98- 95-3 Nitrobenzene

Nitrobenzene (melting point 5.9°C, boiling point 210.9°C, density 1.199, flash point 88°C) is made by the direct nitration of benzene using a nitric acid-sulfuric acid mixture (Fig.l), usually in a cast-iron or steel kettle. 

The majority of the nitrobenzene is used for the production of aniline with a minor amount being used for the production of acetaminophen. 

Nitrobenzene boils at 207° (uncorrected), melts at 3°, and has the specific gravity 1.204 at 20°/4°. The yield should be about 60-70 grams. 

—A low yield of nitrobenzene may be the result of one of the following causes Either the nitration was effected at too low a temperature and a quantity of benzene is recovered in the final distillation, or the temperature of the mixture reaches too high a point and a large amount of dinitrobenzene is formed. In the latter case there will be an appreciable residue in the flask when the temperature reaches 207° in the final distillation. This result is apt to occur if the benzene is added too rapidly at first, and the mixture of acid kept at too low a temperature little nitration occurs, and when the mixture is heated in the water-bath, the reaction takes place rapidly, and the heat generated causes the temperature to rise above 60°, with the result that dinitrobenzene is formed. The effect on the result of the temperature at which nitration is carried out is marked, as can be seen by comparing with the above preparation that of dinitrobenzene described below. 

It is necessary to wash the nitrobenzene fine from nitric acid. If this is not done, the acid which remains dissolved in the nitrobenzene will react further with it when the product is distilled. Brown vapors of oxides of nitrogen will be given off, dinitrobenzene will be formed, and an explosion may occur. The residue of dinitrobenzene should not be distilled it is apt to decompose violently when the flask is nearly empty. It may be dissolved out of the flask and crystallized from alcohol. 

Solubility of nitrobenzene.—Describe the odor of nitrobenzene. Determine whether nitrobenzene is soluble in alcohol, ether, benzene, dilute hydrochloric acid, and a solution of sodium hydroxide. Mix about 2 cc. of nitrobenzene with 5 cc. of concentrated sulphuric acid and pour the mixture into water. How could you most readily separate into its constituents a mixture of benzene and nitrobenzene  

Aniline hydrochloride, C6H5NH2.HCI, is formed as a crystalline solid. Shake up a few drops of the aniline with 2 cc. of water and add a few drops of bromine-water. Tribromoaniline, BrsCshhNhh, is precipitated. 

Nitrobenzene was first obtained by Eilhard Mitscherlich in 1834, by nitration of benzene. In England in 1847, its production from coal-tar benzene was patented and the technology developed by Charles B.Mansfield manufacture began in France in 1848. 

Nitrobenzene is used mainly as an intermediate in the production of aniline. It also forms the basis for the production of 3-chloronitrobenzene, m-nitrobenzene-sulfonic acid and p-aminophenol its earlier importance in the production of the dyestuff intermediate benzidine is insignificant today, due to the carcinogenicity of the latter. The industrial production of nitrobenzene is achieved by isothermal nitration of benzene. Mixed nitration acid, composed of 40% HNO3, 40% H2SO4 and 20% water, is used as the nitrating medium. Whereas the process was originally carried out batchwise, continuous operation predominates today in plants with a capacity up to 100,000 tpa. Stainless steel equipment is used, which is resistant to corrosion due to a passivation effect. 

1 and 2 Cooler 3 Pump circulation reactor 4-6 Centrifugal separators 7 and 8 Stirred vessels 9 Benzene storage tank 

Since nitration proceeds heterogeneously between the aqueous and organic layer, the benzene and mixed acid are mixed intensively. Nitration then takes place predominantly in the acid phase. The reaction, which is carried out at 60 °C, 

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Uses Nitrobenzene is a colorless to pale yellow liquid. It has been used extensively in a variety of industries, (e.g., the manufacture of aniline dyes and soaps, as solvent for paints, for refining lubricating oils, as shoe polish, floor polish, dressings for leather products, the manufacture of explosives). 

Aniline is an important derivative of benzene that can be made in two steps by nitration to nitrobenzene and either catalytic hydrogenation or acidic metal reduction to aniline. Both steps occur in excellent yield. Almost all nitrobenzene manufactured (97%) is directly converted into aniline. The nitration of benzene with mixed acids is an example of an electrophilic aromatic substitution involving the nitronium ion as the attacking species. The hydrogenation of nitrobenzene has replaced the iron- 

The uses of aniline obtained from nitrobenzene are given in Table 11.5. Aniline s use in the rubber industry is in the manufacture of various vulcanization accelerators and age resistors. By far the most important and growing use for aniline is in the manufacture of jP,jP-methylene diphenyl diisocyanate (MDI), which is polymerized with a diol to give a polyurethane. 

Two moles of aniline react with formaldehyde to give p,p-methylenedianiline (MDA). MDA reacts with phosgene to give MDI. The student should develop the mechanism of this electrophilic aromatic substitution. 

We have already been introduced to polyurethane chemistry in Chapter 10, Section 2, where we used toluene diisocyanate (TDI) reacting with a diol to give a polyurethane. Polyurethanes derived from MDI are more rigid than those from TDI. New applications for these rigid foams are in home insulation and exterior autobody parts. The intermediate MDA is now on the Reasonably Anticipated to Be Human Carcinogens list and the effect of this action on the market for MDI remains to be seen. The TLV-TWA values for MDA and MDI are some of the lowest of the chemicals we have discussed, being 0.1 and 0.005 ppm respectively. 

A slurry of 2 (0.5861 g, 84.4 mmol of lithium 14.0901 g, 109.9 mmol of naphthalene 4.7055 g, 36.7 mmol of C0CI2) was prepared, and the supernatant liquid removed by centrifugation. The metal powder was returned to the reaction flask via cannula with use of 100 ml of fresh glyme. Nitrobenzene 

The aqueous extracts were combined, made basic (pH 9), and reextracted with diethyl ether to give a small amount (0.25 g) of brown oil. TLC analysis showed that a multicomponent mixture had resulted. This vras not investigated further. 

TOXIC BY INHALATION, SWALLOWING, OR SKIN CONTACT DANGER OF CUMULATIVE EFFECTS 

Colorless to pale yellow oily liquid with odor of bitter almonds bp, 210-21TC.1 

Combustible. Flash point, 88°C ignition temperature, 496°C. Extinguish with water, dry chemical, foam, or carbon dioxide.2 

Soluble in about 500 parts of water. Soluble in alcohol, benzene, ether, and oils. Volatile 

Alkali. Heating nitrobenzene with sodium hydroxide, potassium hydroxide, and small quantities of water may result in an explosion. 4,5 

The function of the sulphuric acid is that of a dchyclrntin . agent taking up the water foimed iivthe reaction. 

Properties.—Light yellow liquid, with a smell of loittei 

uaterand i c.r. LicI.lI acetic acid. Add a little zinc-dust 

Attach ail uyinphl condenser to. 1 round llask (1 litie). Pour in the methyl alc ohol and add the sodium in small pieces, 

Azoxybenzene from Nitrobenzene by Electrolysis.— Nitrobenzene can be conveniently converted into azoxybenzene by electrolytic reduction. The apparatus required is shown in Fig. 77. 

PNA is mildly irritating to the eyes and may cause some corneal damage.  

In general, higher ambient temperatures increase susceptibility to cyanosis from exposure to methemoglobin-forming agents.  

Exposure of rats to aerosol/vapor of PNA at 30mg/m for 4 weeks produced a significant increase in methemoglobin levels.  

PNA was mutagenic in vitro in some bacterial strains and in chromosomal aberration studies.  

The 2003 ACGIH threshold limit value-time-weighted average (TLV-TWA) is 3 mg/m with a notation for skin absorption. 

Aniline consumption grew at a rate of 5.5 percent per year from 1995 to 2000. It is expected to grow at 3.9 percent per year from 2001 through 2004262. 

Aniline pricing tends to track benzene pricing rather closely. In general the aniline price will move 1.2 cents per pound for each 10 cents per gallon movement in benzene price. Between 1995 and 2001, the aniline sales price varied from US 0.35 per pound to US 0.40 per pound255. 

MDI [4,4 -methylenebis (phenylisocyanate)] accounted for almost 85% of the worldwide demand for aniline in 2000. MDI is used primarily to make rigid polyurethane foam and polyurethane elastomers. MDI growth is expected to be 6% to 8% per year during the 2000 s as its use continues to increase in the construction industry (the largest user of rigid polyurethane foam) and the auto industry (the largest user of reaction-injection molding plastics). The aniline uses are shown in Table 20.3255. 

Miscellaneous includes explosives, epoxy curing agents and pharmaceuticals.  

The use of aniline in rubber chemicals will closely track the Gross Domestic Product growth in the United States, while its use in agricultural chemicals is expected to slowly decline. Aniline-based dyes and pigments are forecast to have little growth255. 

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It is prepared by reduction of nitrobenzene with iron and NaOH. It is also prepared by an electrolytic reduction of nitrobenzene. It is widely used for the preparation of benzidine. 

Crystalline powder, m.p. 174-179 C. Prepared by treatment of thiocarbanilide with sulphur, or by heating aniline, carbon disulphide and nitrobenzene. It is an important rubber accelerator, and on oxidation gives dibenzthiazyl disulphide, also a rubber accelerator. 

It is manufactured by reacting benzene with a mixture of nitric and sulphuric acids. Most of the nitrobenzene produced is used to manufacture aniline, which is obtained by reduction. A considerable proportion is used as a raw material in the dyestufTs industry, either as nitrobenzene as such, or as aniline. 

Further nitration gives w-dinilrobenzene sulphonation gives w-nitrobenzene sulphonic acid. Reduction gives first azoxybenzene, then azobenzene and aniline depending upon the conditions. Used in the dyestufTs industry as such or as aniline. 

CijHgNj. Made by heating o-pheny-lenediamine with glycerol, nitrobenzene and cone, sulphuric acid. It exists as a monohydrate, m.p. 94" C, or anhydrous, m.p. 117°C. It is used as a complexing reagent in the estimation of Fe and in the preparation of numer-... 

Occurs in the high-boiling fraction of coal tar. Most conveniently prepared by Skraup s reaction by healing a mixture of aniline, glycerol, sulphuric acid and nitrobenzene. Used in the manufacture of dyestuffs, and pharmaceutical products. 

Another indication of the probable incorrectness of the pressure melting explanation is that the variation of the coefficient of friction with temperature for ice is much the same for other solids, such as solid krypton and carbon dioxide [16] and benzophenone and nitrobenzene [4]. In these cases the density of the solid is greater than that of the liquid, so the drop in as the melting point is approached cannot be due to pressure melting. 

It reduces nitrobenzene (in the presence of hydrochloric acid) to phenylammonium hydrochloride ... 

Most students will be familiar with simple distillation from their practical inorganic chemistry. Other students should determine the boiling-point of acetone (56°), using a water-bath and water-condenser, or of benzene (81 ), using a sand-bath and water-condenser, and finally of either aniline (184 ) or nitrobenzene (210 ), using for both these liquids a sand-bath and air-condenser. 

Students should carry out the purification by steam distillation of (a) crude nitrobenzene or chlorobenzene, or of (b) crude naphthalene, o-nitrophenol (p. 170) or />-tolunitrile (p. 194) as examples of solid compounds which may also be purified in this way. When the distillation is complete, disconnect the tubing (Fig. 15) between C and D before removing the flame from under D, otherwise the contents of C will be sucked back into D as the latter cools. 

It is found experimentally, however, that if benzene is treated with concen trated nitric acid alone, the yield of nitrobenzene is small. If, however, the nitric acid is first mixed with concentrated sulphuric acid, a high yield of nitrobenzene results. 

After the 45 minutes heating, pour the contents of the flask into a large excess of cold water (about 300 ml.), in which the nitrobenzene, being heavier than water, sinks to the bottom. Stir the mixture vigorously in order to wash out as much acid as... 

Nitrobenzene is a pale yellow liquid, having a b.p. 210 , and dy 1 20. It has an odour which is similar to that of almonds, and which is therefore often confused with that of benzaldehyde. Nitrobenzene is used chiefly for the preparation of aniline. 

Nitrobenzene. (Semi-micro scale), (i 10 of above scale.)... 

This is done so that a small separating-funnel of about 100 ml. capacity may be used for the subsequent washing of the nitrobenzene. 

Both aliphatic and aromatic nitro-compounds can be readily reduced in acid solution to the corresponding primary amine. Thus when a mixture of nitrobenzene and tin is treated with hydrochloric acid, the tin dissolves to give stannous chloride, SnCh, which in these circumstances then reacts with more acid to give stannic chloride, SnCl, and the nascent hydrogen produced from... 

The o-nitrophenol is obtained as bright yellow crystals, m.p. 46°, possessing an odour which resembles both that of phenol and of nitrobenzene. 

Benzonitrile is a colourless liquid, having b.p. 190° and d, 1-02 it is almost insoluble in water, and has an odour resembling that of nitrobenzene and of benzaldehyde. 

When a solution of nitrobenzene in methanol is boiled with sodium hydroxide, the nitrobenzene is reduced to azoxybenzene by the methanol, which is itself oxidised to formic acid. 

AZOXYBENZENE. Required Sodium hydroxide, 23 g. nitrobenzene, 15 ml. methanol, 120 ml. 

Add 23 g. of powdered (or flake ) sodium hydroxide to a solution of 15 ml. (18 g.) of nitrobenzene in 120 ml. of methanol contained in a 250 ml. short-necked bolt-head flask. Fix a reflux water-condenser to the flask and boil the solution on a water-bath for 3 hours, shaking the product vigorously at intervals to ensure thorough mixing. Then fit a bent delivery-tube to the flask, and reverse the condenser for distillation, as in Fig. 59, p. 100, or Fig. 23(D), p. 45). Place the flask in the boiling water-bath (since methanol will not readily distil when heated on a water-bath) and distil off as much methanol as possible. Then pour the residual product with stirring into about 250 ml. of cold water wash out the flask with water, and then acidify the mixture with hydrochloric acid. The crude azoxybenzene separates as a heavy oil, which when thoroughly stirred soon solidifies, particularly if the mixture is cooled in ice-water. 

When a mixture of aniline, nitrobenzene, glycerol and concentrated sulphuric acid is heated, a vigorous reaction occurs with the formation of quinoline. It is probable that the sulphuric acid first dehydrates the glycerol giving acrolein or acraldehyde (A), which then condenses at its double bond with the amino group of the aniline to give acrolein-aniline (B), The latter in its enol... 

The nitrobenzene is thus itself reduced to aniline, which can then react as before. 

Required Aniline, 24 ml. nitrobenzene, 13 ml. anhydrous glycerol, 62 ml. sulphuric acid, 36 ml. zinc chloride, 23 g. 

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