I
KENNETH A. KOBE and JAMES L. LAKEMEYER Department of Chemical Engineering, University of Texas, Austin, Tex.
Effect of Nitrosylsulfuric Acid on Rate of Mononitration of Toluene I
To determine the effect of nitrosylsulfuric acid on the rate of toluene mononitration, a series of nitrations was made with six acids containing 25 mole sulfuric acid, 15 mole % nitric acid, water, and 0 to 9 mole % nitrosylsulfuric acid. At low nitrosylsulfuric acid concentrations nitration rate increased with increasing concentration up to 4.5 mole %, then decreased at higher concentrations of nitrosylsulfuric acid. Nitration rates with a mono mixed acid produced at the Joliet Arsenal were the same as with a synthetic mono mixed acid.
70
tive nitrating agent. The concept of the nitronium ion was first suggested by Euler in 1903, but it is largely because of the work of the Ingold-Hughes group (5-8, 70) that the theory has been accepted. The nitration of aromatics regarded as an electrophilic displacement reaction in which the nitronium ion becomes attached to a carbon atom in the aromatic nucleus and the hydrogen atom is simultaneously expelled as a proton : CH3
CH 3
To determine the mechanism of the formation of the nitronium ion Hughes, Ingold, and Reed (70)observed the kinetic effects of added solutes-sulfuric acid, potassium nitrate, and water-on
zero-order and first-order rates of nitration, Their observations provided the first definite demonstration of the formation of the nitric acidium ion, HzNOa+. The following equations express the mechanism : In sulfuric acid medium: "0s HzS04 ;:2 HzNOs' HSO4H2Nos+ ?:; NO2+ HzO In absence of sulfuric acid : "0s ;2 H + NosHNOs H + ;:t HzNOs' &Nos+ i:; NOz+ HzO Addition of sulfuric acid greatly accelerates nitration in aqueous solution. The kinetics of the reaction suggest that this effect is due not only to the increased concentration of nitronium ions but also to direct attack by nitric acidium ions (2).
+
+
+
+
+
+
Comma
AL nitration of toluene to trinitrotoluene is made in three batch nitrations, one nitro group being introduced into the aromatic nucleus a t a time. Flow of acids is countercurrent to flow of oils. Spent acid from the trinitration stage is fortified with nitric acid to make the mixed acid for the dinitration stage; spent acid from this stage is fortified with nitric acid to make the mixed acid for the mononitration stage. During trinitration sufficient nitrosylsulfuric acid is formed by oxidation of organic material to constitute approximately 17 weight yoof the tri-waste acid. Therefore, the mixed acid employed in mononitration is not a simple mixture of sulfuric acid, nitric acid, and water, but contains from 14 to 17 weight % of nitrosylsulfuric acid. T o study the effect of nitrosylsulfuric acid on the rate of mononitration of toluene, six acids were prepared, each containing 25 mole % sulfuric acid, 15 mole yonitric acid, and water. The first acid contained no nitrosylsulfuric acid, and the rate data obtained with it formed a basis for comparing the rates obtained with the other acids and evaluating the effects of their nitrosylsulfuric acid content. The other acids are identified by their nitrosylsulfuric acid content: 1.5, 2.0, 4.5, 6.0, and 9.0 mole yo.
Mechanism of Aromatic Nitration
In sulfuric acid the nitronium ion, NOz+, has been established as the effec-
F V
Figure 1.
Rate of nitration increases with increase in space velocity VOL. 50, NO. 11
NOVEMBER 1958
169 1
Figure 2 . Rate of nitration i s a t a maximum a t a b o u t 4.0 mote yonitros y I su I f u r i c acid
0
2
I
3
4
5
6
M O L E */- "OS04
The Ingold group has found that nitration can take place as the result of an elec. trophilic attack by the nitrosonium ion, NO+, or a precursor of this, forming the nitroso compound, which is then oxidized to the nitro compound by nitric acid ( 2 ) . This mechanism may be represented by:
CH 3 ()--NO
+ "01
portance only with easily nitrated molecules. Thus there are two mechanisms by which a nitro group may enter an aromatic nucleus-an electrophilic attack by the nitronium ion or by the nitrosonium ion with subsequent oxidation of the nitroso compound by nitric acid. The attack by the nitronium ion is the more powerful. A precursor of the nitronium ion, the nitric acidium ion, may also attack the aromatic nucleus, but the contribution of this mechanism is of less importance than nitrcnium ion nitration.
---* Preparation of Acids
The nitrosylsulfuric acid was prepared by treating cold fuming nitric acid with sulfur dioxide "01
The nitrosonium ion may then be regenerated by protonation of the nitrous acid and breakdown of the nitrous acidium ion formed : HNOz
+ H+;:t
HzNOz'
HzNOz' !i:; N O +
4- HzO
Much of the experimental work was done with p-chloroanisole, but the mechanism appears to be almost universal, However, the nitrosonium ion is a much weaker nitrating agent than the nitronium ion and the mechanism of nitrosonium ion nitration is of practical im-
1 692
+ SO% ;f.
NO+HSO;
by charging a 3-liter reaction flask with approximately 500 ml. of 9270 nitric acid, immersing the flask in crushed ice, and dispersing bubbles of sulfur dioxide through the acid. Soon after the reaction was started, red oxides of nitrogen filled the reactor. One opening of the flask was left unstoppered to permit gases to escape, and the flow of sulfur dioxide was regulated so that the escaping oxides of nitrogen were barely visible above the opening. The reaction was considered complete when oxides of nitrogen were no longer visible with an increased flow of
INDUSTRIAL AND ENGINEERING CHEMISTRY
sulfur dioxide. The charge then had the appearance of a heavy suspension of white solid in the acid, which was largely sulfuric acid. The mixed acid was prepared from 92% nitric acid, 9691, sulfuric acid, distilled water, and nitrosylsulfuric acid. Sulfuric acid was first added to the water, then cooled to room temperature, and the nitric and finally the nitrosylsulfuric acids were added. Before weighing the nitrosylsulfuric acid it was necessary to filter the slurry and drain as much acid as possible from the crystals. The acid containing no nitrosylsulfuric acid could easily be made up so that each component was within 0.25 weight 91, of the desired values. This was not the case with the other acids. The crystals of nitrosylsulfuric acid carried some sulfuric acid, and when they were added to the mixed acid some of the nitrosylsulfuric acid probably reacted with water in the acid, forming sulfuric acid and nitrogen trioxide. After the acid was analyzed, the composition was adjusted and again the acid was analyzed. Analysis of the acid required determinations of total acidity, total sulfuric including sulfuric acid formed from the nitrosylsulfuric acid, and nitric acid (73). Nitrosylsulfuric acid was calculated and water obtained by difference. Nitrosylsulfuric acid was determined directly by titration with a potassium per. manganate solution, before the first complete analysis of an acid to adjust the concentration of this component, because an excess of the crystals of nitrosylsulfuric acid always had to be added. Nitrosylsulfuric acid was also determined directly during the final analysis of several acids. The agreement was usually within 0.1 5 weight % of the value obtained indirectly. The analytical methods are essentially those of Scott (4,73). Equipment and Operation
The reactor was the miniature 1.26-ml. jacketed nitrator described by Brennecke and Kobe (3). Rates of nitration obtained with this reactor approximately doubled with each 10" C. rise in temperature, a good indication that the rates measured are true chemical reaction rates. Agitation is provided hy a fourbladed turbine stirrer, machined out of a single piece of stainless steel, which is driven at a speed in excess of 20,000 r.p.m. The reaction is quenched immediately as the reaction mixture leaves the reactor. Operation requires that the flow of reactants and the temperature be established and maintained a t the desired values long enough to ensure steady-state conditions before a sample is collected. When an experiment was started, the flow of cooling water was adjusted to a rate somewhat less than that required to maintain the desired reaction tempera-
TOLUENE N I T R A T I O N ture. The stirrer was turned on and the flow of reactants started. As the temperature approached the value at which it was to be controlled, the flow of cooling water was increased gradually to slow the rate of temperature rise. After the temperature had become stabilized a t the desired value, the product stream was switched to a sample receiver. Determination of the conversion and rates was based on the fraction of mononitrotoluene in the organic product. Separation and recovery of the product were accomplished by steam distillation. The volume of toluene collected was measured at 32OC. to 0.1 ml.; theweight of mononitrotoluene was determined to 0.1 gram. The separation was sharp, and any error due to intermingling of the compounds was well within the limits of these measurements.
.
A series of runs was made with each acid, only space velocity being varied. The temperature was held a t 35' C. and the ratio of the volume of acid to the volume of toluene was 2 to 1. A number of investigators (7, 9, 72) have reported that nitration takes place in the acid phase only. Therefore the rates have been reported p e r m i t volume of acid phase : =
42
38
34
d
SYNTHETIC MONO M I X
30
CO 26
22
18
Data and Results
R,
46
gram-moles of MNT produced hour-liter of acid phase
Brennecke and Kobe (3) report that the rate of nitration of toluene appears to to proportional to the mole fraction of toluene in the effluent organic phase, XT, for acids containing no more than 30 mole % sulfuric acid and 15 mole % nitric acid. This suggests that R J X , is the rate that would be obtained if the organic phase were pure toluene. Those acids producing higher rates at a particular space velocity are also producing higher conversions. Therefore, a plot of R J X T , which is independent of the organic phase composition, gives a more accurate picture of the relative nitrating strength of the acids than a plot of
-
R,. The plot of rates as a function of space velocity is shown in Figure 1. At steady state the compositions of the acid and the organic phases are constant. If space velocity is increased, conversion will decrease, and a new steady state will be reached in which the acid phase is composed of a higher concentration of nitric acid and less water. Thus the higher the space velocity the more nearly alike are the feed acid and the acid at equilibrium in the reactor, and the more rapid is the conversion. The rates obtained with the various acids increased with increasing concentration of nitrosylsulfuric acid u p to 4.5
14
IO
0
2
4
s
8
IO
12
14
16
18
F V
Figure 3. Toluene conversions at high space velocity fall off sharply at concentration of nitrosylsulfuric acid above 4.5 mole %
mole % and then decreased at higher concentrations. T o illustrate this and to define the maximum, the rates are plotted as a function of the concentration of nitrosylsulfuric acid at various space velocities in Figure 2. The maximum occurs at about 4.0 mole % nitrosylsulfuric acid. Rate data obtained with the acid con-. taining 9.0 mole % nitrosylsulfuric acid scattered considerably on some plots and could not be correlated satisfactorily with data obtained with the other acids. During the nitration runs made with this acid fumes of nitrogen oxide were conspicuous, indicating the nitrosylsulfuric acid and water were reacting to form sulfuric acid and nitrogen trioxide. The behavior of this acid can be explained by referring to Figure 3. At high space velocities the conversions, and therefore the rates, are lower than those obtained with the acid containing no nitrosylsulfuric acid. This could be expected, because the rates fall off sharply as the concentration of nitrosylsulfuric acid increases above 4.5 mole yo. However, as the space velocity is decreased, the conversion increases more rapidly than the conversion increase obtained with the other acids, and a t low space velocities is considerably higher than that obtained with acid containing no nitrosylsulfuric acid. At low space velocities a higher percentage of the nitrosylsulfuric acid ~
breaks down than a t high space velocities, because the acid remains in the reactor for a longer time and more water is present as a result of higher conversion. The slopes of the conversion curves indicate that even at the highest space velocities at which the data had been taken the rates obtained with the 9.0% acid were affected by the breakdown of nitrosylsulfuric acid. When the rates as a function of organic conversion are compared, as shown in Figure 4, the effect of the concentration of nitrosylsulfuric acid appears to be much greater than a t constant space velocity. An acid which produces a high rate a t a particular space velocity exists in a steady state in which the concentrations of toluene and nitric acid are lower than the concentrations existing in the equilibrium reached by a weaker acid a t the same space velocity. The plot of rate us. organic conversion illustrates the relative nitrating strength of the acids a t the same mole concentration of nitric acid. T o determine whether an acid composed of sulfuric acid, nitric acid, water, and nitrosylsulfuric acid contains all the components of a production acid which affect nitration rates, a set of rate data was obtained for a mixed acid for mononitration supplied by the Joliet Arsenal and another set was obtained for a synthetic acid of the same composition as the VOL. 50, NO. 1 1
0
NOVEMBER 1958
1693
I100
I
taken place as a result of an electrophilic attack by the nitrosonium ion with subsequent oxidation of the nitroso compound, but the contributisn of this mechanism could not be significant in a medium in which the nitronium ion mechanism can take place. Hydrolysis of the nitrosylsulfuric acid, forming sulfuric acid and nitrous acid, may take place to an appreciable extent in the acids studied ( 7 7 ) . The acceleration of nitration rates due to increased concentration of sulfuric acid can be explained by the reaction with nitric acid to form the nitric acidium ion and the bisulfate ion. ,4s the concentration of nitrosylsulfuric acid is increased and the concentration of water is decreased, the fraction of nitrosylsulfuric acid hydrolyzed will decrease. Accumulation of the bisulfate ion will repress formation of the nitric acidium ion. resulting in a lower concentration of nitronium ion.
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1000 900
800
700
600
RA
500
Xl
400
Nomenclature
Rate of nitration is a function
mono mixed acid free of organic material. The results of these runs are plotted in Figures 1, 3, and 4. The organic material, largely dinitrotoluene, carried into the reactor with the mono mixed acid enters the organic phase, reducing the concentration of the toluene. For each mole of toluene entering the reactor 0.022 mole of dinitrotoluene also enters and a reduction in rate of approximately 2 7 , could be expected. The rates, compared as a function of space velocity, were almost identical at low space velocities. At high space velocities the rates obtained with the plant mono mixed acid were al-
organic free,
1694
Rt. 7 0 51.54 12.99 19.97 14.39 1.11 100.00
of toluene conversion
most 7% lower than those with the synthetic mono mix. A sample of mononitrotoluene recovered from the organic layer produced by nitration with the plant mono mixed acid was analyzed at the Joliet Arsenal. I t contained no dinitrotoluene, indicating that no significant quantity of the organic material introduced with the plant acid appeared in the mononitrotoluene recovered from the reaction products. The presence of nitrosylsulfuric acid in mixed acid should be interpieted as a concentration of nitrosonium and bisulfate ions. Some nitration may have
Production Mono Mixed Acid -
Organic
mononitrotoluene
MNT
literature Cited (1) Barduhn, A. J., Kobe, K. A,, IND. ENG.CHEM.48, 1305 (1956). (2) Barnett, E., “Mechanism of Organic Chemical Reactions,’‘ Interscience, New
CO
NO +HS04-
= per cent toluene converted to
’‘
20c
&So4 “0s HnO
=
CO
flow rate, ml./min. = space = reactor volume. ml. velocity, minutes -l R, = rate of reaction, gram-moles M N T formed per hour per liter of acid phase XT = mole fraction of toluene in effluent organic phase R,/X, = rate of reaction if organic phase were pure toluene
30C
Figure 4.
MNT
Wt. 70
Synthetir Mono Mixed Acid Wt. 70 Mole yo
52.12 13.14 20.19 14.55
52.09 13.21 20.29 14.41
26.79 10.60 56.91 5.70
100.00
100.00
100.00
... -
INDUSTRIAL AND ENGINEERING CHEMISTRY
...
...
York, 1956. (3) Brennecke, H. M., Kobe, K. A,, IND. ENG.CHEM.48, 1298 (1956). (4) Furman, N. H., “Scott’s Standard Methods of Chemical Analysis,” 5th ed., pp. 2233-7, Van h-ostrand, New York. 1939.
C. K., *Graham,’J., -Peeling, E’. RT A.; Nature 158, 480 (1946). (7) Goddard, D. R., Hughes, E. D., Ingold, C. K., J . Chem. SOC.1950, p.
2559.
D. R., Hughes, E. D., Ingold, C. K., iVature 158, 480 (1946). (9) Hetherington, J. A , , Masson, I., J . Chem. SOC. 1933, pp. 105-14. (IO) Hughes, E. D., Ingold, C. K., Reed, R. I., Ibid.,1950, p. 2400. (11) Kunin, T. I., Zhur. Priklad. Khim. 27, 245-57 (1954). (12) McKinley, C., White, R. R., Trans. A m , Znst. Chem. Engrs. 40, 143-75 (1944). (13) U. S. Rubber Co., Joliet Arsenal, KiYK, “Laboratory Manual,” Sections
(8) Goddard,
I-T-20, GM-11.
RECEIVED for review August 10, 1957 ACCEPTED July 22, 1958 Division of Industrial and Engineering Chemistry, Chemical Processes Symposium, 132nd Meeting, ACS, New York, N. Y . , September 1957.
