Question

Which of the following test is not shown by acetaldehyde?

• A. Lucas test
• B. Benedict’s test
• C. Tollen’s test
• D. Fehling's test

Answer 1

The correct option is A Lucas test

Lucas reagent is a solution of anhydrous $ZnC{l}_2$ & concentrated $HCl$. This solution is used to differentiate between Primary, Secondary & tertiary alcohols.

Aliphatic aldehydes on treatment with Benedict's solution give a colored precipitate while aromatic aldehydes and ketones (except α-hydroxy methyl ketones) do not react with Benedict's solution.

Tollen's reagent oxidizes an acetaldehyde into the corresponding acetic acid.
$C{H}_{3}CHO+2\left[Ag\right(N{H}_3{)}_2{]}^{+}+3O{H}^{-}\to C{H}_{3}CO{O}^{-}+2Ag\downarrow +2H_{2}O+4N{H}_3\text{Aldehyde}\text{Tollen's Reagent}\text{Carboxylate ion}\text{Silver mirror}$
The reaction is accompanied by the reduction of silver ions in Tollen's reagent into metallic silver, which, if the test is carried out in a clean glass test tube, forms a mirror on the test tube.

Fehling's solution oxidizes acetaldehyde to acetic acid.
$C{H}_{3}CHO+2C{u}^{2+}+5O{H}^{-}\to C{H}_{3}CO{O}^{-}+C{u}_{2}O\downarrow +3H_{2}O\text{Aldehyde}\text{Fehling's solution}\text{Carboxylate ion}\text{Red-brown ppt}$
Hence option (a) is correct.