Question

Which of the following is the strongest ortho, para directing group:
A. –OH
B. –Cl
C. \[ - OC{H_3}\]
D. \[ - C{H_3}\]

Answer 1

Hint: All the given groups are ortho para directing groups. The ortho para directing group shows resonance effect +R and +I effect inductive effect and +R effect is greater than +I effect.

Complete step by step answer:
-When a mono-substituted benzene ring is reacted with an electrophilic group then it undergoes three electrophilic aromatic substitution reactions. After each reaction a disubstituted product is formed which are described as ortho, meta and para directing compounds.
-In electrophilic aromatic substitution, the yield of ortho product and the para product is higher than meta product than the group present in the benzene ring is ortho para directing group
-If the yield of a meta product is higher than ortho product and para product, then the group present in the benzene ring is the meta directing group.
-The ortho para directing groups are ring activating groups and meta directing groups are ring deactivating groups. The ring activating group shows +R effect resonance effect and +I Inductive effect and the electron deactivation group contains –I effect inductive effect.
-The strongest ortho para directing group will be whose resonance effect will be higher than the inductive effect.
-The hydroxyl group is the strongest ortho para directing group as hydroxyl group is electron releasing group and it increases the electron density in the benzene ring through resonance and activates the ring towards electrophilic substitution.
-Thus, among the following the strongest ortho para directing group is –OH.
Therefore, the correct option is A.

Note:
There is an exception for halogen groups even though they are electron withdrawing groups and show –I effect even though they are considered as ortho para directing because of lone pairs present on them.