Regiocontrol by choice of route
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص566-567
2025-06-24
660
Choosing the right route to an aromatic product is essential if you want to get one particular isomer. We can illustrate this with the synthesis of the isomers of bromonitrobenzene. Because the bromo substituent is ortho, para-directing and the nitro group meta-directing, it’s possible to make all three isomers, providing we exploit the regioselectivity of electrophilic substitution. Nitration of bromobenzene would give the ortho and para isomers while bromination of nitrobenzene would give the meta isomer. The selectivity of the fi rst reaction is not good: bromine is small and not very electronegative, so steric hindrance is weak and the ortho positions are not deactivated. Furthermore there are two ortho positions but only one para: a typical result is about 37% ortho, 1% meta, and 62% para. Both compounds are industrial products, made by nitration and separated
Bromination of nitrobenzene is remarkably good, considering the unreactivity of nitro benzene in electrophilic aromatic substitution. One recipe uses iron powder and bromine at 140 °C and gives 74% of the meta product. We shall need these reactions in the next section.
Before we move on, consider why this selectivity works: we can get all three isomers because we have one ortho/para director and one meta director. But what if we had two ortho/para directors—say, amino and bromo—and wanted the meta isomer?
The solution in these cases is often to make use of the transformation of the nitro group (a meta director) into an amino group (a para director) by reduction.
Since the amino group can be substituted by, many problems of regio selectivity can be solved by using nitro compounds as intermediates. You could, for example, use the product above to make the otherwise challenging 3-bromoiodobenzene:
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