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SN-Ar-(SUBSTITUTION OF HYDROGEN OF BENZENE:

SN-Ar : More general attack of nucleophile on unsubstituted benzene ring is very difficult as compared to attack by Electrophile hence nucleophilic substitution of hydrogen from benzene is also difficult because..
(1) The pi electron cloud of benzene nucleus repels the approaching nucleophile.
(2) It is difficult to accumulate negative charge (electron) donated by attacking nucleophile.

However in the presence of one the electron withdrawing group like nitro group (-NO2) on benzene ring, then it is sufficiently activate ortho and para position

ILLUSTRATIVE EXAMPLE: 
Nitro phenol give ortho nitro phenol  and also little amount of para nitro phenol on treatment with strong NaOH in the presence of oxidizing agents like KNO3 and K3 [Fe(CN)6].

MECHANISM: (SN-Ar):

­­Important note : Since OH- is  better leaving group than hydride ion (H- ) but the presence of oxidizing agents  like  air , KNO3 and K3 [Fe(CN)6] which encourage the elimination of hydride ion.

Other Examples:

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