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JEE Main 2026 Apr 8 Shift 2, Chemistry Q20: Proteins

JEE Main2026Apr 8, Shift 2Chemistry
Q.

Which of the following amino acid will give violet coloured complex with neutral ferric chloride solution?

  1. A

    Threonine

  2. B

    Serine

  3. C

    Tyrosine

  4. D

    Cysteine

Solution

Neutral FeCl gives a characteristic violet/purple colour with phenols (and other enolisable systems), via formation of an Fe(III)–phenolate coloured complex.

Among the four amino acids: threonine and serine carry aliphatic hydroxyl (CH(OH) or CHOH) groups, cysteine carries a thiol (SH), and only tyrosine has a phenolic OH on its para-substituted benzene ring.

Tyrosine forms the violet complex.

Concept behind this question

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