JEE Main · 2021 · 20 Jul 2021 · Shift 1 · Biomolecules

Ninhydrin condenses with the free $\alpha$-amino group of the protein; the imine so formed loses carbon dioxide and is hydrolysed to 2-aminoindane-1,3-dione, which then condenses with a second molecule of ninhydrin. Both halves of the dye are therefore carbocyclic indane-1,3-dione units carrying ketone groups at the 1 and 3 positions, joined through a single bridging nitrogen rather than through an N–N bond, and that nitrogen stays outside both rings. Loss of the proton from one half puts a negative charge on its five-membered ring, which is delocalised over the adjacent carbonyl, and the extended conjugation through the bridging nitrogen is what gives the deep violet colour. Ruhemann's purple is thus the anion in which the bridging nitrogen is doubly bonded to one 1,3-dioxoindane unit and singly bonded to a second unit that bears the enolate oxygen.