Nitro groups, deactivating

These results should not be surprising. We have already seen that a substituent on a benzene ring has its greatest effect on the carbon atoms ortho and para to the substituent. An electron-donating substituent activates primarily the ortho and para positions, and an electron-withdrawing substituent (such as a nitro group) deactivates primarily the ortho and para positions. 

A nitro group deactivates an aromatic ring to electrophilic attack, but it activates the ring towards nucleophilic substitution. 

Nitration of naphthalene gives 1-nitronaphthalene (12). Further substitution does not occur in the same ring and the main products of dinitration are 1,5- (13) and 1,8-dinitronaphthalenes (14). The initial nitro group deactivates that ring to further electrophilic substitution and attack at the a-positions of the other ring therefore takes place. 

Substituents in the aryl ring, too, can exert their own directional effects a p-nitro group deactivates the benzene ring and directs further attack into the heterocyclic ring in 4-(3, 4-dichlorophenyl)imidazole nitration is mainly at C-5 in 4-(p-methoxyphenyl)imidazole (195) subsequent nitration takes place adjacent to the methoxy function and then at C-5 (Scheme 104) (77CHE1110). The nitration of 2-(p-fluorophenyl)imidazole has been discussed earlier (Section 4.07.1.4.2). 

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