Question

Which of the following does not react with both, acetone and benzaldehyde:
A. Sodium hydrogen sulphite
B. Phenyl hydrazine
C. Fehling’s solution
D. Grignard solution.

Answer 1

Hint: Acetone has the functional group ketone ($R-\stackrel{O}{\stackrel{||}{C}}-R^{\prime }$ ) and benzaldehyde is an aromatic compound with an aldehyde ($\stackrel{o}{\stackrel{||}{-C}}H$) as a functional group. Most of the reaction with aldehyde and ketone involves a nucleophile (Nucleus liking) attack at the carbonyl carbon site due to its partial polarization.

Complete Step by step answer: The given two molecules are ketone and benzaldehyde.
Let’s look at the options and see whether or they will react with any of them.
i. Sodium hydrogen sulphite ($$NaHS{O}_3$$) reacts with both aldehyde and ketone by nucleophilic addition at the carbonyl carbon ($-\stackrel{O}{\stackrel{||}{C}}-$) and forming bisulfite addition compounds.
ii. Addition of a derivative of ammonia ($H_{2}N-Z$) such as phenylhydrazine yields a product of the general formula $>C=N-Z$
iii. Fehling’s solution, an oxidative reaction wherein the carbonyl group of aldehyde is oxidized to carboxylate ion, but Fehling's Solution cannot oxidize aromatic aldehyde, such as benzaldehyde.
iv. Grignard reagent (R-Mg-X) followed by hydrolysis, reacts with both aldehyde and ketone and forms corresponding alcohol, with one carbon greater than the parent chain.
In all the given option only Fehling’s solution [aqueous copper sulphate and alkaline sodium-potassium tartrate (Rochelle salt)] does not react with benzaldehyde and since it is a weak oxidizing agent it will also not react with the acetone (Ketone needs strong oxidizing agents to form corresponding carboxylic acid)

Hence, the correct answer is option(C) i.e., Fehling’s solution.

Additional information:Fehling’s solution and Tollen’s test (ammonical silver nitrate solution) are both weak oxidizing agents and are used to separate a solution containing aldehyde and ketone.

Note: Fehling’s solution reacts with all aliphatic aldehyde, but it does not react with aromatic aldehydes due to the involvement of the carbonyl double bond in resonance. Tollen test is also a weak oxidizing agent and hence cannot oxidize a ketone.