Question

Hydroxamic acid test is employed to detect-
A.Ketones
B.Aldehydes
C.Esters
D.Amides

Answer 1

Hint: We have to know that hydroxamic acid test is one of the approaches for identifying an unknown organic compound through qualitative analysis. Through the qualitative analysis of organic compounds, we can identify several of the functional groups like ketones, aldehydes, esters, carboxylic acids, alcohols, amines, and amides.

Complete answer:
Let us now discuss the procedure of Hydroxamic acid.
We have to take a small quantity of the unknown compound and mix it with one mL of $0.5M$ hydroxylamine hydrochloride in $95\% $ of ethanol and $0.2mL$ of $6M$ sodium hydroxide are added. We heat the mixture until it reaches boiling state and later the solution is cooled mildly. After this, $2mL$ of hydrochloric acid is added. If the solution appears cloudy, again $2mL$ of $95\% $ ethanol is added. We can observe a formation of color when 1 drop of five percent of ferric chloride is added. If we don’t see any change in color, we have to continue adding the reagent in drop wise matter unless the color we observed occupies the whole solution. Finally, we can see a distinct burgundy color (or) magenta color appearance in the solution. This color would confirm the presence of esters functional group in the unknown compound.
We can write the general equation as,
$R - CO - O{R^'} + N{H_2} - OH\xrightarrow{{}}R - CO - NH - OH + {R^'} - OH$
So, finally we can conclude that hydroxamic acid test is used to confirm the presence of functional group esters.

Option (C) is correct.

Note:
We have to know before performing the hydroxamic acid test for ester, we need to perform the ferric chloride test. For the ferric chloride test, we have to dissolve a few drops of unknown compound in one mL of $95\% $ of ethanol and later 1mL of hydrochloric acid is added. When we add $5\% $ of ferric chloride to the solution, we can see the change in color appearance. We can obtain blue, violet, red (or) orange color and this color change indicates the presence of esters.