Question
Tue May 01, 2012 By:

which are meta ortho and para generating groups?

Expert Reply
Wed May 02, 2012
Electron donating groups (EDG) with lone pairs (e.g. -OMe, -NH2) on the atoms adjacent to the p system activate the aromatic ring by increasing the electron density on the ring through a resonance donating effect.  The resonance only allows electron density to be positioned at the ortho- and para- positions.
 
Electron withdrawing groups (EWG) with p bonds to electronegative atoms (e.g. -C=O, -NO2) adjacent to the p system deactivate the aromatic ring by decreasing the electron density on the ring through a resonance withdrawing effect.  The resonance only decreases the electron density at the ortho- and para- positions. Hence these sites are less nucleophilic, and so the system tends to react with electrophiles at the meta sites.

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