There are two types of substituents groups that influence the reactivity of the benzene ring:
- Activating groups
- Deactivating groups
The effect of the substituents on the benzene ring can be explained by the inductive as well as resonance effects. The substituents that increase the electron density on the benzene ring are called activating groups. Whereas the substituents that decrease the electron density on the benzene ring are called deactivating groups. Ortho and para directors are activating groups except for halobenzene. Most of the meta directors are the deactivating groups.
“The substituents that increase the reactivity of benzene ring towards electrophilic aromatic substitution reactions are called activating groups”.
In an electrophilic aromatic substitution reaction, electron-donating groups are considered to be activating groups. This is because they increase the reactivity of the benzene ring. Almost all electron-donating groups are ortho-para directors.
“ The substituents that decrease the reactivity of benzene towards electrophilic aromatic substitution reactions are called deactivating groups “
In electrophilic aromatic substitution reactions, electron-withdrawing groups are considered to be deactivating groups. They decrease the reactivity of the benzene ring. Most electron-withdrawing groups are meta-directors.
Activating ortho-para directing groups with unshared pair of electrons
There are several activating ortho-para directing groups that contain unshared pairs of electrons. Some of the examples are the following:
The -NH2 group of aniline exerts an electron-withdrawing inductive effect on the benzene ring. It also exerts a resonance effect and increases the electron density of the ring. The resonance effect of aniline is stronger than the inductive effect. Therefore, the -NH2 group activates the ring towards an electrophilic substitution reaction.
The resonating structures of the aniline show that they have a negative charge at ortho and para positions. The electrophile attacks the position at which electron density is greatest. So that aniline is ortho-para directing group.
The regioselectivity of the electrophilic aromatic substitution reaction can also be determined by the stability of the intermediate arenium ion.
Ortho and para attack
If electrophile attacks on the ortho and para positions, then four resonating structures are possible for the intermediate in each case.
If electrophile attacks on the meta position, then three resonating structures are possible.
The arenium ion with a greater number of resonating structures is more stable. Thus, in aniline electrophile attacks on the ortho and para position instead of meta position.
Activating ortho-para directing groups without unshared pair of electrons
Alkyl groups are electron-donating groups. They donate electrons to the benzene ring by these two effects:
- Inductive effect
The methyl group activates the benzene ring towards electrophilic substitution reaction by donating the electrons.
The resonating structures of methylbenzene have a negative charge on the ortho and para positions. Therefore, electrophile attacks on the ortho and para position. Thus, toluene is ortho-para directing group.
Deactivating ortho-para directing groups
|Deactivating ortho-para directing groups|
The halogen atoms are the electron-withdrawing group due to greater electronegativity. Therefore, halogen atoms deactivate the benzene ring towards an electrophilic aromatic substitution reaction.
The halogen atoms also possess an electron-donating resonance effect. In chlorobenzene inductive effect is stronger than the resonance effect.
On the other hand, chlorine increases the electron density at the ortho and para positions due to the presence of the electron-donating resonance effect. Hence, chlorobenzene is deactivating the ortho-para directing group.
Deactivating meta-directing groups
In the case of nitrobenzene, the nitrogen atom is bonded to the more electronegative oxygen atom. The oxygen atom pulls the electron to itself and places the positive charge on the nitrogen atom. This positively charged nitrogen atom decreases the electron density at the ortho-para position on the benzene ring.
So, the incoming electrophile attacks on the meta position. Therefore, nitrobenzene is a deactivating meta-directing group.
In benzaldehyde, the carbon is bonded to the oxygen atom. The more electronegative oxygen atom decreases the electron density of the carbon atom. Then, this carbon deactivates the benzene ring towards the electrophilic aromatic substitution reaction. Hence, benzaldehyde is also a deactivating meta-directing group.
What are activating groups examples?
Examples of activating groups are:
- Alkyl group
What is activating effect?
When the electron-donating group is attached to the benzene ring, it donates the electron pair to the benzene. Thus, it increases the rate of reaction towards electrophilic aromatic substitution reaction. This effect is called activating effect.
Are activators electron-donating or withdrawing?
All electron-donating groups are activating groups. They increase the electron density of the benzene ring.
Which of the following is the deactivating group?
Examples of deactivating groups are:
Is bromine activating or deactivating?
Bromine is a deactivating group due to the presence of an electron-withdrawing inductive effect.
What are ortho para directing groups?
Some groups increase the electron density at ortho and para positions. So, the incoming electrophile attacks on these positions. These groups are called ortho-para directing groups.
Why is OH a strongly activating group despite the electronegativity of oxygen?
The -OH group shows an electron-withdrawing inductive effect due to the greater electronegativity of oxygen. At the same time, it shows an electron-donating resonance effect. The resonance effect of -OH is stronger than the inductive effect. Therefore, it is an activating group.
Is NR3 meta directing?
-N+(R)3 is meta directing group due to the presence of a positive charge on the nitrogen atom. It pulls the electron from the benzene ring to itself. It decreases the electron density on the ortho-para position. So, the incoming electrophile attacks on the meta position.
What is the effect of halogen on the activity of a ring?
The halogen atom shows two effects on the benzene ring:
- Electron-withdrawing inductive effect
- Electron-donating resonance effect
The inductive effect of halogens is stronger than the resonance effect. So, they deactivate the benzene ring.
- Advanced Organic Chemistry fifth edition by Francis A. Carey (University of Virginia) and Richard J. Sundberg