Question

The correct order of reactivity towards electrophilic substitution is?
a.) Benzene > phenol > benzoic acid > chlorobenzene
b.) Phenol > benzene > chlorobenzene > benzoic acid
c.) Chlorobenzene > benzoic acid > phenol > benzene
d.) Benzoic acid > chlorobenzene > benzene > phenol

Answer 1

Hint: You should know that in an electrophilic aromatic substitution reaction, the existing substituent groups on the aromatic ring have the ability to influence the overall reaction rate.


Complete step by step solution:
First let us understand what are activating and deactivating groups.
Generally an activating group is simply any substituent that increases the reactivity of the organic compound while a deactivating group tends to decrease the reactivity of the organic compound.
The activating group directs the reaction to the ortho or para position. Whereas, the deactivating group directs the reaction to the meta position.
Electrophilic substitution reaction rate increases if benzene is attached to the electron withdrawing group but rate decreases if the benzene is attached to the electron donating group.
The reactivity of phenol is comparatively more because the methyl group present on the methoxy substituent makes the oxygen less donating in nature thus reducing the activation of the ring.

Generally,
-strongly activating substituents are: Aniline, Phenol, and Anisole
-Moderately activating substituents are: Acetanilide
-Weakly activating substituents: Alkylbenzene , vinylbenzene , diphenyl
-Weakly deactivating substituents: Fluorobenzene , Chlorobenzene , Bromobenzene , Iodo benzene.
-Moderately deactivating substituents: Benzaldehyde , Acetophenone , Methyl benzoate
-Strongly deactivating substituents: Benzene sulphonic acid, Nitrobenzene , Cyanobenzene.
General order of reactivity towards electrophilic substitution reaction: Aniline > Phenol > Anisole > Acetanilide > Toluene > Chlorobenzene > Fluorobenzene > Benzoic acid > Benzaldehyde > Nitrobenzene.

Therefore, the correct order of reactivity towards electrophilic substitution is Phenol > benzene > chlorobenzene > benzoic acid. Hence option B is the required answer.

Note: Keep in mind that electrophile is an electron deficient species, so generally the one which is most reactive towards an electrophilic attack will be electron rich.