Nitration of aromatic and heterocyclic compounds

Nitration in the side chain, exclusively at the -position, occurs, however, on use of hot dilute nitric acid in a sealed tube, but with unsaturated or with longer, branched side chains the oxidizing action of nitric acid becomes evident. 

Considerably more energetic conditions, i.e., stronger acid and higher temperature, are necessary for introduction of several nitro groups than for that of one. In such cases also it is evident that the presence of alkyl substituents facilitates substitution thus trinitrotoluene can be obtained comparatively readily by direct trinitration of the hydrocarbon, whereas to prepare trinitrobenzene in this way would require extreme conditions and this deri- 

It is impossible to introduce more than three nitro groups into one benzene ring the products would contain at least two nitro groups in the ortho-position to one another, and such compounds are very reactive. The two isomeric tetranitrobenzenes should be preparable by oxidation of other nitrogenous groups present in trinitrobenzene derivatives.169 In di- and tri-nitrations the nitro groups normally enter positions meta to one another, in accord with the well-known rules of substitution. However, occasionally, when these rules are overcome by some specific activation by other substituents, 0-dinitro compounds will result for instance, about 4% of m-nitrotoluene is formed on mononitration of toluene and on further nitration this leads to a mixture of 2,3-, 3,4-, and 3,6-dinitrotoluenes. 

However, mononitration of tetralin, which is substituted only in the aromatic ring, by mixed acid at 0° leads to an 80-90% yield of a mixture of about equal parts of 5- and 6-nitrotetralin.174 The danger of an explosion during separation of the nitrotetralin mixture by distillation175 should be carefully noted. Under energetic conditions 5,7- and, surprisingly, 5,6-dinitrotetralin are both obtained.174 

The behavior of anthracene towards nitrating agents is determined largely by the great reactivity at positions 9 and 10. According to the reaction conditions it gives either oxidation products such as anthraquinone or nitro derivatives thereof, or else, by way of primary adducts, 9-nitro- or 9,10-dinitro-anthracene. Directions for the preparation of 9-nitroanthracene are to be found in Organic Syntheses.116 

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