Identify Amino Acid Giving Violet Color with Ferric Chloride

To determine which amino acid gives a violet-coloured complex with neutral ferric chloride ($\text{FeCl}_3$) solution, we examine the functional groups present in each amino acid:

1. Neutral Ferric Chloride Test: Neutral $\text{FeCl}_3$ solution is a specific test for the presence of a phenolic hydroxyl group ($-\text{C}_6\text{H}_4\text{OH}$). Compounds containing a phenolic $-\text{OH}$ group react with $\text{Fe}^{3+}$ ions to form a characteristic colored coordination complex (often violet, blue, or green): $6\text{Ar-OH} + \text{Fe}^{3+} \rightarrow [\text{Fe}(\text{OAr})_6]^{3-} + 6\text{H}^+$

2. Analysis of the Options:

   • Threonine: Contains an aliphatic hydroxyl group ($-\text{CH}(\text{OH})\text{CH}_3$). Aliphatic alcohols do not give a positive neutral $\text{FeCl}_3$ test.
   • Serine: Contains an aliphatic hydroxyl group ($-\text{CH}_2\text{OH}$). Aliphatic alcohols do not react with $\text{FeCl}_3$ to give a violet color.
   • Tyrosine: Contains a phenolic hydroxyl group (a hydroxyphenyl side chain, $-\text{CH}_2\text{-C}_6\text{H}_4\text{-OH}$). Owing to this phenolic group, it reacts with neutral $\text{FeCl}_3$ solution to yield a violet-coloured complex.
   • Cysteine: Contains a thiol group ($-\text{CH}_2\text{SH}$), which does not form a violet phenolic complex with neutral $\text{FeCl}_3$.

Thus, Tyrosine is the only amino acid among the given choices that reacts with neutral ferric chloride solution to give a violet-coloured complex.

Correct Option: C (Tyrosine)