Activating and deactivating group

1. Activating and Deactivating
Group
Mr. Yashvant V. Khandare

2. Activating Group
The group which increases reactivity of
benzene nucleus toward further electrophilic substitution
reaction is called activating group.
All ortho-para directing group except halogens are
activating groups.
These groups donate electrons to benzene ring.
This increases electron density and make the benzene ring
more reactive toward further electrophilic substitution
reaction.
Examples :-
NH2, -NHR, -O-, -OH, -OCH3, -CH3 etc

3. Deactivating Group
The group which decreases reactivity of
benzene nucleus toward further electrophilic substitution
reaction is called activating group.
All meta directing group and halogens are activating groups.
These groups withdraw electrons to benzene ring.
This decreases electron density and make the benzene ring
less reactive toward further electrophilic substitution
reaction.
Hence these groups are called deactivating groups.
Examples :-
NO2, -SO3H, -CN, -CHO, -COOH, -Cl, -Br. etc

5. Direct influence on the basis of modern electronic
theory
1]–OH group
For example Phenol : There are two effects-
i)+M effect :- In phenol, oxygen has a lone pair which is in
conjugation with double bonds of ring hence it takes part in
resonance as shown below-
Because of +M effect, the electron density at ortho and para
position increases. Therefore, OH group act as a ortho/para
directing group.

6. ii)-I effect :-
-OH group withdraws electrons from the benzene
ring from all the positions by means of –I effect as
shown below-
Since –I effect is much smaller than +M effect, There
is ortho-para directing group and activating group.

7. 2] -Cl group
For example chlorobenzene : There are two effects-
i)+M effect : In chlorobenzene, chlorine has a lone pair of
electrons which is in conjugation with the double bonds of
ring and hence takes part in resonance as shown below-
Because of +M effect, the electron density at ortho and
para position increases. Therefore –Cl group acts as a
ortho-para directing group.

8. ii)-I effect :-
-Cl withdraws electron from benzene ring from all the
positions by means of strong inductive effect (-I) which
deactivates the nucleus.
Here –I effect is more powerful than +M effect. Thus is net
deactivation of benzene ring for further electrophilic
substitution reaction.

9. 3]CH3 group
For example toluene:-
i)Hyperconjugation : Methyl group is electron donating
group. It donates electrons to the benzene ring by means
of hyperconjugation (no bond resonance) or sigma bond
resonance as shown below-
Because of hyperconjugation, electron density increases
mainly at ortho and para positions. Therefore –CH3 group
acts as a ortho para directing group.

10. ii) +I effect
Methyl group donate electrons to benzene ring by +I
effect.
Here both the effects hyperconjugation and +I effect
operates in the same direction, as a result, -CH3 group
acts as a ortho-para directing and activating group.

11. 4]NO2 group
For example nitrobenzene:-
i)-M effect : Nitro group is electron withdrawing group.
It withdraws electrons from the benzene ring by means of
–M effect as shown below-
Because of –M effect, electron density at ortho and para
positions decreases. As a result of this electrophile is
attracted towards meta position only which is
comparatively electron rich.
Therefore –NO2 group acts as a meta directing group.

12. ii) -I effect
-NO2 group withdraws electrons from the benzene
ring by means of –I effect and deactivates the nucleus.
Due to both –M effect and –I effect, -NO2 group acts as a
meta directing and deactivating group.