Directive Influence

All six hydrogen atoms in benzene rings are equivalent, as we’ve shown. As a result, any substitution that replaces one of these six hydrogen atoms always results in a single mono substituted product. 

When mono-substituted benzene is further substituted, however, the group on the benzene ring influences the incoming attacking groups.

The directional impact of groups refers to the ability of a group already existing in the benzene ring to lead an entering group to a specific place.

The presence of a substituent on the ring has two effects:

Orientation effect: substituents have an impact on the reaction’s orientation. Ortho, para, and meta, the three potential disubstituted products, are not generated in equal amounts.

The position of the second substitution is determined by the nature of the substituents already present on the benzene ring. 

As a result, every group can be classified into one of two categories: ortho-para directing and meta-directing groups.

The reactivity of the aromatic rings is affected by the substituents. Certain substituents energise the ring, making it more reactive than benzene.

Some groups, on the other hand, deactivate the ring, making it less reactive than benzene. 

The –OH group, for example, makes the ring more reactive than benzene in aromatic nitration, but the nitro group makes the ring less reactive.

Benzene Ring Substitution

When mono substituted benzene is electrophilically attacked, the rate of reaction and attack site differ depending on the functional group connected to it. 

Some groups are known as activating groups because they increase the reactivity of the benzene ring, while others are known as deactivating groups because they lower the reactivity.

We further classify these groupings into two categories based on how they influence the incoming electrophile’s attack orientation.

Ortho-para directors enhance electron density in the “ortho” and “para” positions, while meta directors increase electron density in the “meta” position. 

The following are some examples of functional group directional influence in mono substituted benzene:

Ortho-para directing group: 

As previously stated, these groups target “ortho” and “para” locations for electrophilic assault. All activating groups, such as –NH2, –NHR, –NHCOCH3, –OCH3, –CH3, –C2H5, are “ortho-para” directors.

The electron density at ortho and para positions increases due to the benzene ring’s resonance, as shown in the figure. 

As a result, phenols have a strong electrophilic attack affinity in the “ortho” and “para” locations. As a result, the “–OH” group might be classified as an ortho-para director.

The electron density at the “ortho” and “para” positions increases in comparison to the “meta” position due to the benzene ring’s resonance. As a result, halogens, despite being deactivators due to the “-I” action, are ortho-para directors as well.

Meta directing groups: 

These groups direct electrophilic assault on the linked benzene ring’s “meta” locations. Meta directors are generally deactivating groups, such as –NO2, –CN, –CHO, –COR, –COOH, –COOR, –SO3H, and so on.

The nitro group is a ring deactivating group, meaning it lowers the electron density of the connected benzene ring. 

The electron density at the “meta” position is quite high in comparison to the “ortho” and “para” positions, as shown in the figure. 

As a result, these groups are known as meta directors because they allow electrophilic replacement of the ring at “meta” places.

Toxicity and Carcinogenicity:

Benzene and polynuclear hydrocarbons with more than two benzene rings fused together are poisonous and carcinogenic. 

Incomplete combustion of organic materials such as tobacco, coal, and petroleum produces these.

These are subjected to a variety of metabolic processes that damage DNA and result in cancer.

Conclusion

Ortho and para directing groups are electron releasing groups that guide the incoming group to ortho and para locations, 

where the electron density is higher. As a result, electrophilic substitution occurs primarily at these locations. At certain positions, the aromatic ring becomes reactive.

The electron density is lower in the meta state, hence it is less reactive. The electron withdrawing groups, on the other hand, are meta directing.