Question

What do you observe when acetic acid is added to: neutral $FeCl_3$ solution?

Answer 1

Hint: We know that 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 colors.

Complete answer:
As we know, the ferric chloride test is used to determine the presence of phenols in a given sample. Compounds such as enols, hydroxamic acids, sulfonic acid and oximes gives a positive result when treated with neutral $FeC{{l}_{3}}$ also neutral $FeC{{l}_{3}}$ is used because, if acidic or basic $FeC{{l}_{3}}$ will be used, neutralization reaction will take place and precipitate of the salt will be formed. But acetic acid contains a group as well as a 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 colored compounds like blue, violet, green. This is due to the formation of phenoxide ions which give colored complex compounds that are soluble in water. Here acetic acid will combine with the neutral ferric chloride solution and will give red wine coloration. The reaction proceeds as:
$3C{{H}_{3}}COOH+FeC{{l}_{3}}\to {{\left( C{{H}_{3}}COO \right)}_{3}}Fe+3HCl.$ The ferric acetate formed is responsible for the red wine coloration. Also, we see that the complex ion formed of ferric acetate will further undergo decomposition reaction on heating and will give a brown precipitate of ferric dihydroxy acetate. So, the solution is made neutral by adding the dilute solution of ammonium hydroxide drop by drop with constant stirring till the precipitate is not completely dissolved and this solution formed is called the neutral ferric chloride solution.

Note:
Remember that the neutral ferric chloride solution is especially prepared to test for the presence of phenols and the ferric chloride solution provided in the laboratory is usually acidic due to hydrolysis.