an
Unit Processes Review
Nitration by W. R. Tomlinson, Jr., The Johns Hopkins University, Operations Research Ofice, Bethesda, Md. Nitration as a discipline has developed theory and practice to attain a measure of maturity
WORK
during the past year proceeded in the areas of application, process improvement, and development of fundamental concepts. In the latter area, the status of basic material has reached a satisfying state of firmness and utility. The position of the nitryl ion has been sufficiently elucidated that its importance in nitration can scarcely be overemphasized. While this indication has always been present since its suggestion by Euler in 1903, it could hardly have been claimed a dozen years ago. Even the mechanism by which the nitryl ion is affixed to the atom affected was hardly unquestioned a few years ago. The current position prompts a summary of the situation. The most important advance in the understanding of chemical reactions and some physical processes has been made through the theory of rate processes and its use of the free energy of activation. The familiar rate equation [rate constant = ( k T / h ) in which AF is the free energy of formation of the activated complex in its standard state, permits the chemist to visualize the approach of molecules to each other, their relative orientations changing to permit contact between oppositely charged or attracting groups to form the complex and the final separation into the products. When AF is small or negative (as in ionic reactions), the rate constant is large. With covalently bound atoms, only relatively small, short-range dipole forces favor charge separation and combination; AF is much greater, and rates are lower. For comparing reactivities of covalently bound molecules, it is essential to understand the factors affecting the degree of covalency or ionicity of bonds. The latter factor is related to the size of the dipoles and forces favoring complex formation and thus tending to lower AF. This situation can be related to the signs of the attacking entity and the part attacked and, making it specific to nitration, to the proton shielding effect :
X-NO2
+ H+
The electronegativity of an atom is a useful indication of its electron-attracting power. Carbon and hydrogen have electronegativities of about 2.5 and
2.1, respectively-i.e., carbon attracts electrons more strongly than hydrogen and relative to it may be thought of as an electron sink. Thus, in the carbonhydrogen bond the electron pair may be thought of as closer to the former than the latter, and this displacement can be visualized as being equivalent to a certain degree of ionicity. I n general, a reaction rate depends more strongly on the degree of ionic character of a bond (and its polarity) to be broken than on the strength of the bond itself (other things being equal). Atoms of oxygen, nitrogen, and carbon form a series of decreasing electronegativity, and their bonds with hydrogen decrease in ionicity in the same order. Thus, nitration of alcohols proceeds more readily than that of amines, and in turn these react faster than hydrocarbons under conditions involved in practice. However, in the N- and 0nitration studies cited below, the rates do not reflect this effect; the nitryl ion concentration is so low it is used u p as fast as it is formed. Aromatics Aromatic-heterocyclic compounds are covered in this section if the nitro group substitution involves the aromatic ring; otherwise they are included in the section on cyclics. Aromatic reactivity, substitution, and orientation have long been of intense interest to organic and engineering chemists. This year there was an unusual two-step preparation of 2,3,5trinitrotoluene involving different acid concentrations in the two steps (&A). T h e acid used in the second step contained less water and less nitric acid
than that employed in the first stage. The use of 100% nitric acid a t IO" to 20' C . converted 2,3,5,6-tetralkylbenzenes to the 1,4-dinitro derivatives in 90 to 9S70 yields (37A). Competitive nitration of benzene us. 2-, 3-, and 4-nitrobiphenyls in nitric acid-acetic anhydride a t 0 ' C. showed, through partial rate factors and proportions of the various dinitrobiphenyls formed, that the nitrophenyl group exerts a polar effect similar to that of the halogens, with (mesomeric) electronreleasing and (inductive) electron-attracting effects in evidence (30A). I n a study of triphenyl diethers, mixed acid nitration a t 90' C. produced the pentanitro derivatives; in all cases the end phenyls were substituted in the 2,4,6-positions, but the 1,2-diether led to 4,6-substitution in the central benzene ring, while the 1,3- and 1,4-diethers produced 2,5-substitution in the central benzene ring. The ratio of meta to ortho plus para positions in this central ring was 1 to 1 (35A). I n a Russian patent (27A), p-nitroacetophenone is obtained by treating p-acetylaminoacetophenone as follows : saponification, diazotization, and metathesis with sodium nitrate. French work indicated that 4-acetylbiphenyl does not react with fuming nitric acid in acetic acid at room temperature, but with mixed acid a t 0 ' C . it leads to the 4'-nitro and 2,4'-dinitro derivatives in fair yield ( 7 0 A ) . Using a variety of solvents, benzaldoxime was converted by an equivalent quantity of dinitrogen tetroxide to phenyldinitromethane; other products were formed using the nitrant a t other ratios ( 3 2 A ) . Nitration of benzonitrile with nitric acid in
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VOL. 52, NO. 6
JUNE 1960
545
Unit Processes Review 72% perchloric acid led to 80yc meta substitution, with a n ortho to para ratio of 3.3 to 1 (27A). A Japanese study showed that quinoxaline, its .V-oxide, and the 2,3-dimethyl derivative resisted nitration with even concentrated sulfuric and fuming nitric acid, but the presence of polar substituents, e.g., methoxy, facilitated nitration (36A). Nitration of alkyl-substituted coumarins with strong mixed acids provided 5,7-dinitro derivatives in good yields-e.g., 3-phenylcoumarin (23A).
Aliphatics The reactions of alkyl halides with silver nitrate and pyridine (used as solvent) showed that for chlorides and bromides, the primary halides were more reactive in both respects than the secondary halides; various salts accelerated both reactions, but water, methanol, phenol, and silver chloride had no effect on either reaction (37B). Haloalkyl nitrates were prepared by bubbling alkenes through a mixture of dinitrogen tetroxide and halogen a t low temperature (0' (2.); iodine and dinitrogen tetroxide in chloroform were also cited (3B). Reaction of nitric oxide and chlorine with alkenes at 275' C., with a 4-second contact time, produced halonitro compounds (ZB). Using nitric oxide, with 0.01 % dinitrogen tetroxide as a catalyst a t 150 p s i . , mononitro derivatives were obtained from alkenes; I-pentene produced I-nitro-I-pentene and I-nitro-2-nitrosopentane dimer (6B). Nitrogen dioxide reacted with alkyl radicals to produce nitro compounds, nitrites, and various decomposition products (22B). I n reaction with camphene and trans-stilbene, dinitrogen tetroxide produced 1,2-dinitro compounds and 1,2-nitro alcohols when oxygen was absent; in the presence of oxygen the products were a-nitro ketones, 1,2nitro nitrates, and I,2-nitro alcohols, but not 1,2-dinitro compounds (29B). A n interesting study was made of the addition of dinitrogen tetroxide to cisand trans-stilbene (25B). A 1670 yield of a-nitratoisobutyric acid was obtained
by treating isobutylene with a mixture of nitric acid and dinitrogen tetroxide a t low temperature and saponifying the product with caustic a t 60' C. (70B). T h e effect of irradiation with a Bach quartz mercury vapor lamp on the nitration of propyl and butyl chlorides with ammonium nitrate and 38% sulfuric acid was investigated (7B). I n the absence of irradiation the products were 2-chloro-2-nitro derivatives; irradiation produced 2-chloro-1-nitro derivatives. A patent claims the combination of aluminum nitrate (hydrated) and water is a more effective nitrating agent for nonaromatic hydrocarbons with five or more carbon atoms than aqueous nitric acid; a 40% yield of mononitroisooctane is cited as the result of treating iso-octane at 250' F. (73B). The interesting nitration of nitriles, dinitriles, and ketones with amyl nitrate at low temperature in the presence of a strong, nonhydroxylic base (e.g., potassium ethylate) and a nonhydroxylic solvent (tetrahydrofuran), produced the corresponding a-nitro nitriles, e,@'-dinitro dinitriles, and a,a'-dinitro ketones. The use of potassium tert-butoxide, which is a stronger base than potassium ethylate, substantially improved yields ; yields ranged from 40 to over 907, (8s). Nitration of nitroolefins with 70%, nitric acid was studied under various conditions (9B); 2-nitro-2-butene gave a good yield of 2,2-dinitro-3-nitrobutane. A 38% yield of a-nitroisobutene and 3291, yield of nitro-tert-butyl alcohol were obtained by treating tert-butyl alcohol, isobutene, or a mixture with 70% aqueous nitric acid a t 130' C. and 600 p.s.i.g. for 15 minutes (75B). In the vapor phase nitration of isopentane, the maximum yield of nitroparaffins (nitromethane, nitropropane, and nitrobutanes) and also of nitropentane (17y0)was obtained at 300' C.; effect of carbon tetrachloride on product distribution and yield was discussed (30B). Finely divided fluidized solids were used in the vapor phase nitration with nitric acid of reactive materials requiring short contact times to control exother-
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micity; a cold portion of the finely divided solid was used to quench the reaction. In this way, with methane and 100 to 200 mesh quartz glass, a nitromethane yield of 18 to 25% based on nitric acid was achieved (76B). Nitrate Esters Several patents were concerned with improvements in the nitration process. T h e use of 0.00025 to 0.170 of an inverse separation agent in the nitration of glycol and its mixtures with other alcohols was dercribed; the agent is a polyoxyethylene ether of a polyhydric alcohol such as sorbitol, mannitol, or a glycol of stearic or other fatty acids (17C). If added to the glycol, the inhibitor should be in solution, if added to the mixed acid, it should be insoluble in the glycol nitrated. Starch can be nitrated effectively by dissolving it in concentrated nitric acid and introducing this solution in a finely dispersed form into aqueous nitric acid if the conceiitration is less than 40%; precipitation of the product is aided by the use of fine nozzles of the Schlick type to generate vibrations in the solution (72C). A variation used with cornstarch is to effect complete solution in 55% aqueous nitric acid and to nitrate the clear solution with mixed acid; an ammonia wash is indicated in this case ( 6 C ) . I n a study of the interaction of alkyl chloroformates and silver nitrate in various solvents under mild conditions, it was found (5C) that retention of optical activity is generally the case, Some interesting observations were made on the mechanism of the reaction from a geometrical point of view. T h e 0-nitrations of methanol, p-nitrobenzyl alcohol, ethylene glycol, trimethylene glycol. and glycerol (primary hydioxyl groups) with nitric acid were zero order and identical in absolute rate to one another and to the 'V-nitration of .I-methyl di- or trinitroaniline and to the C-nitrations of benzenoid hydrocarbons, under the same conditions. In the nitration of methanol, small concentrations of sulfuric acid increased, of nitrate ion decreased, and of water did not change the rate; larger concentrations of water increased it, and still larger concentrntions caused a change from zero to first order with a reduction in the absolute rate. L-nder similar conditions, neopentyl alcohol was nitrated in a kinetic form between zero and first order and at a smaller absolute rate. The secondary hydroxyl of glycerol remained until after nitration of the primary hydroxyls and was then nitrated at a smaller rate in a first order mechanism (3C).
KNitro Compounds Two patents concern the production of nitroguanidine and guanidine nitrate
a
( 5 0 , 8 0 ) . T h e ejection of the nitryl ion was found to be rate controlling in the decomposition of nitroguanidines, and mechanistic differences were noted for sulfuric and perchloric acids ( 7 0 ) . Esters of N-monoalkylated carbamic acid were nitrated in high yield by 9570 nitric acid in the presence of a catalyst (0.2 mole per mole or less of material nitrated) consisting of sulfuric acid, boron trifluoride, alkanesulfonic acid, or arenesulfonic acid; e.g., a 96% yield of ethyl-N-tert-butyl-N-nitrocarbamate ( 7 0 0 ) . Dialkylcarbamyl chlorides reacted with silver nitrate to form dialkylnitramines and dialkylnitrosamines in good yield with a fairly even split between the two products; with silver nitrite, the nitrosamines were obtained in good yield ( 6 0 ) . Reactions presumably proceed through unstable intermediate dialkylcarbamyl nitrates or nitrites. An unusual reaction of hexamine in trifluoroacetic anhydride and sulfur dioxide was described where treatment with 99.7y0 nitric acid a t -50' C. yielded N-trifluoroacetyl-N',N"-dinitrotrimethylenetriamine ( 7 0 ) . T h e N-nitration of hr-methyl-2,4,6trinitroaniline by nitric acid in constant excess in relatively dry nitromethane is of zero order, and a t relatively low concentrations of nitric acid the absolute rate of nitration is identical with the common rate of nitration of aromatic hydrocarbons under the same conditions. Small additions of sulfuric acid increase the rate (as in the previous discussion of nitrate esters). T h e nitryl ion mechanism has been established ( 4 0 ) . Heterocyclics Selenophene-2-carboxaldehyde, nitrated in acetic anhydride with fuming nitric acid in the presence of 7% concentrated sulfuric acid, yielded 63% 5nitroselenophene - 2 - carboxaldehyde diacetate, a striking example of sulfuric acid catalysis. If the sulfuric acid was omitted, the yield was 28.5% (74E). I n 30 minutes a t 100' C., phenazine was nitrated in good yield to the 1,6- and 1,9-dinitro derivatives by fuming nitric and fuming sulfuric acid (8E). Nitric acid and acetic anhydride converted 3(5)-phenylpyrazole to a mixture of 3(5) - (4 - nitrophenyl) - 1 - acetylpyrazole and 3(or 5)-phenyl-l-nitropyrazole (ZE). A rather low yield of S-mononitropyridine was obtained by treating a mixture of potassium and sodium nitrates with pyridine a t 300' to 320' C. ( 6 E ) ; 4-nitropyridineethanol was prepared by treating the acetate of 2-pyridineethanolN-oxide in acetic acid with nitric acid (density = 1.50) a t 50' C. (4E). Japanese work has produced 3,5-dinitro-2pyridone from the 5-nitro derivative by nitration with fuming sulfuric and nitric acids a t 70' C. ; the yield was fair (72E).
Nitrating Agents Nitroglycerol nitrated toluene and aniline easily a t 25' to 30' C ; even nitrobenzene yielded m-dinitrobenzene at room temperature in the presence of sulfuric acid. Nitroglycerol was sufficiently stable in concentrated sulfuric acid, so this ruled out the possibility of participation by nitric acid (40F). T h e influence of various gaseous additions (water, ammonia, carbon monoxide, carbon dioxide, and chlorine) on the production of nitric oxide by high frequency discharge a t atmospheric pressure was studied in Russia. Under the conditions employed, only carbon dioxide and chlorine showed any activating influence (55F). * T h e absorption of nitrogen dioxide in a stirred, 30-liter reactor fitted with a gas dispersion disk was more efficient than either the bubble-cap or wettedwall type of units (39F). Increasing the gas-liquid contact time increased efficiency, which is controlled by dinitrogen trioxide formation. T h e kinetics of the process are described. T h e thermal decomposition of nitric acid a t 55', 65', and 75' C. was studied and described by equations. I t was concluded that the data are better explained by a free radical mechanism than by a n ionic onr (76F).
literature Cited (These references were selected from the complete bibliography.) Aromatics (10A) Buu-Ho'i, N. P., Lavit, D., Bull soc. chim. France 1958, pp. 1408-10. (21A) Hammond, G. S., Douglas, K. J., J . Am. Chem. Soc. 81, 1184-7 (1959). (23A) Hurd, C. D., Dowbenko, R., Ibid., 80, 4711-14 (1958). (27A) Kraft, M. Y., Znaeva, K. I., Sytina, E. N., Russ. Patent 118,214 (Feb. 20, 1959). (30A) Mizuno, Y . , Simamura, O., J . Chem. Soc. 1958, pp. 3875-9. (31A) Morningstar, M. G. (to B. F. Goodrich Co.), U. S . Patent 2,864,871 (Dec. 10, 1958). (32A) Novikov, S . S., Khmel'nitskii , L. I., Lebedeva, 0. V.. Zhur. Obschef Khiim. 28. 2296-2304 (1958). (35A) Okon, K., Aluchna, G., Bid. Wojskowej Akad. Technicznej im J . Dabrowskiego 7, No. 38, 49-55 (1957). (36A) Otomasu, H., Nakajima, S., Chem. Pharm. Bull. (Tokyo) 6, 566-70 (1958). (46A) Tonkov, N., Khim. i Znd. ( S o j a ) 30, NO. 1, 24-6 (1958). Aliphatics (1B) Aliev, S. B., Degtyarenko, R. N., Izvest. Vssshikh Ucheb. Zavednii, Neft i Gaz 1958, NO. 5, 109-14. (2B) Bachman, G. B., Chupp, J. P. (to Purdue Research Foundation), U. S. Patent 2,874,195 (Feb. 17, 1959). (3B) Bachman, G. B., Logan, T. J. (to Purdue Research Foundation), Zbid., 2,864,853 (Dec. 16, 1958). (6B) Burkehardt, C. A., Brown, J. F., Jr. (to General Electric Co.), Zbid., 2,867,669 (Jan. 6, 1959).
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(8B) Feuer, H., Savides, C., J . Am. Chem. SOC. 81, 5826-30 (1959). (9B) Frankel, M. B., Klager, K., J . Org. Chem. 23, 494-5 (1958). (10B) Gardner. J. H.. Steadman. T. R. ' (to Escambii Chemical Corp.f, U. S. Patent 2,847,453 (Aug. 12, 1958). (13B) Krause, J. H., Smith, R. K. (to Houdry Process Corp.), Zbid.: 2,864,870 (Dec. 16, 1958). (15B) McKinnis, A. C. (to Collier Carbon & Co.),Zbzd., 2,886,602 (May 12, 1959). (I6B) McKinnis. A. C.. Skinner. D. A. (to ' Union Oil Co.'of Calif.), Zbid.,'2,855,447 (May 5, 1959). (22B) Patsevich, I. V., Topschiev, A. V., Shtern, V. Y., Doklady Akad. hTauk S.S.S.R. 123, 696-9 (1958). (25B) Schechter, H., Gardikes, J. J., Pagano, A. H., J . Am. Chem. SOC.81, 5420-3 (1959). (29B) Stevens, T. E., Zbid., 81, 3593-7 (1959). (30B) Topschiev, A. V., Alaniya, V. P., Poltavtseva, L. I., Doklady Akad. Nauk S.S.S.R. 125, 104-5 (1959). (31B) Vona, J. A., Stiegman, J., J . Am. Chcm. Soc. 81, 1095-9 (1959). Nitrate Esters (3C) Blackall, E. L., Hughes, E. D., others, J . Chem. Soc. 1958, pp. 4366-74. (5C) Boschan, R., J . Am. Chem. Soc. 81, 3341-6 (1959). (6C) Graegeroff, I. A. (to Vsevold A. Amoretty), U. S. Patent 2,883,376 (April 21, 1959). (11C) Wilt, P. E., 111, Young, A. A. (to Atlas Powder Co.), Zbid., 2,883,414 (April 21, 1959). (12C) Zimmerman, W., Sieper, G. A., Reinhardt, L. (to Wassag-Chemie A.-G.), Brit. Patent 815,280 (June 24, 1959). N-Nitro Compounds (1D) Bonner, T. G., Lockhart, J. C., J . Chem. Sac. 1958, pp. 3852-8. (4D) Hughes, E. D., Ingold, C., Pearson, R. B., Zbid., pp. 4357-65. (5D) Levy, J. (to Trubek Laboratories), U. S. Patent 2,871,259 (Jan. 25, 1959). (6D) Norris, W. P., J . Am. Chem. Soc. 81, 3346-50 (1959). (7D) Reed, R., Jr., J . o r g . Chdm. 23, 775-7 (1958). (8D) Robinson, J. N., Miller, F. J. L., McDonnell, B. (to Consolidated Jfining & Smelting Co. of Canada, Ltd.), U. S. Patent 2,895,994 (July 21, 1959). (10D) Thomas, G. R., Zbid., 2,877,263 (March 10, 1959). Heterocyclics (2E) Casoni, D. D. M., Ann. chim. (Rome) 48, 783-7 (1958). (4E) Cislak, F. E. (to Reilly Tar & Chemical Corp.), U. S. Patent 2,868,797 (Jan. 13. 1959). (6E) Kimura, M.: Takano, Y . , Yakugaku Zasshi 79, 549-52 (1959). (8E) Otamasu, H., Chem. Pharm. Bull. (Tokyo) 6, 77-81 (1958). (12E) Takahashi, T., Yoneda, F., Zbid., 6, 442-3 (1958). (14E) Yur'ev, Y . K., Sadovaya, V. K., Zhur. Obsbhei Khim. 28, 2162-4 (1958). Nitrating Agents (16F) Cordes, H. F., Fetter, N R., Happe, J. A., J . Am. Chem. Soc. 80,4802-8 (1958). (39F) Peters, M. S., Koval, E. J., IND. ENG.CHEM.51, 577-80 (1959). (40F) Plazak, E., Ropusznski, S., Roczniki Chem. 32, 681-3 (1958). (55F) Zabolotskii, T. V., Zzvest. Sibzr. Otde!. Akad. Nauk S.S.S.R. 1958, No. 5, 53-6. VOL. 52, NO. 6
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