A Nitration

Nitration in the 5-position of 4-methyl- and 2,4-dimethylselenazoles with HNO3-H2SO4 is more rapid than for thiazoles [4-methyl-5-nitroselenazole. m.p. 45°C (19) 2.4-dimethyl-5-nitroselenazole. m.p. 115-120 C (decomp.) (19)]. Direct nitration of 2-amino 4-methyl-selenazole leads to nng rupture (19). 

However, prior protective acetylation of the amino group leads to a good yield of the 5-nitro compound [2-acetamido-4-methyl-5-nitroselenazole, m.p. 185 C (19)j. Similarly. 2-diethylamino-4-methy)-selenazole with nitric acid gives the. 5-nitro derivative [vellow needles, m.p. 93°C (26)], 

Preparation Sign in at www.cengage.com/login to answer Pre-Lab Exercises, access videos, and read the MSDSs for the chemicals used or produced in this procedure. Review Sections 2.9, 2.10, 2.11, and 2.29. 

Apparatus A 25-mL round-bottom flask, Claisen adapter, thermometer, water-cooled condenser, ice-water bath, apparatus for magnetic stirring and flameless heating. 

Setting Up Prepare a solution of 4.0 mL of concentrated nitric acid and 4.0 mL of concentrated sulfuric acid in the round-bottom flask and cool it to room temperature 

Reaction and Work-Up In portions of approximateiy 0.5 ml, add 4.5 ml of bro-mobenzene to the stirred mixture through the top of the condenser over a period of about 10 min. Do nof allow the temperature of the reaction mixture to exceed 50-55 °C during the addition. Control the temperature by allowing more time between the addition of successive portions of bromobenzene and by cooling the reaction flask with an ice-water bath. 

After the addition is complete and the exothermic reaction has subsided, heat the stirred mixture for 15 min, keeping its temperature below 60 °C. Cool the reaction mixture to room temperature and then pour it carefully and with stirring into 40 ml of cold water contained in a beaker. 

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This is an acid-base reaction, in which the base is the oxide ion (p. 89) the acidic oxide SiOj displaces the weaker acidic oxide CO2 in the fused mixture. But in aqueous solution, where the 0 ion cannot function as a strong basefp. 89),carbon dioxide displaces silica, which, therefore, precipitates when the gas is passed through the aqueous silicate solution. In a fused mixture of silica and a nitrate or phosphate, the silica again displaces the weaker acidic oxides N2O5 and P4OJ0 ... 

The reduction of a nitrate, for example potassium nitrate, by iron(ll) sulphate in the presence of concentrated sulphuric acid gives reasonably pure nitrogen monoxide. The mixture is warmed and at this temperature the nitrogen monoxide produced does not combine with uncharged iron(II) sulphate (see below). 

With a nitrate in alkaline solution, ammonia is evolved quantitatively by Devarda s alloy (Al, 45% Cu, 50% and Zn, 5%). This reaction can be used to estimate nitrate in absence of ammonium ions (see below) ... 

The addition of even a weak acid (such as ethanoic acid) to a nitrite produces nitrous acid which readily decomposes as already indicated. Hence a nitrite is distinguished from a nitrate by the evolution of nitrous fumes when ethanoic acid is added. 

Combined nitrogen is usually convertible either to ammonia by reduction or to a nitrate by oxidation. Hence tests, qualitative or quantitative, already described can be applied for these. 

When concentrated sulphuric acid is added to a nitrate in the presence of aqueous iron(II) sulphate, the nitrogen oxide liberated forms a brown complex [Fe(H20)5N0] which appears as a brown ring at the acid-aqueous interface (test for a nitrate, p 243). 

The 5 3 mechanism does not satisfactorily account for the kinetic observations, as can be seen when the nature of the base (B) in nitrations is recalled it would most likely be a nitrate or bisulphate anion. 

Another circumstance which could change the most commonly observed characteristics of the two-stage process of substitution has already been mentioned it is that in which the step in which the proton is lost is retarded because of a low concentration of base. Such an effect has not been observed in aromatic nitration ( 6.2.2), but it is interesting to note that it occurs in A -nitration. The A -nitration of A -methyl-2,4,6-trinitroaniline does not show a deuterium isotope effect in dilute sulphuric acid but does so in more concentrated solutions (> 60 % sulphuric acid kjj/kjj = 4 8). ... 

Picric acid may be made by gradually adding a mixture of phenol and sulfuric acid at 90—100°C to a nitration acid containing a small excess of nitric acid. The picric acid crystals are separated by centrifugiag, washed, and dried. The wash water is reused to decrease losses owiag to the water solubiUty of the picric acid. A yield of about 225% of the weight of phenol is commonly obtained. 

Quantitative estimation of cyclohexane in the presence of benzene and aUphatic hydrocarbons may be accompHshed by a nitration-dehydrogenation method described in Reference 61. The mixture is nitrated with mixed acid and under conditions that induce formation of the soluble mononitroaromatic derivative. The original mixture of hydrocarbons then is dehydrogenated over a platinum catalyst and is nitrated again. The mononitro compounds of the original benzene and the benzene formed by dehydrogenation of the cyclohexane dissolve in the mixed acid. The aUphatic compound remains unattacked and undissolved. This reaction may be carried out on a micro scale. 

This secondary reaction starts at about 180°C, but the mass must be heated to 350—400°C to bring the reaction to completion and produce a nitrate-free product. The off-gases are extremely corrosive and poisonous, and considerable attention and expense is required for equipment maintenance and caustic-wash absorption towers. Treatment of the alkaline wash Hquor for removal of mercury is required both for economic reasons and to comply with governmental regulations pertaining to mercury ia plant effluents. 

Other Meta.Is, Although most cobalt is refined by chemical methods, some is electrorefined. Lead and tin are fire refined, but a better removal of impurities is achieved by electrorefining. Very high purity lead is produced by an electrochemical process using a fluosiUcate electrolyte. A sulfate bath is used for purifying tin. Silver is produced mainly by electrorefining in a nitrate electrolyte, and gold is refined by chemical methods or by electrolysis in a chloride bath. 

Nitration is defined in this article as the reaction between a nitration agent and an organic compound that results in one or more nitro (—NO2) groups becoming chemically bonded to an atom in this compound. Nitric acid is used as the nitrating agent to represent C-, 0-, and N-nitrations. O-nitrations result in esters. N-nitrations result in nitramines. 

Nitrations are highly exothermic, ie, ca 126 kj/mol (30 kcal/mol). However, the heat of reaction varies with the hydrocarbon that is nitrated. The mechanism of a nitration depends on the reactants and the operating conditions. The reactions usually are either ionic or free-radical. Ionic nitrations are commonly used for aromatics many heterocycHcs hydroxyl compounds, eg, simple alcohols, glycols, glycerol, and cellulose and amines. Nitration of paraffins, cycloparaffins, and olefins frequentiy involves a free-radical reaction. Aromatic compounds and other hydrocarbons sometimes can be nitrated by free-radical reactions, but generally such reactions are less successful. 

Benzene, toluene, and other aromatics that are easily nitrated can sometimes be nitrated using acids having zero NO/ concentrations (see Fig. 1). Two explanations for this are (/) NO/ is actually present but in concentrations too low to be measured by Raman spectra, and (2) NO/ is hydrated to form H2N0" 2> which is also a nitrating agent. 

In addition to the conventional mixed acids commonly used to produce DNT, a mixture of NO2 and H2SO4 (8), a mixture of NO2 and oxygen (9), and just HNO (10) can also be used. TerephthaUc acid and certain substituted aromatics are more amenable to nitrations using HNO, as compared to those using mixed acids. For compounds that are easily nitratable, acetic acid and acetic anhydride are sometimes added to nitric acid (qv). Acetyl nitrate, which is a nitrating agent, is produced as an intermediate as follows ... 

The danger of an explosion of a nitrated product generally increases as the degree of nitration increases, eg, trinitroaromatics are more hazardous as compared to dinitroaromatics or especially mononitroaromatics. Nitroaromatics and some polynitrated paraffins are highly toxic when inhaled or when contacted with the skin. AH nitrated compounds tend to be highly flammable. 

Fig. 1. Schematic of a nitrate ore bed, detailing the various overburden layers. The overburden thickness can vary from 0 to 2.5 m, where chuca = 0.1 0.5 m, panqueque = 0.1 0.4 m, and poody cemented gravel = 1.5-2.5 m. CaUche ranges from 0.8 to 8 m.

The existence of the nitronium ion in sulfuric-nitric acid mixtures was demonstrated both by cryoscopic measurements and by spectroscopy. An increase in the strong acid concentration increases the rate of reaction by shifting the equilibrium of step 1 to the right. Addition of a nitrate salt has the opposite effect by suppressing the preequilibrium dissociation of nitric acid. It is possible to prepare crystalline salts of nitronium ions, such as nitronium tetrafluoroborate. Solutions of these salts in organic solvents rapidly nitrate aromatic compounds. ... 

A very convenient synthetic procedure for nitration involves the mixing of a nitrate salt with trifluoroacetic anhydride. This presumably generates trifluoroacetyl nitrate. 

OXIDIZING AGENT Coiupound that gives up oxygen easily or removes hydrogen from another compound. It may comprise a gas, e.g. oxygen, chlorine, fluorine, or a chemical which releases oxygen, e.g. a nitrate or perchlorate. A compound that attracts electrons. 

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