JEE Main · 2021 · 25 Feb 2021 · Shift 1 · Aldehydes Ketones and Carboxylic Acids

Carbon dioxide is released from sodium bicarbonate only by an acid stronger than carbonic acid, whose $\mathrm{p}{K}_a$ is about $6.4$. The first compound is 2,4,6-triaminophenol: three electron-releasing amino groups push charge into the ring and destabilise its phenoxide, so it is even weaker than ordinary phenol and far weaker than carbonic acid. The second is benzoic acid, $\mathrm{p}{K}_a\approx 4.2$, in which the carboxylate charge is shared equally by two oxygen atoms. The third is picric acid, 2,4,6-trinitrophenol, where three nitro groups delocalise the phenoxide charge onto their oxygen atoms and bring the $\mathrm{p}{K}_a$ down to about $0.4$. Benzoic acid and picric acid liberate carbon dioxide; the triaminophenol does not.