Introduction
Electrophilic aromatic substitution is a type of organic reaction that involves the substitution of an electrophile with an aromatic compound. In this process, the electrophile attacks the aromatic ring and replaces a hydrogen atom. However, the reaction rate and the orientation of the electrophile are influenced by the presence of activators and deactivators on the ring.
Activators
Activators are functional groups that increase the electron density on the aromatic ring, making it more reactive towards electrophiles. These groups have lone pairs of electrons that can donate to the ring through resonance, stabilizing the intermediate cationic species. Examples of activators include amino, hydroxyl, and alkyl groups.
Amino groups, such as the one found in aniline, are strong activators because they have a lone pair of electrons that can donate to the ring through resonance. Similarly, hydroxyl and alkyl groups, such as those found in phenol and toluene, respectively, also have lone pairs of electrons that can donate to the ring through resonance, making them moderate activators.
Deactivators
Deactivators are functional groups that decrease the electron density on the aromatic ring, making it less reactive towards electrophiles. These groups have pi electrons that can withdraw from the ring through resonance, destabilizing the intermediate cationic species. Examples of deactivators include nitro, carbonyl, and halogen groups.
Nitro groups, such as the one found in nitrobenzene, are strong deactivators because they can withdraw electrons from the ring through resonance. Similarly, carbonyl and halogen groups, such as those found in acetophenone and chlorobenzene, respectively, also have pi electrons that can withdraw from the ring through resonance, making them moderate deactivators.
Effect on Reaction Rate
The presence of activators or deactivators on the aromatic ring can significantly affect the rate of electrophilic aromatic substitution. Activators increase the electron density on the ring, making it more reactive towards electrophiles and increasing the reaction rate. Deactivators, on the other hand, decrease the electron density on the ring, making it less reactive towards electrophiles and decreasing the reaction rate.
For example, the reaction rate of electrophilic aromatic substitution on aniline is much higher than that on benzene because of the presence of the amino group, which is a strong activator. Similarly, the reaction rate of electrophilic aromatic substitution on nitrobenzene is much lower than that on benzene because of the presence of the nitro group, which is a strong deactivator.
Effect on Orientation
In addition to affecting the reaction rate, the presence of activators or deactivators can also affect the orientation of the electrophile on the aromatic ring. Activators direct the incoming electrophile to the ortho and para positions, while deactivators direct the electrophile to the meta position.
For example, the amino group in aniline directs the incoming electrophile to the ortho and para positions because of the increased electron density at those positions. Similarly, the nitro group in nitrobenzene directs the incoming electrophile to the meta position because of the decreased electron density at the ortho and para positions.
Conclusion
In summary, the presence of activators and deactivators on the aromatic ring can significantly affect the rate and orientation of electrophilic aromatic substitution. Activators increase the electron density on the ring and direct the incoming electrophile to the ortho and para positions, while deactivators decrease the electron density on the ring and direct the electrophile to the meta position. Understanding the effect of these functional groups is crucial in predicting and controlling the outcome of electrophilic aromatic substitution reactions.