The order of decreasing reactivity towards an electrophilic reagent, for the following
(i) Benzene
(ii) Toluene
(iii) Chlorobenzene
(iv) Phenol
woild be

Solution :
Benzene having any activating group, i.e. $$OH,R,$$  etc, undergoes electrophilic substitution very easily as compared to benzene itself. Thus, toluene $$\left( {{C_6}{H_5}C{H_3}} \right),$$   phenol $$\left( {{C_6}{H_5}OH} \right)$$   undergo elecrophilic substitution very readily than benzene. Chlorine with $$ + E$$  and $$ + M$$ - effect deactivates the ring due to strong $$-I$$ - effect. So, it is difficult to carry out the substitution in chlorobenzene than in benzene, so the correct order is
$$\mathop {{\text{Phenol}}}\limits_{\left( {{\text{iv}}} \right)} {\text{ > }}\mathop {{\text{Toluene}}}\limits_{\left( {{\text{ii}}} \right)} {\text{ > }}\mathop {{\text{Benzene}}}\limits_{\left( {\text{i}} \right)} {\text{ > }}\mathop {{\text{Chlorobenzene}}}\limits_{\left( {{\text{iii}}} \right)} $$