In electrophilic substitution reaction in the monosubstituted benzene, the group already present in the benzene ring decides the position of newly coming group. On the basis of orientation, substituent groups are divided into two types:
1. Ortho- Para director (+M groups) : It is the group in which the atom directly attached to benzene ring is further bonded with other atoms by a single covalent bond.
Groups like –NH2 ,-OH, -OCH3 ,-NHCOCH3 ,-C6H5 ,-CH3 ,-X, etc. make ortho and para position electron rich. So, electrophile is likely to attack in these positions.
Thus, these groups are ortho-para directors for electrophilic substitution reactions.
As they make the ring electron rich, they are ring activators (except halo group X= Cl, Br or I, as they are strong e-withdrawing groups. In halo groups their –I effect is more powerful than +M effect).
2. Meta director (-M groups): It is the group in which the atom directly attached to benzene ring is further bonded with more EN atom by multiple bond.
Groups like –NO2 ,-NH3 + , -CN,-COOH, -COOR, -SO3H, - CHO, -COR, etc. make the ortho and para position e-deficient. Thus, electrophile prefer to attack meta position rather than ortho and para position. So, these groups are meta directors.
As they make the ring e- deficient, they are ring deactivators.
Remember: Halogens are Ortho-Para directors but ring deactivators.
Some important questions
Q.1.Why is phenol ortho-para directing towards electrophilic substitution reaction?
→ The –OH group in phenol is e- releasing (+M) group which increases the e- density in the benzene ring. From the resonating structures we see partial negative(-) charge at ortho and para positions i.e., the e-density in phenol is relatively higher at ortho and para positions than at meta position. Therefore, the incoming electrophile prefers to attack ortho and para position than the meta position. Hence, phenol is ortho-para directing towards electrophilic substitution reaction.
Q.2.Why is aniline ortho-para directing towards electrophilic substitution reaction?
→ The –NH2 group in aniline is e- releasing (+M) group which increases the e- density in the benzene ring. From the resonating structures we see partial negative(-) charge at ortho and para positions i.e., the e-density in aniline is relatively higher at ortho and para positions than at meta position. Therefore, the incoming electrophile prefers to attack ortho and para position than the meta position. Hence, aniline is ortho-para directing towards electrophilic substitution reaction.
Resonating structures of aniline
Q.3.Why is nitrobenzene meta directing towards electrophilic substitution reaction?
→ The –NO2 group in nitrobenzene is e- withdrawing (-M) group which decreases the e- density in the benzene ring. From the resonating structures we see partial +ve charge at ortho and para positions i.e., the e- density in nitrobenzene is relatively lesser at ortho and para positions than at meta position. Therefore, the incoming electrophile prefers to attack meta position than the ortho and para position. Hence, nitrobenzene is meta directing towards electrophilic substitution reaction.
Resonating structures of nitrobenzene
Q.4.Why is benzaldehyde meta directing towards electrophilic substitution reaction?
→ The –CHO group in benzaldehyde is e- withdrawing (-M) group which decreases the e- density in the benzene ring. From the resonating structures we see partial +ve charge at ortho and para positions i.e., the e- density in benzaldehyde is relatively lesser at ortho and para positions than at meta position. Therefore, the incoming electrophile prefers to attack meta position than the ortho and para position. Hence, benzaldehyde is meta directing towards electrophilic substitution reaction.
Hope this will helps you a lot.
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