Effect of nitrating agent and reaction conditions on product selectivity

3 Effect of nitrating agent and reaction conditions on product selectivity 

The nature of the substituents presence in an aromatic substrate has a large effect on which positions are substituted on nitration. The isomeric ratio of products obtained on nitration is also dependent on the nitration conditions, nitrating agent, nitrating medium and its acidity, acid composition and concentration etc., but often to a lesser extent. 

The localization of electron density on heteroatoms is seen during the nitration of anilines which are A-nitrated in aprotic media. Anilines can give abnormal o p-isomer ratios resulting from a process of A-nitration followed by rearrangement to the ring nitrated product, a process which often occurs in situ in the strongly acidic medium of mixed acid (Section 4.5). 

The nitration of phenols can result in anomalous and large differences in product isomer ratios, showing a high dependence on both nitrating agent and reaction medium. Here the situation is complicated by the intervention of an alternative nitration mechanism - that of nitrous acid catalyzed nitration, which proceeds via in situ nitrosation-oxidation (see Section 4.4). 

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