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  4. The reagent used to distinguish between acetaldehyde and benzaldehyde is

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Q. The reagent used to distinguish between acetaldehyde and benzaldehyde is

KCETKCET 2013Aldehydes Ketones and Carboxylic Acids

Solution:

(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)

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

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(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

$\underset{\text{acetaldehyde }}{CH _{3} CHO} +\underset{\text{Tollen's reagent}}{2\left[ Ag \left( NH _{3}\right)_{2}\right]^{+}} +3 OH ^{-} \xrightarrow{{\Delta}}$

$\underset{\text{acetate ion}}{CH _{3} COO ^{-}}+\underset{\text{ silver mirror}}{2 Ag \downarrow}+4 NH _{3}+2 H _{2} O$

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

$\underset{\text{acetaldehyde}}{CH _{3} CHO}\,\,\underset{\text{Fehling's }}{2 Cu ^{2+}}+\underset{\text{ solution}}{5 OH ^{-}} \xrightarrow{\Delta}$

$\underset{\text{acetate ion}}{CH _{3} COO ^{-}}+\underset{\text{cuprous oxide (red) }}{Cu _{2} O} + H _{2} O$

However, benzaldehyde does not reduce Fehling's solution .