Alcohols, Phenols and Ethers Part 1

Acidic strength is inversely related to $\mathrm{p}K_a$: a stronger acid has a lower $\mathrm{p}K_a$.The $-\mathrm{NO_2}$ group is strongly electron withdrawing and stabilises the phenoxide ion, so nitrophenols are more acidic than phenol.For the two nitrophenols, the para isomer $B$ is more acidic than the meta isomer $C$ because the $-\mathrm{NO_2}$ at the para position can delocalise the negative charge more effectively; hence $\mathrm{p}K_a(B) < \mathrm{p}K_a(C)$.Phenol $A$ has no electron-withdrawing substituent, so it is less acidic than both nitrophenols: $\mathrm{p}K_a(C) < \mathrm{p}K_a(A)$.The para $-\mathrm{OMe}$ group in $D$ is electron donating by resonance and destabilises the phenoxide ion, making $D$ the least acidic: $\mathrm{p}K_a(A) < \mathrm{p}K_a(D)$.Therefore, the increasing order of $\mathrm{p}K_a$ values is $B < C < A < D$.