Nitration, aromatic acidity

Aromatic aldehydes and ketones have been extensively reported from alkaline nitrobenzene oxidations but not from oxygen or nitric acid oxidations. A number of nitrophenols and nitrated aromatic acids have been... 

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 ... 

Nitrous acid or nitrite salts may be used to catalyze the nitration of easily nitratable aromatic hydrocarbons, eg, phenol or phenoHc ethers. It has been suggested that a nitrosonium ion (NO + ) attacks the aromatic, resulting initially in the formation of a nitro so aromatic compound (13). Oxidation of the nitro so aromatic then occurs ... 

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. ... 

The central C-C double bond of dibenz[, /]oxepin displays the properties of an activated aromatic system and undergoes substitution reactions. Nitration and acid-catalyzed dcutcration gives the dibenzo[i>,/]oxepins 1 with the respective substituent in position 10.161... 

The various methods that are used for the production of aromatic acids from the corresponding substituted toluenes are outlined in Figure 1. The first two methods -chlorination/hydrolysis and nitric acid oxidation - have the disadvantage of relatively low atom utilization (ref. 13) with the concomitant inorganic salt production. Catalytic autoxidation, in contrast, has an atom utilization of 87% (for Ar=Ph) and produces no inorganic salts and no chlorinated or nitrated byproducts. It consumes only the cheap raw material, oxygen, and produces water as the only byproduct. 

Dinitrogen pentoxide (prepared by the oxidation of N204 with 03) in nitric acid is a potent nitration system. It can be used for nitrating aromatic compounds at lower temperatures than conventional system. It is also convenient for preparing explosives that are unstable in nitrating media containing sulfuric acid (Eq. 2.7).20... 

Adsorption of nitric and sulfuric acids on ice particles provides the sol of the nitrating mixture. An important catalyst of aromatic nitration, nitrous acid, is typical for polluted atmospheres. Combustion sources contribute to air pollution via soot and NO emissions. The observed formation of HNO2 results from the reduction of nitrogen oxides in the presence of water by C—O and C—H groups in soot (Ammann et al. 1998). As seen, gas-phase nitration is important ecologically. 

A commonly used strategy for the higher nitration of phenolic substrates is to sulfonate the electron-rich aromatic ring before nitration. Sulfonic acid groups are electron withdrawing and... 

Nitronium salts are efficient and powerful nitrating agents. Nitronium tetrafluoroborate is the most commonly used nitronium salt for nitration and is commercially available as a solid or as a solution in sulfolane in which the ions are highly solvated and exist as an ion pair. Nitronium tetrafluoroborate shows poor solubility in most organic solvents and so the more soluble nitronium hexafluorophosphate is sometimes preferred for nitrations. Olah and co-workers - have studied the nitration of a vast array of aromatics with nitronium salts. Solutions of nitronium salts in aprotic organic solvents are useful for the nitration of acid sensitive or readily oxidized substrates. Nitronium tetrafluoroborate has been used for the... 

Topchiev first reported on the use of metal nitrate-Lewis acid mixtures for aromatic nitration. Many of these nitrations are heterogeneous due to the poor solubility of metal nitrates in organic solvents. 

TNT is the abbreviation of the aromatic nitrated aromatic compound 2,4,6-trinitrotoluene. It is a pale-yellow crystalline solid that was first synthesized in 1863 by the German chemist Joseph Wilbrand (1811—1894), but it was not immediately used as an explosive. TNT is made by nitrating toluene using nitric acid, sulfuric acid, and oleum (a mixture of sulfuric acid and S03). Nitration of toluene occurs in stages, with the nitro units added sequentially in a stepwise process as the reaction proceeds. The last nitro unit is accomplished by using oleum (SO, dissolved in sulfuric acid). After nitration, unused acids are recycled, and the product is washed with sodium sulfite and water to remove impurities. 

Some nitramines may be prepared without treating amines with nitric acid. The classical example is the so-called E-method of cyclonite preparation in which a nitramine is formed by dehydration of a mixture of paraformaldehyde and ammonium nitrate, i.e. without using either amine or nitric acid (this will be discussed more fully on p. 109). When a nitramine is required with a non-nitrated aromatic ring which readily undergoes nitration with nitric acid, Bamberger s method [45], involving the oxidation of diazo compounds (13), may be applied. 

The molecular composition of picric acid was established in the early ninteenth century when it was the only highly-nitrated aromatic compound then known. It was evident that its oxygen content was insufficient for complete combustion (to C02 and H20). This was considered to be an adequate foundation for the erroneous theory that, because of its insufficient oxygen content, the substance has no explosive properties. It was believed that explosive properties are achieved only by mixing picric acid with oxidizing agents such as chlorates, or sodium or potassium nitrates. 

Keywords aromatic compound nitric acid, acetic anhydride, zeolite, nitration, aromatic nitro compound... 

Stable nitronium salts, which are readily prepared from nitric acid (or nitrates) with HF and BF3 (and other Lewis acids such as PF5, SbF5, etc.) [Eqs. (5.171) and (5.172)], will nitrate aromatics in organic solvents generally with close to quantitative yield. Because HF and PF5 (or BF3) can be easily recovered and recycled, the method can be considered as a nitric acid nitration using a superacid catalyst [Eq. (5.173)]. 

Methyla-arylacetates. These esters have been obtained by oxidative rearrangement of alkyl aryl ketones with thallium(III) nitrate in acidic methanol or trimethyl orthoformate (4,496 5, 656 7, 362). A new method, which avoids the toxic TTN, is based on the Woodward version of the Prevost reaction. Thus, treatment of the ketone with iodine (or bromine) and silver nitrate (2 equiv.) in refluxing methanol containing trimethyl orthoformate results in methyl a-arylacetates in 90% yield from simple substrates. Yields are lowered by electron-withdrawing substituents on the aromatic group and by a-branching in the alkyl group.2... 

These systems nitrate aromatic compounds by a process of electrophilic substitution, the character of which is now understood in some detail ( 6.1). It should be noted, however, that some of them can cause nitration and various other reactions by less well understood processes. Among such nitrations that of nitration via nitrosation is especially important when the aromatic substrate is a reactive one ( 4.3). In reaction with lithium nitrate in acetic anhydride, or with fuming nitric acid, quinoline gives a small yield of 3-nitroquinoline this untypical orientation (cf. 10.4.246) may be a consequence of nitration following nucleophilic addition.5... 

Nitration offers a method of making unreactive paraffins into reactive substances without cracking. Because nitric acid and nitrogen oxides are strong oxidizing agents, oxidation always accompanies nitration. Aromatic nitration reactions have been important particularly for the manufacture of explosives. Nitrobenzene is probably the most important nitration product. 

Ingold, Hughes and Reed [39] have studied the kinetics of nitration with nitric acid in the presence of nitromethane and found the reaction to be of zero order in the case of readily nitrated aromatic compound, and of first order with compounds... 

According to Michael [1], on nitrating aromatic hydrocarbons an intermediate addition product is formed, which has one hydrocarbon hydrogen atom attached to one oxygen atom of nitric acid, and a carbon atom of the aromatic ring directly attached to the nitrogen atom of the acid. The aldol formed gives off water in the presence of an excess of nitric or sulphuric acid. The mechanism was depicted by the author as follows ... 

The rule was checked by Tronov and Ber [15] when nitrating aromatic compounds with nitric acid in nitrobenzene. They found there was no simple correlation between the influence of a substituent on the reaction rate and its orienting activity. The rule is true only for the groups with strong orienting activity (e.g. N02, CN, CH2CN, C6H5CO). 

A number of other authors have also nitrated aromatic amines, acting with sodium nitrite on the amine dissolved in hydrochloric or acetic acid. Thus, tri-phenylamine and its methyl derivative in acetic acid were nitrated by Haussermann and Bauer [159], and Rupe, Braun and Zembruski [160] and Tsuruta and Oda [161] nitrated derivatives of dimethylaniline in hydrochloric acid to yield mononitro products. 

Nitric esters in mixtures with sulphuric acid. Raudnitz [171] was the first to draw attention to the nitrating properties of solutions of ethyl nitrate in sulphuric acid. Later in 1941, Titov [34] expressed the view that such a solution contains the nitronium ion, and L. P. Kuhn (p. 15) proved this by cryometric measurements. A solution of ethyl nitrate in sulphuric acid has been used for nitrating aromatic compounds. 

The ability of higher nitrated aromatic compounds to form dark coloured products in alkaline medium can be used for the titration of certain weak acids with bases. For example trinitrobenxene acquires an orange and red colour at pH 12.2 and 13.6 respectively. 

This property of nitro compounds was first observed by Foord [78] with regard to o- nitrophenol, 2,4-dinitrophenol, picric acid, m- dinitrobenzene, 2,4-dinitro-toluene, 1,3,8-trinitronaphthalene, 2,4-dinitroaniline and other higher nitrated aromatic compounds. 

While the process at the cathode always ends finally in withdrawal of oxygen or in taking up of hydrogen, the number of possible reactions at the anode—aside from solution-phenomena, which are without interest here—is a much greater one. For, each ion which is capable of substituting can pass into the reactive state at the anode and produce reactions which cannot be numbered with the real oxidations. In the first place numerous substitutions can occur in difficultly oxidizable bodies, especially aromatic compounds, for instance the chlorination of phenols and phthale ins, nitration of acids, diazotizing of amines, etc. Substitution and oxidation processes often occur simultaneously, as in the electrolytic formation of iodoform from alcohol. 

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