Directive Influence of the Functional Group in Mono Substituted Benzene

When monosubstituted benzene is subjected to electrophilic attack, the reaction rate  and attack site depend on the functional groups already attached. A substituent already present on the ring has two effects: on orientation and reactivity. 

Substituents affect the orientation of the reaction in three possible di-substitution products. The type of substituent already present on the benzene ring determines the location of the second substitution. Thus, each group may further fall into one of two classes: the ortho para-oriented group and the meta-oriented group.

Substituents also affect the reactivity of aromatic rings in different ways. Some substituents activate the ring and make it more reactive than benzene. On the other hand, some groups deactivate the ring and make it less reactive than benzene. 

For example, in aromatic nitration, the -OH group makes the ring more reactive than benzene, and the nitro group makes the ring less reactive.

Mono Substituted Benzene

Benzene enters a typical electrophilic substitution reaction and forms a monosubstituted benzene as a product. When this mono-substituted benzene undergoes further electrophilic substitution, it forms three possible di-substituted products in different quantities. 

The large amount of disubstituted product produced depends on the reactivity of the monosubstituted benzene. The reactivity of monosubstituted benzene, in turn, depends on the nature of the groups present on the ring. The group present on the ring directs the newly arrived electrophile to one of the remaining five positions. 

Some groups increase the reactivity of the benzene ring and are known as activated groups, while others decrease the reactivity and are known as deactivated groups. 

Groups fall into two categories based on how they influence the attack direction of  incoming electrophiles. Ortho-para director groups are groups that increase electron density at the ortho’ and para positions. Meta director groups are groups that increase the electron density at the meta position.

Ortho Para directing groups

“Ortho Para” directors.:

As already mentioned, these groups direct electrophilic attacks to the “ortho” and “para” positions. All activation groups are “Ortho Para” directors.

Meta-directional groups: These groups control electrophilic attacks on the “meta” position of the associated benzene ring. In general, deactivation groups are meta-directors.

In these resonant structures, the  charge residue (-) is at the ortho and para positions of the ring structure. It is clear that the lone pair of electrons of the atom  attached to the ring participates in  the resonance, forming a ring richer in electrons than benzene. The electron density at the ortho and para positions increases  compared to the meta position. Therefore, the phenol group activates the benzene ring for electrophilic attack at the “ortho ” and “para” positions, so the -OH group is the ortho para director and activator.  

In aryl halides, the strong –I effect of  halogens (electron attraction tendency) reduces the electron density of the benzene ring,  deactivating the benzene ring due to electron-requester attack. However the presence of lone pairs on halogens involved in the resonance with pi electrons of the benzene ring increases electron density at ortho and para position. Hence the halogen group is an ortho para director and deactivator.

Meta-directing group:

• These groups lead the next group to the meta position. Examples: -NO2, -CN, -CHO, COR, -COOH, -COOR, -SO3H, etc … 
• These groups reduce the electron density of the benzene ring, especially at the O- and p-positions. 
• Therefore, additional replacement missions are placed in the meta position.  Meta Directing group. 
• Therefore, these groups are known as deactivated groups.

In these resonant structures,  (+) charge residues are present in the ring structure. It is very clear that resonance delocalizes the positive charge of the atoms in the ring and makes the ring less electron-rich than benzene. Here, the overall density of the benzene ring is reduced due to the –I effect of the –CHO group there, in that benzene is deactivated due to electrophilic attack. However, the resonant structure indicates that the electron density is more in the meta position. Compared to o & position. Therefore, the CHO group is a meta-director and deactivation tool.

Carcinogenicity and Toxicity:

• Benzene and polynuclear hydrocarbons containing two or more condensed benzene rings  are toxic and carcinogenic (carcinogenic). 
•  These are formed by the incomplete combustion of organic matter such as tobacco, coal and petroleum. They are affected by various biochemical reactions that damage DNA and cause cancer.

Conclusion 

As we have learned, all  six hydrogen atoms in the benzene ring are equivalent. Therefore, substituting one of these six hydrogen atoms with an arbitrary substitution always results in a single one-substitution product. However, if the monosubstituted benzene is  further substituted, the groups present in the benzene ring will affect the  attacking groups.  The ability of a group already present on the benzene ring to direct an incoming group to a particular position is called the directive effect of the group.

The electron donating group directs the incoming group to the ortho and para positions. This is called the ortho and para orientation group because the electron densities are in the o and p positions. Therefore, electrophilic substitution occurs primarily at these positions. Aromatic rings become reactive at these positions. Since the electron density is low at the  meta position, the reactivity is low. This makes the electron attractive meta-directional.