I
HENRY M. BRENNECKEI and KENNETH A. KOBE University of Texas, Austin, Tex.
Mixed Acid Nitration of Toluene A miniature continuous-flow quenched reactor extends rate data on mononitration a thousandfold
A R O M A T I C nitration is one of the oldest and most common industrial and laboratory reactions. Nevertheless, the reaction is still known only imperfectly and many misconceptions of its nature are widely held. Only a small amount of useful kinetic data are available to the engineer, and this is restricted to a small range of process operating conditions. The extension and interpretation of the existing data are therefore of importance. Nitration Mechanism
nitration reaction thoroughly. They have shown conclusively that nitration can be considered in many cases as an electrophilic attack of the nitronium ion, NO,+, on the benzene nucleus. A comprehensive survey of this theory has been presented by Gillespie and Millen (5),while Haun and Kobe (8) have shown how a n understanding of the theory is helpful in the interpretation and correlation of experimental results. Aromatic nitration by the nitronium ion can be shown by chemical equations:
The stoichiometry of the reaction is misleadingly simple :
+ ArH + ArNOz + HzO
HNOa
(1)
This chemical equation is misleading because sulfuric acid does not appear in it, and for this reason sulfuric acid has often been assigned the role of a dehydrating agent in the law of mass action (6). This interpretation has no foundation in fact; it has long been known that aromatic nitration is an irreversible reaction. In the past 20 years a group of workers headed by C. K. Ingold and Robert Robinson, and known as the “English School” in reference to their outstanding work on the electronic effects involved in aromatic substitution, having studied the 1 Present address, Burnside Laboratory, E. I. du Pont de Nemours & Co., Penns Grove, N. J.
100 zMOLE
H20 H / 2
Figure 1. toluene
1298
P E R C E N T “03
SO
4
The conversion, and therefore the reaction rate, approaches zero as the mole ratio of water-sulfuric acid approaches unity. This is significant because when this ratio appreciably exceeds 1.0 the nitronium ion is spectroscopically undetectable in the sulfuric acid-nitric acid-water ternary solution. Bennett showed that various acid mixtures which gave the same conversion contained practically the same concentration of nitronium ion, as determined by Raman spectra. Hetherington and Masson ( 9 ) had obtained analogous results in the twophase nitration of nitrobenzene. Their
MOLE RATIO
Rate of nitration of dinitro-
(3) (4)
These equations are written in to show how the reaction proceeds in theory and to illustrate the reason for the preferred positions of the nitro group of the ring. I n a large number of ring substitution reactions the English workers have shown that the orientation can be explained on the basis of probable electron densities around the ring, and that the nitration reaction is apparently a perfect example of these principles. Nitronium Ion Mechanism
Unfortunately, most of the evidence presented by the English workers in support of the nitronium ion mechanism was obtained in homogeneous solutions under conditions that could not be compared directly with conditions generally employed in commercial nitrations. The work of Bennett and others (2) was one exception; a 50/50 mixture of di- and trinitrotoluene was nitrated by shaking with mixed acids of various compositions for a fixed time. The reaction was then quenched with cold water, and the proportion of dinitrotoluene which had been converted to trinitrotoluene was determined. The data obtained are plotted in Figure 1.
INDUSTRIAL AND ENGINEERING CHEMISTRY
results are plotted on the ternary sulfuric acid-nitric acid-water diagram of Figure 2. The change of acid concentration with time is shown as an arrow on the plot. Hetherington and Masson found that the reaction rate became negligibly small at certain concentrations, and that a line drawn through these limiting concentrations almost coincided with the boundary of the area in which Chedin ( 3 ) was able to detect the nitronium ion by its Raman spectra. The nitronium ion mechanism explains the dinitration and trinitration reactions. However, there are no com-
Figure 2. Concentration nitration of nitrobenzene
limits
for
w
parable experimental data connecting the mononitration reaction with nitronium ion concentrations. Perhaps the situation can best be shown by means of Figure 3, which is a diagram in molar concentrations of the ternary system sulfuric acid-nitric acid-water. Superimposed on the plot are nitronium ion concentrations calculated from the Raman spectra of Chedin. The nitronium ion is spectroscopically detectable only in highly acid solutions, and most kinetic evidence has been obtained for acid concentrations lying within the nitronium ion envelope. Also shown on the plot are typical acid concentrations used in commercial nitrations, as well as the rate data of McKinley and White (72). It is evident that the acids used for mononitration lie outside the nitronium ion envelope; therefore it is doubtful that the nitronium ion mechanism applies. Other nitrating agents have been proposed for the more dilute media, notably the “pseudo” acid (molecular nitric acid) and the nitracidium ions, HzNOi, H 3 N O y of Hantzsch (7). However, there is no direct evidence to support any of these nitrating agents. O n the contrary, Lowenff, Murray, and Williams ( 7 7) have offered evidence showing that the nitronium ion may be the nitrating agent even in concentrations too weak to be spectroscopically detectable.
Process Rate Data
*
Lewis and Suen (70) in 1940 nitrated benzene in a two-phase system using a continuous-flow, stirred reactor. Mixed acid and benzene were metered into a reactor in which the temperature was controlled by a n internal water-cooled coil; stirring gave intimate and immediate mixing of the feed. Significant observations by these workers were: The rate doubled with each 10’ C. rise in temperature, indicating that ionic chemical reaction was the ratecontrolling factor, and the reaction took place predominantly in the acid phase. General Rate Equation. McKinley and White (72) used exclusively a continuous-flow, steady-state reactor to obtain the first rate data on the nitration of toluene. Acid and toluene were fed at constant rates to a stirred, glass reactor, the effluent product was separated continuously by gravity settling, and each phase was analyzed chemically. Precautions were taken to make certain that the stirring was good, so that mass transfer effects would not influence the rate results, and the uniformity of the steady-state emulsions was checked by electrical resistance measurements. The most important result of this work was a correlation which makes possible the prediction of nitration rates:
CHEMICAL PROCESSES “ 0 3
Hetherington . . . . - . . . . & _Masson’s ,
VA
Figure 3.
7f
\
Comparison of mixed acid compositions
R, KXTXAP where R, = rate, moles of nitrotoluene per hour per liter of acid phase P,K = constants, but functions of sulfuric &id concentration XT = mole fraction of toluene in organic phase X A = mole fraction of nitric acid in aoid phase This equation has been altered slightly from the one originally given by McKinley and White; the mole per cent nitric acid in the original equation has been changed to mole fraction, with the necessary adjustment of the constants. A correlation of this type is a most convenient one for continuous nitration, since the sulfuric acid concentration, and therefore the rate constants, are fixed by the initial conditions, and are independent of the degree of completion of the reaction. As it is impossible to preset the conditions in a continuous reactor, this correlation was derived by taking a large number of runs, 60 in all, and assuming various empirical relations until one was found which fitted the data. Because the data were badly scattered, in that no variable other than temperature could be held constant, all data points were corrected to standard values with the same correlation. I t was found that the data could be correlated most effectively if it was assumed that the reaction took place in the acid phase only. I n addition to this correlation, McKinley and White made further important observations. The temperature coefficient for this reaction was 2.2 per 10’ C. This indicated that rates were probably true chemical rates and not influenced by rates of mass transfer. T h e weight ratio of nitrotoluene to dissolved nitric acid in the organic phase is a function only of the weight per cent nitric acid in the acid phase. Nitration rates based on the acid
-
phase are independent of the ratio of the phase volumes. The rate of nitration of toluene is four or five times as rapid as that of benzene. Extension of Rate Data. Figure 3 shows the striking correlation between the rate of dinitration and the concentration of the nitronium ion. However, the mononitration rate data which have so far been reported have been restricted to acids that lie well below the concentrations in which the nitronium ion has been detected; consequently, there is no parallel correlation for this reaction. Therefore, it is of considerable interest to extend the rate data to stronger acid concentrations, to see if there is such a direct correlation. I t is also desirable to extend the rate data to much higher levels, because the present data are restricted to rates well below those that could be attained in continuous-flow reactors and are somewhat below the rates attained a t present in commercial batch reactions.
Continuous Flow Reactor Previous investigators were limited to rather low rates of reaction, on the order of 2 gram-moles per hour-liter, for two reasons. The nitration reaction is highly exothermic, and the several hundred
G
I INCH-
Figure 4.
Miniature jacketed nitrator
VOL. 48, NO. 8
AUGUST 1956
1299
AUTOTRANSFORMER
WATER L I N E MANOMETER
TOLUENE ROTAMETER
ACID ROTAMETER
MOUNTING RACK AND A L I G N M E N T MECHANISM
Figure 5.
milliliter glass reactors employed could not remove heat rapidly enough to permit the study of higher rates. Secondly, the gravity separation of the two phases with subsequent analysis by titration could not be used when a n appreciable proportion of the reaction would take place in a few minutes. The simplest way to increase the rate of heat transfer is to increase the ratio of area to volume of the reactor by making it as small as possible. Accordingly, it was decided to use a miniature, jacketed, steel kettle with water quench. This miniature rcactdr is shown in cross section in Figure 4. The inner chamber, A , was 0.50 inch in internal diameter, 0.50 inch from bottom to plug face at top, and 0.10 inch thick, made of 304 stainless steel. The shoulders of the chamber rested on the outer jacket, B, which was 0.25 inch thick. The plug, C, served the multiple purpose of seal for the reaction chamber and entrance for the thermocouple, D, and stirrer shaft, E. The Teflon gasket, F, prevented leakage from the water jacket into the reaction chamber. The cooling water circulated in the space between the inner chamber and the outer jacket, and passed out through the discharge tube, G. The reactants were introduced through two assemblies as shown a t I. The outer jacket was drilled and tapped to receive two standard, stainless steel Ermeto male connectors made of l / ~ inch pipe and I/s-inch tubing. These were reamed out to pass the I/s-inch stainless steel tubes with a m nimum of clearance. The tubes were threaded with machine screw threads and screwed into the inner chamber; the 0.10-inch thickness of this chamber wall was found to be necessary to provide a leakproof
1 300
Continuous flow reactor system
seal. Both the acid and the toluene were introduced a t the bottom of the reactor, but 180' apart. The reactants left through the short stub of '/g-inch tubing as shown a t H , where the reaction was quenched by the relatively large volume of the cooling water in the discharge tube, by means of a standard l/s-inch Ermeto male connector used in the normal manner. The thermocouple, D,well consisted of a '/g-inch borosilicate glass tube fitting snugly into the '/s-inch hole in the plug. The end of this tube was sealed by fusion, arid a n iron-constantan thermocouple was forced into the molten glass until it protruded into a well defined tip. The thermocouple junction was then separated from the reaction mixture by only a thin wall of borosilicate glass. Stirrer E was a turbine type. The stirrer shaft and turbine head were turned out in one piece on a lathe; the turbine itself was formed by cutting four deep, roughly V-shaped notches in the cylinder, forming four heavy lobed blades. The l/s-inch stirrer shaft was introduced through a hole in the center of the plug with only about 0.001inch clearance. The small shoulder on the stirrer shaft immediately below the plug was designed to decrease leakage and facilitate vertical positioning of the stirrer. The stirrer driver was a small motor which had a n in-place speed in excess of 20,000 r.p.m. This motor and the reactor were fixed to a rack of heavy angle iron in such a way that practically perfect, rigid alignment could be attained. This made it possible to drive the stirrer shaft through the 1-inch thick block without the benefit of a flexible coupling. This alignment rack as well as the feed system is shown in Figure 5. The
INDUSTRIAL AND ENGINEERING CHEMISTRY
RECEIVERS
acid and organic reactants were contained in glass chambers, which were enclosed in steel shells for safety reasons. The reactants were forced out of these vessels with a n air-pressure drive controlled by a pressure regulator and air bleed. Usually this pressure was low, only 3 to 6 inches of mercury. The rate of flow was regulated by needle valves on the rotameters. All lines were 1,'sinch stainless steel tubing, with some '/e-inch tubing on the acid side. This larger tubing was necessary to prevent frequent line stoppages due to small amounts of corrosion products. The glass to metal connections for both the acid and toluene lines were made with short sections of Tygon tubing, which apparently had an almost indefinite life under these conditions. The rotameters appeared to be incapable of giving precise control and required constant attention. This difficulty was probably aggravated by the small flow rates involved and the intermittent nature of the operation. Small amounts of solid impurities were always present in the acid line, making control of the acid flow especiallv difficult. The reactor was designed to operate a t a slight internal pressure, as the leakage of air into the reaction mixture could not be tolerated. In order to prevent excessive kakage from the reactor, it was necessary to regulate not only the feed pressure but also the pressure on the exit line as well. This control was provided by the application of a gentle suction to the exit line. This exit line was brought out to a three-way stopcock, so that the flow could take either of two routes into one of two collecting bottles. These bottles were connected to a vacuum line; fine pressure control was provided by a n air bleed.
C H E M I C A L PROCESSES
Startup a n d Operating Procedure.
In order to start an experiment, the
ts
mixed acid was first admitted to the inner chamber at the predetermined flow rate. The pressure on the acid egg and the receiver was regulated so as to give slight liquid leakage around the stirrer shaft. Toluene was then admitted and the temperature and amount of cooling water were controlled to give the desired temperature of reaction. After all conditions were stable, the speed of the stirring motor was varied to make certain that it was operating a t speeds in excess of the limiting speed. This limiting speed is defined as the speed at which a further increase in the speed would have no effect upon the reaction rate; it was usually about 10,000 r.p.m. The reaction temperature was used as the criterion for steady state conditions. When all factors were steady, the flow of the rotameters was checked at regular intervals by recording the levels in the calibrated feed chambers. This usually required from 5 to 45 minutes. Then the flow of product was switched to a fresh sample jar and a sample was taken until a t least 20 ml. of product had been collected. I n general, the larger the sample the easier it was to make an accurate analysis. Steam Distillation Apparatus. As there was always liquid leakage around the stirrer shaft, analysis could not be made on the basis of total amount of nitrotoluene formed. Only the percentage of mononitrotoluene in the reaction product was determined. All rates and calculations were based upon this one value. The apparatus used, in Figure 6, is a simple steam distillation apparatus with some additional provisions for fractionation. The discharge container was chilled with crushed ice to reduce toluene evaporation to a negligible amount. Generally, the discharge stream contained more or less finely dispersed bubbles of the reaction product; to minimize losses the whole mixture was poured directly into the steam distillation flask. The steam distillation apparatus was fitted with a short packed section which
was topped with a small partial condenser to provide reflux. Separation was based on the 84' boiling point of the toluene-water azeotrope and the 100' boiling point of the straight steamdistilled nitrotoluene. In operation the flask was heated slowly until toluene vapors began to rise above the partial
Figure 6. Steam distillation apparatus
condenser; the heat was maintained at that level until all the toluene had come over; the heat was then increased and the partial condenser was disconnected. The large Friedrich condenser then served to condense the vapors and the mononitrotoluene collectzd in the
trap. The toluene was measured by volume, and the nitrotoluene was collected and weighed. Dinitrotoluene was not determined, but was sometimes detected by the manner in which it steamdistilled in tiny droplets in contrast to the rather large droplets of the mononitrotoluene. Dinitrotoluene was rarely noticed, however, and was not a significant factor in the analysis. The trap for the mononitrotoluene was provided with an underwater return for the condensed steam in order to speed up the accumulation of the product. The nitrotoluene has a powerful tendency to remain floating on the surface of the water in the trap, despite its great difference in density. The separation obtained in this way was quantitative. At constant low heat the toluene-water azeotrope came over fairly rapidly a t first, then more slowly, and then ceased altogether. At that point the character of the condensing vapors in the partial condenser had changed radically from the large, colorless, mobile toluene droplets to the tiny scattered yellow droplets of nitrotoluene. An increase in heat input was necessary to bring these vapors past the partial condenser. In the trap the cut point was clear. A colorless drop of toluene was succeeded by a yellow colored drop of great enough density to sink; there could have been only very slight intermingling of materials. The tiny reactor made possible the measurement of rates of reaction a thousandfold greater than had been measured by McKinley and White. Rates were measured at as high acidities as was practicable, but even this reactor could not measure rates of mononitration using acids strong enough to contain detectable quantities of nitronium ion. This limitation restricted the range of acid concentrations studied to a much greater extent than originally had been planned. Range of Conditions. The data were plotted like those of McKinley and White (72), both as a matter of convenience and for purposes of comparison:
? .
200
P E R C E N T ORGANIC CONVERSION
0 20 40 60 80 100 P E R C E N T ORGANIC CONVERSION
Figure 7. Left.
26 mole
% sulfuric acid
Center.
0 20 40 60 80 100 P E R C E N T ORGANIC CONVERSION
Rate of nitration
30 mole
% sulfuric acid
Right.
35 mole
VOL. 48, NO. 8
% sulfuric acid
AUGUST 1956
1301
Run No.
Toluene, MI.
2 4 6 7 9 10 12" 13" 16 17 18
Acid, Mi.
19
20 39 20 20 30 20 21 32 24 20 20 27
38 78 39 40 60 40 43 60 24 20 60 96
2 3 4 5 7 8 10 13 14 16" 17" 18
20 23 32 18 20 22 21 19 20 20 20 24
40 60 32 54 60 22 42 38 40 40 40 48
2" 4 5 7 9 11 13 14 15 16 18"
22 25 23 20 20 21 20 20 21 21 20 Contain 25 mole
43 25 69 40 40 42 60 40 42 63 40
Table 1. Rates of Nitration Recovery Time, Toluene, MNT, Tzmfi., F/ ml. grams c. AIT Min. -l Min. Series A. Nitration Acid 25 Mole yo H$O, and 1 5 Mole yo HNO:, 7.10 35 1.9 1.45 32 9.4 7.15 35 31 . O 7.7 8.50 45 L.U 7.20 45 7 n 12.7 75 7.5 11.90 6 5.2 2.0 1.49 32 13.85 8.3 35 2.0 1.50 34 14.30 35 1 9 12.17 6 17.5 4.35 35 10 9 51 4 24.8 4 29 14.8 35 1 0 1 59 20 10 17 35 3 0 3 18 20 12.6 9 06 21,l 35 3 5 12 20 8 Series B. Nitration .4cid 30 Mole % H ~ S O and I 15 Mole % HNOy 6.5 12.72 35 2.0 1 19 40 10.98 35 2.0 11.90 18.1 22.0 6.8 11.4 13.1 13.54 45 2.0 1.19 42 4.3 14.67 3.7 55 2.0 1.19 38 9.77 45 2.0 9.52 5 10.5 18.08 35 2.0 1.22 39 3.9 11.17 35 2.0 9.52 5 6.9 12.64 10.0 55 2.0 9.52 6 Series C. Nitration Acid 35 MoIe % HzSOa and 15 Mole % "OB 16.95 35 1.9 1.31 40 2.8 9.54 35 1. o 3.97 10 11.1 20.74 35 3.0 2.43 30 5.3 3.5 2.6 10.0 3.8 5.7 17.20 55 2.0 7.14 7 6.7 14.55 35 3.0 7.14 7 9.4 14.64 35 2.0 9.52 5 6.8
v,
determined by the steam distillation analysis of the product. All other values were calculated from the figure and knowledge of the quantity and composition of the charge streams. F j V = space velocity, total liquid feed rate divided by reactor volume, usually given in reciprocal minutes.
=
Run Toluene, No, 2t4l. 1 2 3
4 5 6 7 8 9 10
20 20 20 20 20 21 20 19 20 20
Acid
The basic data are given in Figure 7 which shows the rate curves obtained for three different acid strengths: 25, 30, and 35 mole yo sulfuric acid (Table I ) . The strongest acid that could be used
H 2 S 0 4 ,%
HAT03, %
Mi.
27.00 27.00 27.00 40.89 40.89 34.02 34.02 37.00 37.00 37.00
15.00 15.00 15.00 15.00 15.00 22.60 22.60 22.34 22.34 22.34
40 40 40 40 40 42 40 27 29 30
~~
1 302
Campletton,
RJXT
70
157 664 195 515 1108 244 130 495 645 164 229 501
249 776 334 739 1650 584 300 808 735 204 374 670
36.9 15.2 46.1 30.5 32.9 58.1 56.4 38.8 12.1 18.6 38.8 25.2
202 1080 890 342 926 304 238 254 1150 284 812 1330
51 1 1590 1180 797 1593 452 821 1040 1950 1090 1840 2640
60.4 32,O 24.8 57.2 38.8 32.7 71 . O 75.5 42.0 78.2 55.8 49.5
214 875 344 250 293 1020 238 490 1828 970 975
1190 1460 1390 980 2480 2100 1250 1880 4010 2160 2680
82.0 40.0 75.2 75.5 88.2 51.5 80.9 73.9 66.9 55.0 62.9
o-nitrotoluene.
rate based on acid phase, grammoles of nitrotoluene/houri liter ofacid phase. Acid phase volume is based on that of the reactants charged, hence differs slightly from actual measured volumes (72). A / T = ratio of volume of acid to volume of toluene charged. XT = mole fraction of toluene in the organic phase. Value calculated assuming no interphase solubility, therefore again differing slightly from actual analyzed value. = per cent conversion of the orC O ganic phase. This value was
R,
Rates R O
INDUSTRIAL AND ENGINEERING CHEMISTRY
Table II. Rates of Recovery Toluene, MST, grams ml. Series D 6.40 14.6 14.8 8.12 9.65 13.3 14.71 5.7 4.8 18.05 14.10 7.5 4.5 20.78 8.4 13.41 7.3 15.13 18.87 5.1
in the reactor over a wide range of conditions of 357, sulfuric. Each acid was used over the greatest possible range in space velocities a t 45O, three different temperatures-35', and 55' C.-all a t a n acid-toluene ratio of 2 to 1. Each was used a t three different acid-toluene ratios-I, 2, and 3all at 35' C. In addition, one series of runs was made for each acid using a feed consisting of 25 mole 70 o-nitrotoluene, using a n acid-organic volume ratio of 2 to 1 and at 35' C. The experiments made at 45O and 55' C. were intended as a check to determine whether or not the rates obtained displayed the
Nitration
F j V, Min.-'
Time.
,4/T 2.00 2.00 2.00 2.00 2.00 2.00 2.00 1.36 1.42 1.45
9.52 4.76 2.38 4.76 2.38 7.14 1.19 5.21 3.23 1.32
5 10 20
:ampletion,
%
Mill.
10 20 7 40 T
12 30
682 401 241 905 500 1195 262 1140 771 369
91 5 571 375 3020 1950 2930 1160 2590 2000 1430
25.4 29.8 36.4 66.6 74.4 59.2 77.5 55.8 62.0 74.2
C H E M I C A L PROCESSES 3000
2000 1400 1000
600 400
S U L F U R I C ACI
Rf?/xT
200
100
70 PERCENT
ORGANIC CONVERSION
Figure 8. Effect of acid strength on rate of reaction a t 35' C.
30 0
20
PERCENT
temperature dependence characteristic of ionic reactions. This temperature factor had been reported by Lewis and Suen, and McKinley and White, to be about 2. This same temperature factor was found to hold under these conditions as well. The dashed iines on these three charts intersecting the three lines for various acid-toluene ratios represent lines of constant acid phase concentration. The slope of these lines may be explained by the increasing solubility of the nitric acid in the organic phase as the fraction of nitrotoluene increases. I n Figure 8 some additional data on acids of various concentrations are plotted. All these data (Table 11) were obtained a t 35' C. using a pure toluene feed. McKinley-White Correlation
The rate equation of McKinley and White does not fit these data. Although their data did not cover the acid con-
c
40
60
80
100
ORGANIC CONVERSION
Figure 9. Actual and calculated nitration rates
centrations employed here, the constants were plotted in such a manner that the equations could be used to calculate much higher rates with a minimum of extrapolation. I n Figures 9 and 10 are shown the results of the comparison of experimental results with rates calculated by the above correlation. I t is apparent that it is unwise to extrapolate the MCKinley and White correlation beyond the acidity ranges from which it was derived. I n Figure 10 some data of Barduhn and Kobe (7) a t very low acid concentrations are shown. These data also do not agree with the McKinley and White correlation. Errors in the difficult nitric acid analysis are especially critical in these regions of very low nitric acid concentrations. The data of McKinley and White and of Barduhn and Kobe are all based upon analyzed acid-phase concentrations, whereas the other data are based upon calculated compositions, disregarding interphase solubilities. If nitric acid solubility in the organic phase is taken into account, the discrepancy between calculated and actual rates is even greater. Part of the data can b: correlat-d by an equation of the type:
P E R C E N T ORGANIC CONVERSION
Figure 10. Nitration rates in large and small reactors
perimental values. However, rates a t other A / T ratios cannot be predicted without taking interphase solubility into account. Data for any one acid a t the various A / T ratios and for the two organic feeds could be correlated by assuming an empirical relationship between the nitric acid distribution coefficient and the composition of the two phases. However, any such assumed relationship held only for the case for which it was derived, so that a general practical correlation for all the data was not discovered. Change in Mechanism with Increasing Acidity
No relationship was derived which suitably correlated all the data. One reason for this may have been that the reaction mechanism changed as the acidity increased from the lowest to the highest values studied. Effect of Organic Phase Composition. Throughout this paper the factor R J X T has been used consistently as a matter of convenience. Its use implies that the rate is a direct function of the composition of the organic phase. T h a t this is not necessarily true is shown in
log R J X T = m(X.iivr) -t b
For the case of three different acids fed at the same acid-toluene ratio these constants would be : PERCENT
ORGANIC CONVERSION
Figure 1 1 . Effect of organic composition and acid strength on rate of nitration -Pure
feed ---toluene 25 mole % o-nitrotoluene
-
organic conversion 1 XT Same data, organic conversion 1
----
0.25
70
feed,
%
- XT
Acid Strength Sulfuric- Nitric 25-15 30-1 5 35-15
m -2.22 -1.74 -1.41
b 3.24 3.74 4.04
If these constants are plotted and extrapolated to the stronger acid of 40.9 mole 70 sulfuric-15 mole 7 0 nitric, calculated rates agree very well with ex-
20
40
60
00
100
P E R C E N T 0 R G A N I C CONVE R S l O N
Figure 12. Reaction rates with strong acid deficient in nitric acid VOL. 48, NO. 8
AUGUST 1956
1303
Table 111. Rates of Nitration Series E, Nitration Acid 40.89 Mole % HzS04 and 11.40 Mole % “ Run No. 2 3 6 7 8 9
I
Toluene, MI. 20 20 22 24 20 21
Acid,
Ml. 40 40 42 48 40 42
Recovery Toluene, MNT, ml. grams 4.6 15.99 4.6 17.64 9.1 15.74 6.5 15.46 5 8 1 3 15 9.0 12.32
Figure 11, which gives a set of lines for each of the three acids. For the weaker acids the line falls below that of the toluene curve, but almost coincides with it for the 35 mole yo sulfuric acid feed. If the nitronium ion mechanism were just beginning to make itself felt, the organic phase composition would be relatively unimportant, as the ratecontrolling step would be the reaction of the nitric acid with the sulfuric acid. Effect of Nitric Acid Strength. I n Figure 12 are shown the results obtained when the nitration was performed with a strong acid deficient in nitric acid (Table 111). High rates of reaction persisted up to the point of extinction of the nitric acid. As acidity is increased and the nitronium ion mechanism assumes more and more influence, one would expect both the nitric acid and the organic concentrations to have less and less influence upon the reaction rate. Effect of Acid Strengths. Figures 7 and 8 show that the following trends hold : Concentration of nitric acid becomes less important as over-all acidity rises, At higher acidities, cancentration of sulfuric acid is more important than nitric acid concentration. Both trends are consistent with the idea that the nitronium ion mechanism
FP,
Temp.,
c.
35 55 45 55 45 35
AIT 2.0 2.0 1.9 2.0
2 0 2 0
Min.-’ 1.70 4.75 7.25 9 52 4.56 7.15
at least partly controls the rate when the sulfuric acid concentration is in the neighborhood of 35 mole %, but that some other mechanism may apply at lower acidities. Correlation of Crookes and White
The relative effects of nitric and sulfuric acid strength upon the rate of reaction can be expressed numerically by empirically weighting the mole concentrations of water, sulfuric acid, and nitric acid in the acid phase. Crookes and White ( 4 ) have correlated the data of McKinley and White by plotting R,/XTCAr against the logarithm of the factor ( X , 5/3 Xa - ‘/a XHzo). Here X, is the mole fraction of nitric acid and X , the mole fraction of sulfuric acid in the acid phase in moles per liter. The success with which this factor fits the data is shown in Figure 13. The data points a t low acidities lie considerably above the line, while the data of the miniature reactor scatter rather badly, generally falling below the line. Different weighting factors or the inclusion of another term might give an even better correlation. However, a correlation of this type cannot be used for calculated acid phase compositions, as it ~ r o u l d predict the same rate for
+
0 3
Time,
Completion,
Rates
Min.
R, 350 1000 1266 1740 -856 990
28 10 7 6 10
7
RJXT 1290 4020 2960 4950 2360 2040
% 72.9 74.9 57.3 64 9
63.7 51 5
any acid-toluene phase ratio, which is not the case. Summary
A m i n i a t u r e c o n t i n u o u s -f lo w, quenched reactor can be used effectively to study relatively rapid rates of reaction. Rates as high as 2000 grammoles per liter-hour have been measured in a reactor of 1.26-ml. capacity, and the effects of process variables have been studied over a wide range of conditions. Available rate data on the mononitration reaction have been extended to rates a thousandfold greater than those previously reported. The rate equation of McKinley and White should not be extrapolated beyond the conditions from which it was derived. The nitronium ion mechanism begins to control the rate of reaction in acids containing more than 30 mole yo sulfuric acid, but some other mechanism is rate-controlling in weaker acids. Acknowledgment
The work reported here was performed as part of Army Ordnance Corps research contract DAI-23-072-501-ORD (P)-6. The kindness in releasing these data for publication is appreciated. Literature Cited
( 2 ) Barduhn, A. J., Kobe, K. A,, IND. ENG.C H E M48, . 1305 (1 956). ( 2 ) Bennett, G. M., others, J . Chem. SOC. 1947, p. 1185. ( 3 ) Chedin, J., Ann. Chin. 8 , 243-315 (1937). ( 4 ) Crookes, R. C., White, R. R., Chem. EnR. Propr. 46, 249-57 (1950). (5) Gillespie. R. J., Millen, D. J., Ouart. ReLs. 2, 277-306 (1948). ( 6 ) Groggins, P. H., “Unit Processes in Organic Svnthesis,” 2nd ed., p. 1, h.lcGraw-Hil1, New York, 1938. ( 7 ) Hantzsch. A., Ber. 5 8 , 941-61 (1925). ( 8 ) Haun, J. K., Kobe. K. A,, IND.EXG. C H E M . 43, 2355 (1951). ( 9 ) Hetherington, J. A,, Masson, I., J . Chem. SOC.1933, wp. 105-114. (IO) Lewis, ‘iV. K., Suen, T. J., IND.END. CHEM.32, 1095-101 (1940). (1 1) Lowen, A., Murray, M. A , , Williams, Gwyn, J . Chem. Sac. 1950, p. 3318. (12) McKinley, C., White, R. R., Trans. Am. Inst. Chem. Engrs. 40, 143-75 (1944). \ -
LOG (XN+ 5 / 3 X ~Figure 13.
1 304
% XH~O)
Correlation of Crookes and White
INDUSTRIAL AND ENGINEERING CHEMISTRY
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RECEIVED for review November 14, 1955 .L\CCEPTED May 19, 1956
