Question

Arrange the following compounds in the increasing order of their acid strength:
p-cresol, p-nitrophenol, phenol.

Answer 1

Hint: The acidic strength depends on the inductive effect. The +I effect group (electron releasing group) decreases the acidic strength and –I effect group (electron withdrawing group) increases the acidic strength.

Complete step by step answer:
The acid strength of an acid is defined as the efficiency of acid to dissociate into its constituent anion and cation. The acidic strength of acid depends on the inductive effect of the group attached to the substituent compound.
The inductive effect is defined as the formation of permanent dipole moments in a molecule which is generated by the unequal sharing of bonding electrons.
When an electron withdrawing group and an electron releasing group is attached to the compound, negative and positive charge is incorporated in the compound due to the group added. This results in permanent dipole moments in the molecule resulting in an inductive effect.
The inductive effect is divided into +I effect inductive effect and –I effect inductive effect based on the group attached to the compound.
When an electron releasing group is attached which can donate its electrons then + inductive effect is formed. When an electron withdrawing group is attached which cannot donate the electrons, -Inductive effect is observed.
The +I effect group decreases the acidic strength and –I effect group increases the acidic strength.
In p-cresol the methyl group is attached at para position to the hydroxyl group. The methyl group is an electron releasing group which shows +Inductive effect so it will decrease the acidic strength.
In phenol, no group is attached. It will show neither +I effect and –I effect.
In p-nitrophenol the nitro group is attached at para position to the hydroxyl group. The nitro group is an electron withdrawing group which –I effect so it will increase the acidic strength.
Thus, p-nitrophenol has maximum acidic strength and p-cresol has least acidic strength.
The order is shown below.
p-nitrophenol > phenol > p-cresol.

Note: The inductive effect only takes place in compound carrying sigma bond and the electromeric effect is seen in compound carrying pi bond.