Nitroparaffins

Nitroparaffias (or nitroaLkanes) are derivatives of the alkanes ia which one hydrogen or more is replaced by the electronegative nitro group, which is attached to carbon through nitrogen. The nitroparaffins are isomeric with alkyl nitrites, RONO, which are esters of nitrous acid. The nitro group ia a nitroparaffin has been shown to be symmetrical about the R—N bond axis, and may be represented as a resonance hybrid  

Nitroparaffins are classed as primary, RCH2NO2, secondary, R2CHNO2, and tertiary, R2CNO2, by the same convention used for alcohols. Primary and secondary nitroparaffins exist ia tautomeric equiUbtium with the enoHc or aci forms. 

The nitroparaffins are named as derivatives of the corresponding hydrocarbons by usiag the prefix nitro to designate the NO2 group (1), eg, 1,1-dinitroethane, CH2CH(N02)2- The salts obtained from nitroparaffins and the so-called nitronic acids are identical and may be named as derivatives of either, eg, sodium salt of i7ti-nitromethane, or sodium methanenitronate [25854-38-0]. 

Nitromethane, nitroethane, 1-nitropropane, and 2-nitropropane are produced by a vapor-phase process developed ia the 1930s (2). 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

Introduction of a nitro group into a hydrocarbon considerably enhances the thermodynamic acidity of the C—H bond. For example, nitroethane has pK 8.60 [99]. The effect of the nitro group is comparable to the effect of the two keto groups in acetylacetone (pK 8.9) [17]. The nitrocarbanion (or nitronate ion) is strongly stabilized by inductive and mesomeric electron withdrawal. In solution the ionization (73) is slightly complicated by the presence of the aci-nitro isomer, similar to the enolic form of a ketone, viz. 

The pK values for the nitro and aci-nitro form show that at high pH ( 7) in aqueous solution the first equilibrium predominates. At low pH ( 3) the equilibrium is between nitroethane and the aci-nitro isomer and only low concentrations of the latter are present, (aci-nitroethane)/(nitroethane) = 9 x 10 5. When acid is added to a solution of ethane nitronate in alkali, aci-nitroethane forms more rapidly than nitroethane (proton transfer to oxygen is fast compared with proton transfer to carbon) and the second equilibrium may be studied before any nitroethane has formed. The first equilibrium may be studied in equilibrated solutions at pH 7—11 [99]. 

Substituent effects on the rate of reaction operate on the slow step with a nearly pyramidal transition state whereas substituent effects on the equilibrium will be largely determined, it is argued, by changes in the stability of the nitronate ion. Hence it is not unreasonable that substituents have different effects on the rate and equilibrium. The effect of a substituent on the free energy of the transition state may be smaller, greater, or opposite to the effect on the free energy of the nitronate ion. A similar argument was used to explain the inverse orders of rates and acidity in the series nitromethane, nitroethane and 2-nitropropane [109(b), 109(c)]. 

The foregoing results indicate that it is difficult to obtain information about transition state structure from the size of the Bronsted exponent in the ionization of nitroparaffins. The current view is that proton transfer is about half-complete when the transition state is reached [76(b), 106]. It is difficult to reconcile the results for nitroparaffins with the hope that the Bronsted exponent may in general give an indication of transition state structure and there is a general tendency to treat nitroparaffins as exceptional and still use this concept in other cases. 

The rate of reaction of the anion of 2,6-dinitrotoluene with hydronium ion, water, carboxylic acids, phenols and alcohols (77) has been studied by generating the anion photochemically in aqueous solution and observing spectrophotometrically its neutralization by HA [110], viz. 

The nitroalkanes are useful in extractive distillation processes based on the azeotropes the nitroalkanes form with many industrial solvents. Use of the nitropropanes in solvent blends result in shorter drying times for flexographic and gravure inks. 

Nitroparaffins afford an unique reaction medium for Friedel-Crafts reactions since these solvents will dissolve Lewis acid catalysts such as anhydrous aluminum chloride (AICI3), boron trifluoride (BF3), titanium tetrachloride (TiCl4), and stannic tetrachloride (SnC ). The role of nitromethane as a metal stabilizer for various chlorinated and fluorinated solvents involves its ability to complex with metal salts like aluminum chloride from the solvent-metal reaction. 

Other miscellaneous uses for the nitroalkanes include their use as fuels in racing cars and model engines, their ability to displace water from metal surfaces during metallic pigment grinding operations, and as chemical intermediates. 

The nitroparaffins produced by the vapor phase nitration of propane or ethane are available from two United States production facilities Angus Chemical Company (Sterling, LA) and W. R. Grace and Company (Deer Park, TX). This chapter discusses the naming nomenclature for the nitroparaffins, their physical properties, the various industrial uses, possible environmental concerns, and the safe handling of the nitroalkanes. 

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Some nitro-compounds are themselves coloured and can be used as dyestuffs, e.g. picric acid. In this case the nitro-group can be considered to be the chromophore. For aliphatic nitro-compounds see nitroparaffins. 

The lower nitroparaffins are used as propellants, as solvents and as chemical intermediates, e.g. nilromethane is an excellent solvent for polar materials especially metal salts. 

Table 5.4 gives the specific energies of selected organic liquid compounds. Compared with the isooctane chosen as the base reference, the variations from one compound to another are relatively small, on the order of 1 to 5%, with the exception of some particular chemical structures such as those of the short chain nitroparaffins (nitromethane, nitroethane, nitropropane) that are found to be energetic . That is why nitromethane, for example, is recommended for very small motors such as model airplanes it was also used in the past for competitive auto racing, for example in the Formula 1 at Le Mans before being forbidden for safety reasons. 

The normal form A can pass by tautomeric change under the influence of alkali into the acidic hydroxy form B, which in turn can 3deld the sodium salt C. Nitroparaffins are therrfore pseudo-acids, and are soluble in alkaline solution. 

The lower nitroalkanes (sometimes refered to as nitroparaffins) are easily reduced by a multitude of systems, but by far the easiest, and also the highest yielding, is the Iron/Hydrochloric acid system. The reaction is ... 

Just as most other aldehydes do, furfural condenses with compounds possessing active methylene groups such as aUphatic carboxyUc esters and anhydrides, ketones, aldehydes, nitriles, and nitroparaffins. 

Both vapor-phase and Hquid-phase processes are employed to nitrate paraffins, using either HNO or NO2. The nitrations occur by means of free-radical steps, and sufftciendy high temperatures are required to produce free radicals to initiate the reaction steps. For Hquid-phase nitrations, temperatures of about 150—200°C are usually required, whereas gas-phase nitrations fall in the 200—440°C range. Sufficient pressures are needed for the Hquid-phase processes to maintain the reactants and products as Hquids. Residence times of several minutes are commonly required to obtain acceptable conversions. Gas-phase nitrations occur at atmospheric pressure, but pressures of 0.8—1.2 MPa (8—12 atm) are frequentiy employed in industrial units. The higher pressures expedite the condensation and recovery of the nitroparaffin products when cooling water is employed to cool the product gas stream leaving the reactor (see Nitroparaffins). 

An important side reaction in all free-radical nitrations is reaction 10, in which unstable alkyl nitrites are formed (eq. 10). They decompose to form nitric oxide and alkoxy radicals (eq. 11) which form oxygenated compounds and low molecular weight alkyl radicals which can form low molecular weight nitroparaffins by reactions 7 or 9. The oxygenated hydrocarbons often react further to produce even lighter oxygenated products, carbon oxides, and water. 

Only 20—40% of the HNO is converted ia the reactor to nitroparaffins. The remaining HNO produces mainly nitrogen oxides (and mainly NO) and acts primarily as an oxidising agent. Conversions of HNO to nitroparaffins are up to about 20% when methane is nitrated. Conversions are, however, often ia the 36—40% range for nitrations of propane and / -butane. These differences ia HNO conversions are explained by the types of C—H bonds ia the paraffins. Only primary C—H bonds exist ia methane and ethane. In propane and / -butane, both primary and secondary C—H bonds exist. Secondary C—H bonds are considerably weaker than primary C—H bonds. The kinetics of reaction 6 (a desired reaction for production of nitroparaffins) are hence considerably higher for both propane and / -butane as compared to methane and ethane. Experimental results also iadicate for propane nitration that more 2-nitropropane [79-46-9] is produced than 1-nitropropane [108-03-2]. Obviously the hydroxyl radical attacks the secondary bonds preferentially even though there are more primary bonds than secondary bonds. 

Conversions per pass of NO2 to nitroparaffins tend to be significantly less than when HNO is used. When propane is nitrated with NO2, conversions are as high as 27%, but they are much less for nitrations of methane and ethane. The remaining NO2 reacts mainly to produce NO, and a considerable number of oxidation steps occur. The theoretically maximum conversion of NO2 to nitroparaffins is 66.7%. 

HNO conversions to nitroparaffins pass through a maximum at paraffiniHNO molar ratios of approximately 4 1 to 6 1 (32). At higher ratios, a high fraction of the HNO reacts to form alkyl free radicals. At lower ratios, a large fraction of HNO decomposes, as ia reaction 5. 

In a commercial unit, a spray nitrator (39) is operated adiabaticaHy. The Hquid HNO feed is sprayed direcdy into the hot and preheated propane feed. The heat of nitration provides the heat to vaporize the HNO and to preheat it to the desired temperature for nitration. At one time, several spray nitrators were operated in series, with additional HNO being sprayed into each nitrator (32). In such an arrangement, the optimum propane HN02 ratios did not occur, and considerable amounts of nitroparaffins degraded. 

Eor vapor-phase processes, the product stream from the nitrator must be separated. The nitroparaffins, excess propane, and NO plus NO2 (which are converted back to HNO ), are recovered. The oxygenated products are removed, but there are generally insufficient amounts for economic recovery. 

The vapor-phase process of SocifitH Chemique de la Grande Paroisse for production of nitroparaffins employs propane, nitrogen dioxide, and air as feedstocks (34). The yields of nitroparaffins based on both propane and nitrogen dioxide are relatively high. Nitric oxide produced during nitration is oxidized to nitrogen dioxide, which is adsorbed in nitric acid. Next, the nitric dioxide is stripped from the acid and recirculated. 

Nitromethane [75-52-5] is produced in China. Presumably a modified Victor Meyer method is being employed. Nitromethane is transported in dmms or smaller containers. Two tank cars of nitromethane exploded in separate incidents in the 1950s. Both explosions occurred in the switching yard of a railroad station. In both cases, essentially adiabatic vapor compression of the nitromethane—air mixture in the gas space of the tank car resulted in the detonation of the Hquid nitromethane. Other nitroparaffins do not, however, detonate in this manner. 

A nitro alcohol is formed when an ahphatic nitro compound with a hydrogen atom on the nitro-bearing carbon atom reacts with an aldehyde in the presence of a base. Many such compounds have been synthesized, but only those formed by the condensation of formaldehyde (qv) and the lower nitroparaffins (qv) are marketed commercially. The condensation may occur one to three times, depending on the number of hydrogen atoms on the nitro-substituted carbon (R and R = H or alkyl), and yield nitro alcohols with one to three hydroxyl groups. 

The nitro alcohols available in commercial quantities are manufactured by the condensation of nitroparaffins with formaldehyde [50-00-0]. These condensations are equiUbrium reactions, and potential exists for the formation of polymeric materials. Therefore, reaction conditions, eg, reaction time, temperature, mole ratio of the reactants, catalyst level, and catalyst removal, must be carefully controlled in order to obtain the desired nitro alcohol in good yield (6). Paraformaldehyde can be used in place of aqueous formaldehyde. A wide variety of basic catalysts, including amines, quaternary ammonium hydroxides, and inorganic hydroxides and carbonates, can be used. After completion of the reaction, the reaction mixture must be made acidic, either by addition of mineral acid or by removal of base by an ion-exchange resin in order to prevent reversal of the reaction during the isolation of the nitro alcohol (see Ion exchange). 

Most organic compounds, including aromatic hydrocarbons, alcohols, esters, ketones, ethers, and carboxyUc acids are miscible with nitroparaffins, whereas alkanes and cycloalkanes have limited solubiUty. The lower nitroparaffins are excellent solvents for coating materials, waxes, resins, gums, and dyes. 

The thermal characteristics of higher nitroparaffins are quite different from those of nitromethane. The nitropropanes provide neatly twice as much heat as does nitromethane when burned in air or oxygen. When the only source of oxygen is that contained within the molecule, nitropropanes yield only 20% as much energy as nitromethane on burning. 

The chemical reactions of the nitroparaffins have been discussed in depth (11—19), and their utility for the synthesis of heterocycHc and other compounds has been noted (20,21). 

Salts. Nitroparaffins dissociate to form ambidentate anions, which are capable of alkylation at either the carbon or oxygen atom (22). 

Alkali salts of primary nitroparaffins, but not of secondary nitroparaffins, react with acyl cyanides to yield a-nitroketones by C-acylation (27). 

Most other acylating agents act on salts of either primary or secondary nitroparaffins by O-acylation, giving first the nitronic anhydrides which rearrange to give, eg, nitrosoacyloxy compounds (28). 

Reaction with nitrous acid can be used to differentiate primary, secondary, and tertiary mononitroparaffins. Primary nitroparaffins give nitrolic acids, which dissolve in alkali to form bright red salts. 

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