JEE Main · 2023 · 30 Jan 2023 · Shift 2 · Alcohols Phenols and Ethers

The acidity of a para-substituted phenol is set by how the substituent treats the phenoxide formed on ionisation. A nitro group withdraws electrons strongly by resonance, spreading the negative charge and making that phenol the most acidic, hence the lowest ${\mathrm{pK}}_a$. A dimethylamino group pushes electron density into the ring, intensifying the charge on oxygen and leaving that phenol the least acidic, hence the highest ${\mathrm{pK}}_a$. An isopropyl group is only weakly electron releasing, so its phenol is a little less acidic than phenol itself, while unsubstituted phenol lies between the isopropyl and nitro cases. Arranged by increasing acidity the dimethylamino compound stands highest in ${\mathrm{pK}}_a$, followed by the isopropyl compound, then phenol, then the nitro compound, giving the order $b>d>a>c$.