MCQ Detailed View

Acetaldehyde (CH₃CHO) is the simplest aliphatic aldehyde after formaldehyde. It shows characteristic reactions due to the presence of the –CHO functional group and an α-hydrogen atom. Various qualitative tests are used in organic chemistry to identify aldehydes and related compounds.

Iodoform test: Acetaldehyde gives a positive iodoform reaction. The test identifies compounds with the structure –COCH₃ or those that can be oxidized to this group. When acetaldehyde reacts with iodine in the presence of a base, a yellow precipitate of iodoform (CHI₃) is formed.

Benedict’s test: This is a classical test for aldehydes and reducing sugars. Benedict’s reagent contains copper(II) sulfate in alkaline medium. Aldehydes like acetaldehyde reduce Cu²⁺ to Cu₂O, producing a brick-red precipitate. Thus, acetaldehyde responds positively.

Tollen’s test: Acetaldehyde, like most aldehydes, reduces ammoniacal silver nitrate solution to metallic silver. The deposition of silver on the inner walls of the test tube forms the well-known silver mirror. This confirms the presence of an aldehyde group.

Lucas test: The Lucas reagent (concentrated HCl with ZnCl₂) is used specifically to differentiate between primary, secondary, and tertiary alcohols. It works on the principle of forming alkyl chlorides with different rates depending on carbocation stability. Since acetaldehyde is not an alcohol, it cannot undergo substitution with Lucas reagent. Therefore, it does not give this test.

From the above analysis, acetaldehyde can successfully show iodoform, Benedict’s, and Tollen’s reactions, but it cannot show the Lucas test, because this test is exclusive to alcohols, not aldehydes.