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final temperature was 50” C. N o such oxidation products could be found in the nitro-p-xylene layer formed in this work. SUMMARY
p-Xylene has been nitrated with a yield of 89% mononitrop-xylene using the following conditions: Temperature, C . Nitration time, min. D.V.S. “01: p-xylene, mole ratio
30 30 3.0 1.10
No dealkylation could be found in the nitration under these conditions. Only small amounts of dinitro-p-xylene are formed. LITERATURE CITED
(1) Atanasiu, I. A., BulE. Chim. SOC.Rom4ne Chim., 39, 71-82 (1937-38). (2) Atanasiu, I. A., and Belcot, C., Bull. sect. sei. acad. Roumaine, 19. 28-38 (1937). (3) Batchelder, G. W ; Nagle, W. M., Vyverberg, J. C., and Willis, J. M., U. S. Patent 2,400,904 (May 28, 1946). and Haeffely, P., Bull. soc. chirn., 35, 983 (1924). (4) Battegay, M., (5) Blankama, J. J., Chem. Weelcblad, 10,136-7 (1913). (6) Brady, 0. L . , and Day, J. 3.E., J . Chem. Soc., 1934, 115
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Brown, C. L., Smith, TV. M., and Scharman, W. G., IWD. EWG. CHEIvf., 40, 1538-42 (1948). Castner, J. B., U. S. Patent 2,386,128 (Sept. 18, 1945); I b i d . , 2,438,204 (March 23, 1948). Doumani, T. F., and Kobe, K . A., J . Org. Chem., 7, 1-5 (1942). Glinzer, E., and Fittig, R., Ann., 136, 307-8 (1865). Groggins, P. H., “Unit Processes in Organic Synthesis,” p. 25, New York, McGraw-Hill Book Co., 1947. Heilbron, I. M., and Bunbruy, H. M., “Dictionary of Organic Compounds,” Vol. 3, p. 779, London, Eyre and Spottiswoode, 1943. Jannasch,P., Ann., 171, 81 (1874). Ibid., 176,55-6 (1875). Kobe, K. A., and Doumani, T. F., IND. ETG.CHEM.,31, 257-63 (1939). Kobe, K. A. and Doumani, T. F., Organic Sgntheses, 21, 98-8 (1941). Konowalow, M.,and Gurewitsch, Ch., J . Rzm. Phys. Chem. Soc., 27, 537-41 (1905); Chem. Zentr., 76, 11, 818 (1905). Legge, D. I . , J . Am. Chem. SOC.,69, 2086-90 (1947). Newton, A., I b i d . , 65, 2434-9 (1943). Nightingale, D. V.,Chem. Rem., 40, 117-40 (1947). Nolting, E., and Forel, S.,Ber., 18, 2680 (1885). Wahl, H., Ann. chim., [ l l ]5, 43-4 (1936). Wahl, H., Compt. rend., 198, 2107 (1934). RECEIVED September 30, 1948. Presented before the 13th Unit Prooesses Symposium of the Division of Industrial and Engineering Chemistry at the 114th Meeting of the AMERICAN CHEMICAL SOCIETY,St. Louis, Ma.
Nitration of Nitro-p-xylene KENNETH A. KOBE AND T. BROCKETT HUDSON University of Texas, Austin, Tex.
Nitro-p-xylene is easily nitrated to dinitro-p-xylene i n 95% yield in 15 minutes at a temperature of 80’ C., D.V.S. (dehydrating value of sulfuric acid) value of 8.0, and 10% excess nitric acid. The effect of these process variables is shown graphically. The orientation is anomalous, with 60 to 8070 of 2,3-dinitro and 40 to 20% 2,6-dinitro-p-xylene being formed.
P
REVIOUS work of Kobe and Levin (IO)in which a small amount of dinitro-p-xylene was produced indicated t h a t
PREVIOUS WORK
Dinitro aromatic hydrocarbon may be made in either one or two steps (4,7), but it is more common t o carry out the nitration in separate steps t o reduce oxidation and also reduce the solubility of the product in the strong acid required for the one step process. For the nitration of nitro-p-xylene it would be expected that the mixed acid would be less concentrated than for the dinitration of toluene, but more concentrated than for the mononitration of p-xylene. Likewise, the temperature of nitration should be lower than for dinitration of toluene but higher than that used for the mononitration of p-xylene.
Most of the work previously reported on the dinitro compounds has been incidental t o the nitration of p-xylene. Glinzer and Fittig (6) heated mononitro-p-xylene and fuming nitric acid t o produce a solid mixture of dinitro compounds from which two compounds were separated, one melting a t 123.5’ and the other a t 93 C. Jannasch (8), Jannasch and Stunkel ( 9 ) ,and Choufoer ( 2 )all reported the preparation of dinitro-p-xylene and the formation of only these two isomers. However, in 1885, Lellmann (11) reported the three isomeric dinitro compounds, the 2,3- melting a t 93”, the2,6-meltingat 124”]andthe2,5-meltingat 147’to 148OC. His nitration method was t o mix 25 grams of p-xylene and 100 grams of nitric acid (density] 1.51) and allow them t o stand for several days. From 100 grams of solid product he separated only 2 grams of the 2,5-dinitro-p-xylene. Giua (6) nitrated p-xylene with mixed acid and found only the 2,3- and 2,6-isomers. None of these investigators reported any data on yields and gave little d a t a on the operating conditions.
SITRATOR. The nitration was carried out in a 1-liter flask having 3 necks (Figure 1). Through the central neck passed a stainless-steel propeller-type stirrer which turned in such a direction as to force the liquid downward. I t turned at 1725 r.p.m. The mixed acid was added through a thistle tube entering through a side neck. The acid discharged about 0.125 inch above the propeller so that it was mixed instantaneously with the reaction mixture. A thermometer entering through the other side neck was immersed in the nitration mixture near the propeller. The flask rested in a cradle made of 6 turns of 0.25-inch copper tubing spaced about 0.5 inch apart. The coil of tubing was placed in a round steel container] approximately 4 X 6 inches in diameter. The small amount of free space in the container was filled with water. The coil was so connected that either steam or water or both could be passed through the coil, giving a bath
approximately 75% was the 2,3- and 25% was the 2,6-dinitro-pxylene. This orientation is not t h a t which would be predicted, hence it was decided t o investigate the nitration of 2-nitro-pxylene t o determine the optimum conditions for producing the dinitro compound and the effects of the process variables on the orientation.
O
APPARATUS AND MATERIALS
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TABLE I. NITRATIONOF NITRO-~-XYLENE" Mixed Acid Composition, % HNOo Te,mp., Time Yield, Run H2SOd HNOa HzO D.V.S. Ratio C. Min.' % 1.1 100 80.8 7.1 8 12.1 2 80.8 93.8 80 8 1.1 7.1 80.8 3 12.1 fin 8 86.2 7.1 1.1 12.1 80.8 4 91.3 90 8 1.1 7.1 12.1 80.8 5 95.0 1.1 100 5.9 10 84.4 9.7 6 92.0 80 5.9 10 1.1 84.4 9.7 7 93.2 80 10.9 1.1 5.4 86.3 8.3 8 79.3 80 6 1.1 8.1 15.4 76.5 9 80 94.5 1.1 7 7.4 79.4 13.2 10 15 90.7 6 80 1.1 8.1 76.5 15.4 11 94.6 15 1.1 80 8 7.1 12.1 80.8 12 96.0 15 10 1.1 80 5.9 84.4 9.7 13 97.2 15 1.1 80 12 5.0 87.5 14 7.5 All runs were made with 30.75 grams of nitro-p-xylene, 87.5 grams of sulfurio acid (d = 1.84),and 20.05 grams of nitric acid (d = 1.421,except run 2 in which twice these amounts were used.
__
~~
60
65
75
70
Q
REACTION
Figure 2.
*
80
85
TEMPERATURE
90
95
100
IN OC.
Effect of Temperature on Yield Reaction time, 30 minutes
temperature that could be varied from 25' to 100' C. a t a rate of about 20" per minute and maintained a t 1 2 ' C. a t any given temperature level. REAGENT.The nitro-p-xylene was produced in an earlier investigation (10) and was used without further treatment because of its high purity. The other reagents were of analytical grade.
100
2
9o
k
METHOD OF NITRATION
W
The nitro-p-xylene was added t o the nitrator flask and heated t o a temperature 5 " to 10" C. below the desired temperature for the experiment. The agitator was started and the mixed acid added dropwise a t such a rate that the temperature quickly rose t o the desired value. This usually required an initial rapid rate which was reduced to a ronstant rate and the nitration temperature was then controlled by the water bath. At the end of the run the thistle tube, thermometer, and stirrer were removed from the flask and the reaction mixture carefully poured with stirring into a 600-ml. beaker full of cracked ice. A yellow-orange amorphous solid preoipitated and was filtered off. The solid was washed with 1 liter of tap water, the pieces crushed, and washed further with 2 liters of dilute sodium bicarbonate solution followed by 0.5% sodium hydroxide solution until the filtrate came through uncolored after which a water wash was used t o remove the alkali. The washed solid was dried in a desiccator for several days and the yield calculated. The results are given in Table I. PROCESS VARIABLES
The process variables investigated were temperature, D.V.S. (dehydrating value of the sulfuric acid), explained in preceding paper (IO), and reaction time. These values of yield from Table I are shown in Figures 2 and 3. It is apparent that 80" C. represents the reaction temperature ACID
ro
DRAIN
Figure 1.
m
I One-Liter Flask Used for Nitration
8 4: 70
6
7
8
9
DEHYDRATING VALUE
Figure 3.
IO
II
12
OF SULFURIC ACID
Effect of D.V.S. on Yield
R e a c t i o n t e m p e r a t u r e , 80' C.
for optimum yield. Although the yield can be increased from 93.8 to 95.0% by increasing the temperature to 100" C. the D.V.S. must also be increased from 8 to 10, and the increased yield is not worth the increased acid. The nitration should be conducted rapidly, in 15 minutes rather than 30, to minimize oxidation reactions, thereby increasing the yield from 93.8 to 94.6%. Although a maximum yield of 97.5% dinitro-p-xylene can be obtained a t a D.V.S. value of 12, the large amount of sulfuric acid required to secure this value makes a yield of 94.6% a t a D.V.S. of 8 appear to be optimum. SEPARATION O F ISOMERS
All three dinitro-p-xylenes were reported formed on direct nitration by Lellmann (II), who obtained only 2 grams of 2,5compound from 100 grams of solid product. All other investigators report only the 2,3- and 2,6-dinitro-p-xylenes. The reaction conditions used in this work are not as severe as those used by previous workers who reported only the two dinitro compounds, and certainly not as severe as the conditions of Lellmann, and so it was considered doubtful that any 2,5-dinitro-p-xylene would be produced. It was first thought that by the use of successive recrystallizations and melting points the relative yields of 2,3- and 2,6-dinitrop-xylenes could be determined. Blanksma (1) presented a liquid-solid phase diagram for the 2,3- and 2,6-isomers, but he did not give his technique for preparing and analyzing his samples. He reported the formation of two eutectic compositions a t 20% and 60% of the 2,3-compound. Analysis was attempted using this diagram. Twelve grams of the solid product from the nitration were dissolved in 450 ml. of 70% ethyl alcohol and the solution allowed to stand for several days and evaporate slowly until long pale yellow crystals appeared. These were filtered from the solution, which was returned to the beaker for further evaporation. Melting points were taken in an
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it was necessary to hand pick these crystals from the crystal mass, recrystallize, hand pick, and continue this process until a pure prismatic crystal melting sharply a t 93” C. was obtained. Standard solutions of each isomer were made in hexane and a satisfactory optical density was found a t 3 X molar concentration. The extinction coefficient curves for these solutions were determined (Figure 4)and wave lengths of 272, 277, and 295 mp were selected as points for analysis. Known solution checks were made that show!d that the composition could be determined within +2%, which was considered satisfactory for this study. When solutions of crystallization fractions from various nitrations were made and placed in the spectrophotometer the data were obviously in error and the composition in terms of the 2,3and 2,6-isomers could not be obtained. Apparently, traces of phenolic or other compounds are present t h a t give decidedly different optical densities. Sumerous purification methods were tried without giving concordant results. The relative amounts of the two isomers can be given only as a range. The melting point of the product mas 83” t o 8 5 ” C., which on the basis of Blanksma’s diagram, shows 60 to 80% of the 2,3isomer. From the constancy of the optical densities of the products from different runs it is concluded that temperature and D.T;.S. over the ranges investigated here have no effect on the relative yield of isomers. No 2,B-ilinitro-1;-xylene or trinitro-p-xylene were found iri the products from any run. SUM3IARY
260
270
280
290
300
310
WAVE L E N G T H , mp
Figure 4. Extinction Coefficient Curves for 2,3- and 2,6-Dinitro-p-xylenes
aluminum block (12) fitted with magnifying eyepiece and using calibrated thermometers. This method was finally abandoned because of the. slow tedious process of recrystallization in which accuracy was lost in transferring the numerous crystallization fractions. Chemical processes have been patented for separating ortho, para isomers from the meta dinitro compound. Weiland and Gubelmann (IS) treat the mixture with alkali to decompose the ortho, para compounds and leave the meta dinitro compound. Coward (3)uses aqueous sulfite solutions to reduce the ortho, para compounds and separates them from the meta compound. Neither method gave satisfactory results in separating the 2,3- from the 2,d-dinitro compound. Spectrophotometric analysis was then attempted. Pure samples of the 2,3- and 2,6-dinitro isomers were then separated from the reaction solid. The 2,6- was separated in pure form relatively easily because of its low solubility in cold ethyl alcohol, in which the 2,a-isomer is soluble. An excessive number of crystallizations was necessary t o secure the pure 2,a-isomer and
Xitro-p-xylene is easily nitrated t o the dinitro-p-xylene in 95% yield under the follon-ing conditions: Temperature, C . Nitration time, min. D.V.S. “0s: nitro-p-xylene,mole ratio
80
15 8.0
1.10
The relative yield of the two dinitro-p-xylene isomers is 60 to 80yo of 2,a-isomer and 40 t o 20% of 2,6-isomer. No 2,5-isomer or trinitro-p-xylene was found in the nitration products. ’*
LITERATURE CITED
(1) Blanksma, J. J., Chem. Weekblad, I O , 1936-7 (1913). (2) Choufoer, H. J., Proc. Acad. Sci. Amsterdam, 28, 119-26 (1925). (3) Coward, H. W., U. S. Patent 2,040,123 (May 12, 1936).
(4) Davis, T. L., “Chemistry of Powder and Explosives,” pp. 133-5, 141-150, New York, John Wiley & Sons, 1941. (5) Giua. M., Gam. chim. ital., 49, 11, 149 (1919). (6) Glineer, E. and Fittig, It., Ann., 136,307-8 (1865). (7) Groggins, P. H., “Unit Processes in Organic Synthesis,” pp. 4851, New York, MoGraw-Hill Book Co., 1947. (8) Jannasoh, P., Ann., 171,81 (1874). (9) Jannasch, P., and Stunkel, C., Ber., 14, 1146 (1881). (lo) Kobe, K. A,, and Levin, H., IND.ENO.CHEX, 42, 352 (1950). (11) Lellmann, E., Ann., 228, 250-3 (1885). (12) Morton, A. A., “Laboratory Technique in Organic Chemistry,” pp. 32-3. New York, McGraw-Hill Book Co., 1938. (13) Weiland, H. J., and Gubelmann, I., U. S. Patent 1,665,005 (April 3, 1928). RECEIVED July 19, 1949.
