Directive effect in substituted benzene derivatives

         (1) Directive effect in mono substituted benzene derivatives : The substituent already present on the benzene ring directs the incoming substituent to occupy ortho (2 or 6), meta (3 or 5) or para (4) point. This direction relays on the nature of the first substituent  and is called directive or the orientation effect.

         The substituent already present can increase or decrease the rate of further substitution, i.e., it either deactivates or activates the benzene ring towards further substitution. These effects are known as activity effects.

There are two types of substituents which produce directive effect are,

         (i) Those which direct the incoming group to ortho- and para-positions simultaneously  (Neglecting meta all together).

(ii) Those which show the incoming set to meta-position only (Neglecting ortho- and para-positions all together).

         Theory of ortho - para directing group  

         The above mechanism is followed when S is -OH, -NH₂, -Cl, -Br, -I, -OR, -NR₂, -NHCOR etc.

         In methyl or alkyl group, the +I effect of the methyl group or alkyl group initiates the resonance effect.

         Thus, alkyl or methyl group shows all electrophiles to ortho and para positions.

         Theory of meta directing group : The substituent, S takes electrons from para  and ortho positions. Thus, m-position becomes a position of relatively high electron density and further substitution by electrophile occurs at meta position. For example, -NO₂ group is a meta directing (Electron withdrawing). Its process can be illustrate as : 

         All meta-directing sets have either a partial positive charge or a full positive charge on the atom directly attached to the ring.

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