JEE Main · 2019 · 09 Jan 2019 · Shift 1 · General Organic Chemistry

Acid strength here is settled by how well the conjugate base spreads out its negative charge. Removing the proton from $\mathrm{CH}(\mathrm{CN}){}_3$ leaves a carbanion flanked by three cyano groups, and the charge delocalises extensively onto all three $\mathrm{C}\equiv \mathrm{N}$ units, giving an unusually stable anion. The halogen-bearing haloforms have no such resonance outlet, so $\mathrm{CH}(\mathrm{CN}){}_3$ is the strongest acid of the set.