Chemistry - GOC Question with Solution | TestHub

$\mathrm{P}$ is indole: its nitrogen lone pair participates in the pyrrole-like aromatic sextet and is weakly basic. Q is quinoline, whose pyridine-like lone pair is outside the pi sextet. In $\mathrm{R}$, the two facing pyridine-like nitrogens form a proton-sponge arrangement: protonation relieves lone-pair repulsion and the conjugate acid is stabilized by an intramolecular $\mathrm{N}-\mathrm{H}$···$\mathrm{N}$ interaction. This raises basicity above Q. Hence $\mathrm{R}$ > Q > $\mathrm{P}$, $\mathrm{B}$.