When few drops of concentrated trioxonitrate(V) acid is added to an unknown sample and wanned an intense…

Analyze the question: Concentrated trioxonitrate(V) acid (HNO₃) is warmed with an unknown sample, and an intense yellow colour appears. We need to identify the functional group responsible.

Concept: This is the xanthoproteic test, used to detect proteins and amino acids that contain aromatic rings (such as tyrosine, phenylalanine and tryptophan).

• Concentrated HNO₃ nitrates the benzene ring in the amino acid side chain.
• This produces yellow nitro-compounds.
• Adding alkali (NaOH or NH₃) deepens the colour to orange.

So the yellow colour points to a protein or amino acid, whose functional group is the amino acid unit: an amino group (–NH₂) and a carboxyl group (–COOH) attached to the same carbon.

Checking the options:

• A. NH₂RCHCOOH is the general amino acid structure (R–CH(NH₂)–COOH). This is the group found in proteins, so it matches the xanthoproteic test.
• B. CHO is an aldehyde. Aldehydes are tested with Tollens' reagent or Fehling's solution, not with HNO₃.
• C. CO is a ketone or carbonyl group. It is tested with 2,4-DNPH and gives no yellow colour with conc. HNO₃.
• D. CNH₂ is an amide or amine type group and does not give this test.

Strictly speaking, it is the aromatic ring in the side chain (R) that is nitrated, so the test works only for amino acids and proteins with aromatic R groups. Among the options given, however, A is the only one that represents an amino acid or protein, so it is the best answer.

Exam tip: Remember the protein tests:

• Xanthoproteic (conc. HNO₃, warm) gives a yellow colour.
• Biuret (NaOH plus dilute CuSO₄) gives a purple/violet colour.
• Millon's test gives a red/pink precipitate (for tyrosine).

If the question says "yellow with conc. HNO₃", think protein or amino acid.