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Benzene: Directive Influence Of Substituents

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Benzene: Directive Influence Of Substituents - Lesson Summary

Benzene undergoes typical electrophillic substitution reactions and forms mono-substituted benzene as the product. When this mono-substituted benzene is subjected to further electrophillic substitution, it forms three possible disubstituted products in varying amounts. The disubstituted product formed in major amounts depends on the reactivity of the mono-substituted benzene.

The reactivity of the mono-substituted benzene depends on the nature of the group present on the ring. A group can be either electron repelling or electron withdrawing.

The group present on the ring orients the new incoming electrophile to any of the remaining five positions, with respect to position one positions 2 and 6 are equivalent and said to be the ortho positions and positions 3 and 5 are equivalent and said to be the meta positions and finally the position 4 is said to be the para position.

The position that the incoming electrophile will occupy depends upon the availability of electron density in the ring, which in turn again depends on the nature of the group already present in the benzene ring known as the directive influence of a group. This influence could either be ring activating or ring deactivating. Ring activating groups (G) orient the new incoming electrophile to ortho and para positions.
 

Ex: In phenol due to the plus resonance effect of Hydroxyl group, the ring gets activated and the incoming electrophile is directed to the ortho and para positions.
 
 
Thus ring activating groups are said to form ortho and para substituted compounds in major amounts. The groups like –NH2, –NHR, –NHCOCH3, –OCH3 are increasing electron density on the benzene ring and are called ring activating groups.

Ring deactivating groups which relatively decrease the electron density at ortho and para positions of the ring orient the new incoming electrophile to Meta position. In nitro benzene nitro group, the electron density decreases at ortho and a Para position, thus the incoming electrophile is directed to the Meta position.


Ring deactivating groups include –CN, –CHO, –COR, –COOH, –COOR and –SO3H directs the incoming group to Meta position. 

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