Question

The reagent used to distinguish between acetaldehyde and benzaldehyde is

• A. Tollen's reagent
• B. Fehling's solution
• C. $2-4$-dinitrophenyl hydrazine
• D. Semicarbazide

Answer 1

Answer: (B)

(i) Reaction with $2,4$ -dinitrophenylhydrazine (Brady's reagent) : Acetaldehyde and benzaldehyde react with $2,4$ -dinitrophenylhydrazine (DNP) to form yellow, orange or red ppt of $2,4$ -dinitrophenylhydrazones (DNP derivatives)



(ii) Reaction with semicarbazide : Both acetaldehyde and benzaldehyde react with semicarbazide to form.



(iii) Reduction ot Tollen's reagent both acetaldehyde and benzaldehyde reduce Tollen's reagent to metallic silver which deposit on the walls of the test tube as bright silver mirror

$\underset{\text{acetaldehyde }}{C{H}_{3}CHO}+\underset{\text{Tollen’s reagent}}{2{\left[Ag{\left(N{H}_3\right)}_2\right]}^{+}}+3O{H}^{-}\stackrel{\Delta }{\to }$

$\underset{\text{acetate ion}}{C{H}_{3}CO{O}^{-}}+\underset{\text{ silver mirror}}{2Ag\downarrow }+4N{H}_3+2H_{2}O$

(iv) Reduction of Fehling's solution : Acetaldehyde reduces Fehling solution to a red ppt of cuprous oxide.

$\underset{\text{acetaldehyde}}{C{H}_{3}CHO}\underset{\text{Fehling’s }}{2C{u}^{2+}}+\underset{\text{ solution}}{5O{H}^{-}}\stackrel{\Delta }{\to }$

$\underset{\text{acetate ion}}{C{H}_{3}CO{O}^{-}}+\underset{\text{cuprous oxide (red) }}{C{u}_{2}O}+H_{2}O$

However, benzaldehyde does not reduce Fehling's solution .