Question

The correct order of acid strength of the following substituted phenols in water at ${28^\circ }$ is
A. p-nitrophenol $<$ p-fluorophenol $<$ p-chlorophenol
B. p-chlorophenol
C. p-fluorophenol $<$ p-chlorophenol $<$ p-nitrophenol
D. p-fluorophenol $<$ p-nitrophenol $<$ p-chlorophenol

Answer 1

The presence of the electron withdrawing group increases the acidity of the compounds while the presence of the electron donating group increases the basicity of the compounds. All the three groups floro, chloro, nitro are electron withdrawing groups and show both inductive as well as mesomeric effect. The resultant of these two effects i.e. inductive and mesomeric will decide the order of acidic strength .

Complete answer:
-Acidic strength depends how stable the anion would be after the loss of the hydrogen atom . Both inductive and mesomeric effects affect the acidic strength of the compound.
-All the three groups are electron withdrawing groups .
-In case of halogens, the inductive effect is more dominant while in others mesomeric effect is dominant.
-Nitro group which shows $- M$ dominates all other effects. If this is added to any group it will withdraw the electron cloud with great extent.
-Now, in the case of fluoro and chloro both show $- I$ effect. When we draw the resonating structures of the conjugate base of both p-chlorophenol and p-fluorophenol , we will see that p-chloro phenoxide ion will have extra resonance structure. This happens because chlorine has extra vacant d-orbital for delocalisation of electrons which is not possible in fluorine atoms . Hence ,the conjugate base of p- chlorophenol is more stable and hence more acidic.
-p-chlorophenol is more acidic than p-fluorophenol.
Hence, the correct option is (C).

Note:
Inductive effect – The phenomena in which the electron cloud gets shifted toward the more electronegative atom leading to the creation of dipole moment. It is a permanent effect.
Mesomeric effect- It is a temporary effect and this is also the polarisation of the bond.