Activating and deactivating group

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

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.