Question

Which one of the following compounds forms a red colored solution on treatment with neutral $FeC{l_3}$ solution?
(A) $C{H_3}COC{H_3}$
(B) $C{H_3}OC{H_3}$
(C) $C{H_3}C{H_2}OH$
(D) $C{H_3}COOH$

Answer 1

Hint: We know that the ferric chloride test is used to determine the presence of phenols in a given sample. Compounds such as enols, hydroxamic acids, sulfinic 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.

Complete answer: Here acetic acid will combine with the neutral ferric chloride solution and will give red wine coloration.
The reaction proceeds as:
$3C{H_3}COOH + FeC{l_3} \to {(C{H_3}COO)_3}Fe + 3HCl$
So, 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.

Additional information: The neutral ferric chloride test is used to identify phenols as it forms a violet color ${[Fe{(O{C_6}{H_5})_6}]^{3 - }}$ ion which is formed when phenol reacts with neutral $FeC{l_3}$

So, the correct answer will be Option D. i.e. $C{H_3}COOH$.

Note: The ferric chloride solution provided in the laboratory is usually acidic due to hydrolysis. 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. To solve questions like these you must always remember the test for different types of functional groups present.