Nitration of aromatic hydrocarbons

NITRATION OF AROMATIC HYDROCARBONS Aromatic hydrocarbons may be nitrated, i.e., the hydrogen atoms replaced by nitro (NOj) groups, with concentrated nitric acid in the presence of concentrated sulpliuric acid, for example  

The function of the sulphuric acid is to furnish a strongly acid medium and to convert the nitric acid into the highly reactive nitronium ion NOj+, which is the real nitrating agent  

The mechanism of the aromatic substitution may involve the attack of the dectrophilic NOj ion upon the nucleophilic aromatic nucleus to produce the carboniiim ion (I) the latter transfers a proton to the bisulphate ion, the most basic substance in the reaction mixture 

Nitrations are usually carried out at comparatively low temperatures at higher temperatures there may be loss of material because of the oxidising action of the nitric acid. For substances which do not nitrate readily with a mixture of concentrated nitric and sulphuric acids ( mixed acid ), the intensity of the reaction may be increased inler alia by the use of fuming sulphuric acid (containing up to 60 per cent, of sulphur trioxide) or by fuming nitric acid. Thus nitrobenzene is converted by a mixture of fuming nitric acid and concentrated sulphuric acid into about 90 per cent, of wi-dinitrobenzene and small amounts of the o- and p-isomers the latter are eliminated in the process of recrystallisation  

Nitration of bromobenzene with mixed acid yields largely p-bromo nitrobenzene accompanied by a little of the o-isomeride  

---

Impurities can sometimes be removed by conversion to derivatives under conditions where the major component does not react or reacts much more slowly. For example, normal (straight-chain) paraffins can be freed from unsaturated and branched-chain components by taking advantage of the greater reactivity of the latter with chlorosulfonic acid or bromine. Similarly, the preferential nitration of aromatic hydrocarbons can be used to remove e.g. benzene or toluene from cyclohexane by shaking for several hours with a mixture of concentrated nitric acid (25%), sulfuric acid (58%), and water (17%). 

The reaction with HNO3 is quantitative, and the presence of large concentrations of the nitronium ion, NOi ", in solutions of HNO3, MNO3 and N2O5 in H2SO4 enable a detailed interpretation to be given of the nitration of aromatic hydrocarbons by these solutions. 

For nitration of aromatic hydrocarbons with acetylnitrate, there is a clear linear correlation between the IPs of these hydrocarbons and rate constants relative to benzene (Pedersen et al. 1973). Table 4.4 jnxtaposes spin densities of cation-radicals and partial rate factors of ring attacks in the case of nitration of isomeric xylenes with nitric acid in acetic anhydride. 

The nitration of aromatic hydrocarbons is one of the most widely studied and well-documented reactions in organic chemistry. Aromatic nitro compounds are of huge industrial importance in the synthesis of pharmaceutical drugs, agrochemicals, polymers, solvents and perfumes, and for the synthesis of other industrially important chemicals containing amine and isocyanate functionality. However, early research into aromatic nitration was fuelled exclusively by their use as explosives and intermediates in the synthesis of dyestuffs. The former is the subject of this chapter. 

Boron phosphate is used as an acid catalyst for dehydration of alcohols to olefins isomemization of olefins nitration of aromatic hydrocarbons polymerization of aldehydes and other synthetic reactions. It also is used as a flux in silica-based porcelain and ceramics special glasses and acid cleaners. 

Dewar, M. J. S., T. Mole, and E. W. T. Warford, Electrophilic Substitution. Part VI. The Nitration of Aromatic Hydrocarbons Partial Rate Factors and Their Interpretation, J. Chem. Soc., Part 111, 3581-3586 (1956). 

Our basic knowledge of electrophilic nitration of aromatic hydrocarbons results from the work of Ingold,24 who established that the nitronium cation is the true reactive electrophile in these reactions.64-67 In the traditional nitration process with the mixed acid the nitronium cation is generated according to Eqs. (10.4) and (10.5) ... 

A procedure for quantitative regiose1ective nitration of aromatic hydrocarbons in the laboratory, Tetr. 

Tetranitromethane forms with hydrocarbons (such as benzene and toluene) one of the most brisant, destructive mixtures known. TNM is very poisonous, affecting the membranes of the eyes and nose. It is always formed as a by-product in nitration of aromatic hydrocarbons and always accompanies crude TNT (Refs 30 35). 

Nitration of aromatic hydrocarbons is usually carried out with the above mixed acid reagent at comparatively low temperatures (e.g. about 50 °C, as used in the preparation of nitrobenzene and 1-nitronaphthalene, Expt 6.17). Unnecessarily high temperatures should be avoided since polynitration is then more likely and oxidative breakdown of the aromatic ring system may occur. 

There are numerous applications for bubble columns, for example, gas-liquid columns include the absorption of isobutylene in sulfuric acid, and liquid-liquid columns are used for nitration of aromatic hydrocarbons. 

Cornells, A., Delaude, L., Gerstmans, A. and Laszlo, R A procedure for quantitative regioselective nitration of aromatic-hydrocarbons in the laboratory, Tetrahedron Lett., 1988, 29, 5909-5912. 

The kinetics of nitration of aromatic hydrocarbons with nitric acid in the presence of acetic anhydride was first studied by Tronov, Kamay and Kovalenko [110], Their studies will be discussed later. 

Menke [2] inferred from the results of his studies on nitration of aromatic hydrocarbons with mixtures of nitric acid and acetic anhydride that the latter acted not only as a dehydrating substance but also as a catalyst. 

Since industrial nitration occurs, in most cases, in two-phase system a number of workers have investigated the kinetics in both phases organic and acid. Hethe-rington and Masson [84], McKinley and R. R. White [118], Barduhn and Kobe [119] all reported that nitration of aromatic hydrocarbons takes place only in the acid phase. However, other workers (W. K. Lewis and Suen [120]) have shown, when nitrating benzene, that the reaction rate in the organic phase is an appreciable fraction (10-15%) of that in the acid phase. 

A similar scheme for the nitration process was given by Reddelien [5] who expressed the view that nitration of aromatic hydrocarbons with mixtures of nitric and sulphuric acids gave addition products, e.g. 

In favour of the view, that postulates the formation of an addition product during the first stage of nitration this fact should be known to all who are practically engaged in nitration of aromatic hydrocarbons. Immediately before contacting the nitrating acid (HN03 or nitric and sulphuric acids mixture), benzene and toluene give brown coloured products amid nitric acid vapours. On dissolution in the acid these products decolourize at once. It is quite possible they arc addition products formed by nitric acid vapours with the hydrocarbon. 

Tronov, Kamay and Kovalenko [66] have measured the rate of nitration of aromatic hydrocarbons and their halogenides with a mixture of nitric Mid acetic acids. The compounds examined were arranged according to increasing rate of nitration the relative rate is given in brackets, taking 1 for benzene ... 

During the nitration of aromatic compounds a certain amount of diazo compounds as by-products can also be formed. This for the first time was described by Weselsky as early as in 1875 [84] in the case of nitration of phenols with nitrogen dioxide and of nitration of aromatic hydrocarbons by Titov and Baryshnikova [85], Titov [39] explained the reaction as the result of reaction of nitroso compounds with NO ... 

Nitration of aromatic hydrocarbons with a side chain gives mononitro compounds with the nitro group attached either to the ring or to the side chain, with a predominance of the former. For example, from toluene at temperature 14-15°C, 46% of nitrotoluenes and 9% of phenylnitromethane were obtained. 

Cohen and Wibaut [151] in their work, already mentioned (p. 44), on the nitration of aromatic hydrocarbons with a mixture of nitric acid and acetic anhydride, confirmed the catalytic action of nitrous acid in this case too. As the reaction proceeded the concentration of nitrous acid increased owing to the oxidizing action of nitric acid on the hydrocarbon. 

Methods of continuous nitration of aromatic hydrocarbons may be based on one of the following principles ... 

Tetranitromethane is also formed during the nitration of aromatic hydrocarbons under very vigorous conditions, for example, when benzene or toluene is nitrated to the trinitro derivative. 

The ability of azoles to electrophilic substitution reactions is determined by the activity of reagents, the basicity of substrates, and the acidity of media. This caused some uncertainty in the interpretation of results and complicated a comparison of the reactivity of various azoles. The situation has changed after Katritzky and Johnson [1] have reported the criteria allowing, with a sufficient degree of reliance, the establishment in what form (base or conjugative acid) the compound reacts. The information on the mechanism of nitration of azoles was basically borrowed from the extensive literature on the nitration of aromatic hydrocarbons [2-8] therefore, we have found expedient to discuss briefly some works in this field. 

---