Vapour phase nitration

The simpler nitrop>arafIins (nitromethane, nitroethane, 1- and 2-nitroproj)ane) are now cheap commercial products. They are obtained by the vapour phase nitration of the hydrocarbons a gaseous mixture of two mols of hydrocarbon and 1 mol of nitric acid vapour is passed through a narrow reaction tube at 420-476°. Thus with methane at 476° a 13 per cent, conversion into nitro methane is obtained ethane at 420° gives a 9 1 mixture of nitroethane (b.p. 114°) and nitromethane (b.p. 102°) propane at 420° afifords a 21 per cent, yield of a complex mixture of 1- (b.p. 130-6°) and 2-nitropropane (b.p. 120°), nitroethane and nitromethane, which are separated by fractional distillation. 

Traditionally, aromatic nitrations have been performed with mixtures of concentrated nitric and sulphuric acids leading to considerable effluent problems associated with the spent acid. A heterogeneous alternative for. sulphuric acid has been reported (Bertea et al., 1995), namely dealuminated Mordenite, which is sufficiently robust to be able to catalyse the vapour-phase nitration of benzene with 65% aqueous nitric acid. 

In a plant for the continuous nitration of chlorobenzene, maloperation during startup caused the addition of substantial amounts of reactants into the reactor before effective agitation and mixing had been established. The normal reaction temperature of 60°C was rapidly exceeded by at least 60° and an explosion occurred. Subsequent investigation showed that at 80° C an explosive atmosphere was formed above the reaction mixture, and that the adiabatic vapour-phase nitration would attain a temperature of 700° C and ignite the explosive atmosphere in the reactor. See l,3-Bis(trifluoromethyl)benzene, above... 

Nitroalkanes can be formed from the direct nitration of aliphatic and alicyclic hydrocarbons with either nitric acid ° or nitrogen dioxide in the vapour phase at elevated temperature. These reactions have achieved industrial importance but are of no value for the synthesis of nitroalkanes on a laboratory scale, although experiments have been conducted on a small scale in sealed tubes. 

The vapour phase nitration of hydrocarbons proceeds via a radical mechanism and so it is found that tertiary carbon centres are nitrated most readily, followed by secondary and primary... 

Both nitric acid and nitrogen dioxide, in the liquid and vapour phase, have been used for the nitration of the alkyl side chains of various alkyl-substituted aromatics without affecting the aromatic nucleus.Thus, treatment of ethylbenzene with nitric acid of 12.5 % concentration in a sealed tube at 105-108 °C is reported to generate a 44 % yield of phenylnitroethane. The nitration of toluene with nitrogen dioxide at a temperature between 20-95 °C yields a mixture of phenylnitromethane and phenyldinitromethane with the proportion of the latter increasing with reaction temperature. ... 

In the solid state, dinitrogen pentoxide is ionic, existing as N02+N03 and sometimes called nitronium nitrate. The same is true of dinitrogen pentoxide in polar solvents like nitric acid where complete ionization to nitronium and nitrate ions is observed. In the vapour phase, and in nonpolar solvents, a covalent structure is observed. This dichotomy of behavior in both physical state and in solution means that no single nitrating agent is as diverse and versatile as nitrogen pentoxide. 

Nitromethane was first prepared in 1872 by Kolbe, and is produced commercially by high-temperature vapour-phase nitration of propane. The process, which uses nitric acid as the nitrating agent, is based on a free-radical reaction in which the active species is the NO2 radical (Markofsky, 1991 Angus Chemical Co., 1998). 

Nitro compounds. Aliphatic nitro compounds are acidic. They are freed from alcohols or alkyl halides by standing for a day with concentrated sulphuric acid, then washed with water, dried with magnesium sulphate followed by calcium sulphate and distilled. The principal impurities are isomeric or homologous nitro compounds. In cases where the nitro compound was originally prepared by vapour phase nitration of the aliphatic hydrocarbon, fractional distillation should separate the nitro compound from the corresponding hydrocarbon. Fractional crystallisation is more effective than fractional distillation if the melting point of the compound is not too low. 

Its practical application depends on the manufacture of nitromethane, which can be now obtained by vapour phase nitration (Hass et al, [78] T. Urban ski and Sion [79]). 

The most detailed research on nitration in the gaseous phase has been carried out by Wilson and Miles [69]. They concluded that there were no prospects of introducing this method in practice, since the nitrocellulose produced is non-homogeneous, and the nitration is inadmissibly slow. Thus, to reach a nitrogen content of 10% N by vapour phase nitration, at a vapour pressure of 16 mm Hg, a period of 48 hr was needed. 

Wilson and Miles suggest the following second order equation for the rate of nitration with nitric add in vapour phase ... 

T. Urban ski and Siemaszko [75] examined the nitration of cellulose with N02C1 in the vapour phase. Gaseous N02C1 was drawn through cellulose at 20°C for 4 hr, and the nitrocellulose than subjected to the usual treatment. A product with 12.2%N was obtained with 168% yield. The product does not contain chlorine. Its stability is higher than that of nitrocellulose of the same nitrogen content prepared by means of the usual nitrating mixture. 

The nitration of aromatics in the vapour phase has two main advantages namely the continuous removal/desorption of water and the possibility to use a fixed bed reactor, which allows a continuous nitration process. 

Smith et a/.[24] have investigated deactivation and shape-selectivity effects in toluene nitration in the vapour phase with N02 as nitrating agent using zeolites, MCM-41 and sulfated zirconia as catalysts. Almost all the catalysts exhibited deactivation over a period of about 5 h on stream, due mainly to coke formation. 

Bertea et /.173-751 reported the vapour phase nitration of benzene with aqueous nitric acid (65%) at 170 °C over post-synthetic dealuminated Y, ZSM-5 and Mordenite zeolites by high temperature and acid treatment. This treatment reduces the framework as well as the nonframework aluminium content, which results in... 

Regioselective vapour phase nitration of o-xylene to 4-o-NX with dilute nitric acid (30%) using HBeta catalyst at 150 °C has been reported.[80,81] Under these... 

Germain, A., Akouz, T. and Figueras, F. Vapour-phase aromatic nitration with dinitrogen tetroxide over solid acids kinetics and mechanism, J. Catal., 1994, 147, 163-170. 

Bertea, L. E., Kouwenhoven, H. W. and Prins, R. Vapour-phase nitration of benzene over zeolitic catalysts, Stud. Surf. Sci. Catal., 1994, 84, 1973-1980. 

Sato, H., Hirose, K., Nagai, K., Yoshioka, H. and Nagaoka, Y. Vapour phase nitration of benzene over solid acid catalysts II. Nitration with nitric acid (1) montmorillonite and mixed metal oxide catalysts, Appl. Catal., A, 1998, 175, 201-207. 

The nitration of aliphatic hydrocarbons may be accomplished in the vapour phase, at 410-430°C, using nitric acid vapour. A number of papers describing this method of nitration were published by Hass and co-workers [29] between 1936 and 1940. 

Hass Mid Shechter [33] have formulated thirteen general rules governing the vapour-phase nitration of pMaffins (and cycloparaffins). Here they are in a summarized form ... 

Detailed experiments on nitration of n-hexane with nitrogen dioxide in the vapour phase were recently carried out by Slebodzinski, T. Urbanski et al. [64], They nitrated n-hexane using the molar ratio... 

McKee Mid Wilhelm [92] found that the highest yield of nitration of benzene with nitrogen dioxide in the vapour phase was obtained at 310°C. In the presence of activated silica the yield was higher than that obtained without a catalyst Mid could Mnount to 83.6%. Nitrophenol was also formed in a yield rising with temperature, so that at 330°C it was the principal reaction product. The authors supposed that the catalytic action of activated silica was probably a specific one Mid did not consist only in binding the water formed during the nitration process. 

Nitration in the vapour phase, hi methods of this kind (e.g. the Wilhelm method [13]) a continuous stream of vapours of the aromatic hydrocarbon and nitric acid or nitrogen dioxide is introduced into the reactor. Sulphuric acid is not used in this reaction. 

Nitromethane, CH3N02, is a volatile liquid. It may be obtained by direct vapour phase nitration of paraffinic hydrocarbons either with nitric acid vapours (Hass et al. [1-8]) or with nitrogen dioxide (T. Urbanski and Slebodzinski [9]). It may also be prepared by the action of sodium- or potassium nitrite on chloroacetic acid... 

Zeolites have been used before in the vapour phase nitration of aromatic compounds using nitrogen dioxide.20 However, the conditions were harsh and there was no regioselectivity. Initially, therefore, we attempted to reproduce the mild conditions of Suzuki for nitration of chlorobenzene. Liquid N2O4 (approx. 10 ml) was condensed into a trap at -78 °C and was then warmed to 0 °C. Fe(acac)3 (0.355 g) and chlorobenzene... 

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