The correct structure of Rhumann's Purple, the compound formed in the reaction of ninhydrin with proteins is :

Solution

Chemistry ma’am · 3 steps

Ruhemann's purple is two indanedione halves joined by one N, carrying one charge spread over both halves.

1. How it forms. Ninhydrin (indane-1,2,3-trione) oxidatively deaminates the free -NH₂ group of an amino acid, releasing CO₂ and an aldehyde. Its nitrogen ends up on a reduced ninhydrin unit, which then condenses with a second ninhydrin molecule.
2. What to look for. So the product has one N bridge between two carbon-only five-membered rings, both from ninhydrin. The colour needs one long conjugated chain across both halves, so every ring carbon must be sp² and the anion carries a single charge, as an enolate O⁻.

   Option	Bridge	Charge and O⁻ type	Fully conjugated?
   (a)	one N	-2, two alkoxide O⁻ on sp³ C–H	no, sp³ carbons break it
   (b)	one N	-2, one enolate plus one alkoxide on sp³ C–H	no, the right ring has an sp³ carbon
   (c)	N=N-N⁺, N inside the rings	phthalimide rings, not indanedione	not a ninhydrin skeleton
   (d)	one N, C=N	-1, one enolate O⁻, three C=O	yes, across both rings

3. So. Only (d) has two indane-1,3-dione units joined through a single C=N bridge, with one delocalised negative charge. This is the deep purple anion, so the answer is (d).

The idea to keep. Ninhydrin test: the amino acid gives only its N. Two ninhydrin halves join through that one N, and the single negative charge spread over the whole conjugated system gives the purple colour.

Answer from the NTA final answer key. Solution by PrepWiser.