Regioisomerism in Aromatics with One Substituent

When aromatics, even single-ring compoimds such as benzene, have one substituent already, the introduction of the next also gives rise to regioisomerism. In addition, the first one guides the introduction of the second by steric and electronic effects. 

In this context, the second-stage nitration of 1-mononitronaphthalene was investigated. The isomeric ratio of the two regioisomers, 1,5-dinitro- to 1,8-dinitro-naphthalene, was constant at 1 3.5 for macroscopic batch reactors, whereas it changes to 1 2.8 in micro reactors [166]. 

For toluene fluorination, the impact of micro-reactor processing on the ratio of ortho-, meta- and para-isomers for monofluorinated toluene could be deduced and explained by a change in the type of reaction mechanism. The ortho-, meta- and para-isomer ratio was 5 1 3 for fluorination in a falling film micro reactor and a micro bubble column at a temperature of-16 °C [164,167]. This ratio is in accordance with an electrophilic substitution pathway. In contrast, radical mechanisms are strongly favored for conventional laboratory-scale processing, resulting in much more meta-substitution accompanied by imcontroUed multi-fluorination, addition and polymerization reactions. 

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