Nitration, aromatic examples

Aromatic substrates containing Lewis basic substituents can undergo ort/io-lithiation. Quenching these anions with dinitrogen tetroxide at low temperature is an example of nucleophilic aromatic nitration." Similar examples have been reported with anions generated from Grignard reactions with arylhalides." ... 

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 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. 

As known (Vol. I, p. 332) the reaction of sodium sulpliiie with unsymmetri-cal trinitrotoluenes is a major reaction in the puriOcation of crude TNT. known under the name of Scllite Process - sellite being the name of sodium sulphite solution. Sodium sulphite solutions have pll > 8.0. It is known how sensitive higher nitrated aromatic compounds are to bases and sodium sulphite (particularly at a temperature, for example, above 40 45 ( ) can produce a damaging effect on TNT It is therefore advisable to add a certain amount of sodium bisulphite to sodium sulphite to keep the pH value as low as possible. According to Clift and Fedoroff [71] 0.1 -0.3% NaHSOy was added in the U.S.A, as a buffer to keep the pH value down. 

Although the nitronium ion is the nitrating agent, there are known examples when nitration can occur in media in which the concentration of NOj is too small to be detected spectroscopically (Vol. I, pp. 25,48). This was pointed out by Bunton and Halevi [4] who succeeded in nitrating aromatic compounds with 40-60% aqueous nitric acid. Bunton and co-workers (5, 6) showed that the nitronium ion was an intermediate in both oxygen-exchange and aromatic nitration in the sense of reactions ... 

Nitro dyes are nitrated aromatic compounds which dye protein fibres and are classified as acid dyes. Naphthol Yellow (C.I. acid yellow 1), formula (6), is an example ... 

Propargylation. Propargylic alcohols can be converted into stabilized cations by reaction with Co2(CO)s and then with tetralluoroboric acid. The resulting cation propargylates activated aromatic compounds. The —Co2(CO)g unit is removed by treatment with ferric nitrate. An example is shown in equation (1). The steps can be conducted in sequence without purification of intermediates. The complex of propargyl alcohol itself reacts at both the ortho- andpara-positions of anisole. 

You ve seen plenty of electrophilic aromatic substitution reactions in this chapter that are reliable and widely used—bromination and nitration, for example. But others pose... 

The rates of electrophilic substitution on other even aromatic systems, and the effects of substituents, can be deduced in the usual way. Thus since -1- / substituents destabilize odd AH cations while — / substituents stabilize them, substitution should tend to take place in such a way as to ensure that + / substituents are at inactive positions in the arenonium intermediate and - / substituents at active positions. For example, in benzene, where the positions ortho and para to the point of substitution are active in the intermediate benzonium ion (89), +/ substituents (e.g., CF3) are best placed in the meta position, while — / substituents (e.g., CH3) should be ortho or para. Benzotrifluoride (PhCF3) consequently substitutes mainly meta, nitration, for example, giving (90), while toluene (PhCH3) substitutes mainly ortho and para, giving, e.g., (91) and (92) on nitration. 

The nitration, sulphonation and Friedel-Crafts acylation of aromatic compounds (e.g. benzene) are typical examples of electrophilic aromatic substitution. 

It should be noted that aliphatic compounds (except the paraffins) are usually oxidised by concentrated nitric acid, whereas aromatic compounds (including the hydrocarbons) are usually nitrated by the concentrated acid (in the presence of sulphuric acid) and oxidised by the dilute acid. As an example of the latter, benzaldehyde, CjHsCHO, when treated with concentrated nitric acid gives ffi-nitrobenzaldehyde, N02CgH4CH0, but with dilute nitric acid gives benzoic acid, CgHgCOOH. 

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 ... 

Returning to Table 12 2 notice that halogen substituents direct an incoming electrophile to the ortho and para positions but deactivate the ring toward substitution Nitration of chlorobenzene is a typical example of electrophilic aromatic substitution m a halobenzene... 

MDA reacts similarly to other aromatic amines under the proper conditions. For example, nitration, bromination, acetylation, and dia2oti2ation (1 3) all give the expected products. Much of the chemistry carried out on MDA takes advantage of the diftmctionality of the molecule in reacting with multiftmctional substrates to produce low and high molecular weight polymers. 

Liquid/hquid reactions of industrial importance are fairly numerous. A hst of 26 classes of reactions with 61 references has been compiled by Doraiswamy and Sharma Heterogeneou.s Reactions, Wiley, 1984). They also indicate the kind of reactor normally used in each case. The reactions range from such prosaic examples as making soap with alkali, nitration of aromatics to make explosives, and alkylation of C4S with sulfuric acid to make improved gasoline, to some much less familiar operations. 

Particular reactions can occur in either or both phases or near the interface. Nitration of aromatics with HNO3-H2SO4 occurs in the aqueous phase (Albright and Hanson, eds.. Industrial and Laboratoiy Nitration.s, ACS Symposium Series 22 [1975]). An industrial example of reaction in both phases is the oximation of cyclohexanone, a step in the manufacture of caprolactam for nylon (Rod, Proc. 4th Interna-tional/6th European Symposium on Chemical Reactions, Heidelberg, Pergamon, 1976, p. 275). The reaction between butene and isobutane... 

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 normal course of a kinetic investigation involves postulating likely mechanisms and comparing the observed rate law with those expected for the various mechanisms. Those mechanisms that are incompatible with the observed kinetics can be eliminated as possibilities. Let us consider aromatic nitration by nitric acid in an inert solvent as a typical example. We will restrict the mechanisms being considered to the three shown below. In an actual case, such arbitrary restriction would not be imposed, but instead all mechanisms compatible with existing information would be considered. 

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