MCQ Detailed View

Acetaldehyde (CH₃CHO) is a simple aliphatic aldehyde. Its chemical behavior can be identified using classic organic tests that help differentiate between functional groups like alcohols, aldehydes, and ketones.

The Iodoform test is positive for compounds that contain either a methyl ketone group (–COCH₃) or secondary alcohols that oxidize to methyl ketones. Acetaldehyde contains the required –COCH₃ structure, so it gives a positive iodoform test, forming a yellow precipitate of CHI₃.

Benedict’s test is used to detect reducing sugars and aldehydes. Aldehydes like acetaldehyde can reduce the blue cupric ion (Cu²⁺) in Benedict’s reagent to red cuprous oxide (Cu₂O), giving a brick-red precipitate. So, acetaldehyde gives a positive Benedict’s test.

Tollen’s test is specific for aldehydes. Acetaldehyde reacts with Tollen’s reagent (ammoniacal silver nitrate), reducing Ag⁺ to metallic silver. This forms a silver mirror on the inner wall of the test tube. So, acetaldehyde gives a positive Tollen’s test.

Lucas test is used to distinguish between primary, secondary, and tertiary alcohols. The reagent is a mixture of concentrated HCl and ZnCl₂. It reacts with alcohols, not aldehydes. Acetaldehyde does not contain an alcohol group, so it does not react in this test. Therefore, it gives a negative Lucas test.

Among the listed tests, the only one that acetaldehyde cannot perform is the Lucas test. This helps identify the functional group of the compound and its chemical behavior under specific reaction conditions used in qualitative organic analysis.