Question

Which of the following give violet colour with neutral ferric chloride solution ?
A.Acetic acid
B.Benzoic acid
C.Salicylic acid
D.Formic acid

Answer 1

Hint: When neutral ferric chloride reacts with any enol group , that is $=\stackrel{|}{C}-OH$ then it gives characteristic colours like violet , blue , green depending upon the nature of the phenol.

Complete step by step answer:
Neutral ferric chloride solution is especially prepared to test for the presence of phenols.
To prepare neutral ferric chloride solution, first we add ammonium hydroxide solution to $FeC{l}_3$ , so a precipitate is formed , then again we add $FeC{l}_3$ solution so that the precipitate dissolves .The clear solution thus obtained can be used as neutral ferric chloride solution to test for phenolic , that is, $=\stackrel{|}{C}-OH$ group .
Among the above given compounds, acetic acid contains a $COOH$ group, benzoic acid contains a $COOH$ group attached to a benzene ring and formic acid too contains a $COOH$ group attached to a hydrogen atom.
But salicylic acid contains a $COOH$ group as well as an $OH$ group attached to the benzene ring which means it is a phenolic compound and therefore it will give violet colour with neutral ferric chloride solution.
The theory behind this is that phenolic compounds react with neutral ferric chloride solutions to give coloured compounds like blue, violet, green .This is due to the formation of phenoxide ions which give coloured complex compounds that are soluble in water.
So option C is correct.


Note:
The reaction with neutral ferric chloride is used as a test to distinguish between alcohols and phenols , as alcohols do not react with neutral ferric chloride whereas phenols react to give different colours .