0 0.132 0.329 0.658 1.32 1.64 1.97

0 0 0 0 Mixed Mixed Mixed Mixed 1

0.0125 0.0221 0.0444 0.0444 0.0444 0.0444 0.0444

0 0 0 0.329 0.986 1.320 1.64

0 0 0 Mixed Mixed Mixed 1

1 -1 sec *o mole 1 or Jc^ sec

Some Aspects of Aromatic Nitration in Aqueous Systems - ACS

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