二地 黄 g文民 品裂 16-ihei irtes towrd ubstd Iheialeaetrecdingpowecrotsubstuente mong these are: 29hg8g2a ctrophilcsub n b t nitrobenzene is used instead: 人 ce6be6ohehcanusaueawy 66 Synthetic Strategies toward Substituted Benzenes 16-5 To obtain substitutions in positions incompatible with the directing sense of substituents requires a couple of synthetic “tricks”. Among these are: •Chemical interconversions of ortho, para with meta directors (nitro ⇔ amino or carbonyl ⇔ methylene); •Additional knowledge about practicality of certain electrophilic substitutions; •Employment of reversible blocking strategies with sulfonic acid groups (-SO3H). The sense of the directing power of substituents can be changed. The simplest way to introduce a nitrogen substituent into an arene is by nitration. The nitro group (meta-directing, deactivating) can be easily converted into the amino group (ortho, para-directing, activating) by reduction: Catalytic hydrogenation or Exposure to acid in the presence of active metals (iron or zinc amalgam) Oxidation of the amino group back to a nitro group can be accomplished using trifluoroperacetic acid. To prepare 3-bromobenzamide, direct bromination of benzamide leads to ortho and para substitution. If nitrobenzene is used instead: A similar conversion strategy can be used for alkanoyl ⇔ alkyl. In the synthesis of 1-chloro-3-ethylbenzene from benzene, neither chlorobenzene nor ethylbenzene is suitable as the immediate precursor to the product; each is ortho-, para￾directing. Ethanoyl benzene is a meta director which can subsequently be reduced to the ethyl functional group