Q.The reagent which does not react with both acetone and benzaldehyde is __________.

The key idea is that Fehling's solution oxidises only aliphatic aldehydes — it does not react with ketones (like acetone) or aromatic aldehydes (like benzaldehyde).

Step 1: Acetone is a ketone; benzaldehyde is an aromatic aldehyde.

Step 2: Sodium hydrogen sulphite adds to the carbonyl group of both aldehydes and ketones. Phenyl hydrazine forms hydrazones with both. Grignard reagents add to both carbonyls. …

Concept & Intuition

This question tests your understanding of the reactivity of carbonyl compounds — specifically, how aldehydes and ketones differ in their reactions with common reagents.

Acetone is a ketone ($\ce{CH3COCH3}$), while benzaldehyde is an aromatic aldehyde ($\ce{C6H5CHO}$). Both have a carbonyl group ($\ce{C=O}$), but the key difference lies in the hydrogen atom attached to the carbonyl carbon: aldehydes have at least one $\ce{H}$ (making them easier to oxidise), while ketones have two carbon groups attached.

Many reagents attack the electrophilic carbonyl carbon via nucleophilic addition. Others are specific to the oxidisability of the aldehyde group. The reagent that fails to react with both must be one that works only on aldehydes (or only on ketones), or on neither.

Let’s examine each option.

Step-by-Step Analysis

1. Sodium hydrogen sulphite ($\ce{NaHSO3}$)

This reagent adds across the $\ce{C=O}$ bond via nucleophilic addition. The bisulphite ion ($\ce{HSO3-}$) attacks the electrophilic carbonyl carbon, forming a crystalline bisulphite addition product.

• Acetone: Reacts readily — gives a white crystalline solid.
• Benzaldehyde: Also reacts — though more slowly due to steric hindrance from the phenyl ring, it still forms the addition product.

2. Phenyl hydrazine ($\ce{C6H5NHNH2}$)

Phenyl hydrazine is a derivative of hydrazine that reacts with carbonyl compounds to form phenylhydrazones ($\ce{C=N-NHC6H5}$). This is a classic condensation reaction (nucleophilic addition–elimination).

• Acetone: Forms acetone phenylhydrazone.
• Benzaldehyde: Forms benzaldehyde phenylhydrazone.

3. Fehling’s solution

Fehling’s solution is an oxidising agent — it contains $\ce{Cu^{2+}}$ ions complexed with tartrate in alkaline medium. It specifically oxidises aliphatic aldehydes to carboxylate salts, while the $\ce{Cu^{2+}}$ is reduced to $\ce{Cu+}$, giving a red precipitate of $\ce{Cu2O}$. …

Concept: Nucleophilic Addition to the Carbonyl Group

Both acetone (a ketone) and benzaldehyde (an aldehyde) contain a polar $\ce{C=O}$ bond. This makes them susceptible to nucleophilic addition reactions. The reagent that will not react with either must lack the ability to act as a nucleophile toward the carbonyl carbon under normal conditions.

Method: Reagent Functionality Analysis

Step 1: Identify the reactive site in each reagent

• Sodium hydrogen sulphite ($\ce{NaHSO3}$): The bisulphite ion ($\ce{HSO3-}$) is a nucleophile. It adds to the carbonyl carbon of both aldehydes and ketones to form a crystalline bisulphite addition product.
• Phenyl hydrazine ($\ce{C6H5NHNH2}$): The terminal $\ce{-NH2}$ group is a strong nucleophile. It reacts with both aldehydes and ketones to form phenylhydrazones.
• Fehling's solution: Contains $\ce{Cu^{2+}}$ ions in alkaline tartrate. It is an oxidising agent, not a nucleophile. It oxidises only aldehydes (not ketones) to carboxylic acids, reducing $\ce{Cu^{2+}}$ to $\ce{Cu2O}$ (red precipitate).
• Grignard reagent ($\ce{RMgX}$): The carbon attached to Mg is strongly nucleophilic (carbanion character). It adds to the carbonyl carbon of both aldehydes and ketones.

Step 2: Find the reagent that does not react with both

• Sodium hydrogen sulphite — adds to both acetone and benzaldehyde (bisulphite addition product).
• Phenyl hydrazine — reacts with both (forms phenylhydrazones).
• Grignard reagent — adds to both carbonyls.
• Fehling's solution — an oxidising agent, not a nucleophile. It oxidises only aldehydes (benzaldehyde → gives red $\ce{Cu2O}$) and does not react with ketones (acetone). So it does not react with both — it is selective for the aldehyde only. …

Here is a breakdown of the common mistakes students make on this question, along with the correct reasoning to avoid them.

The Core Concept: Reactivity of the Carbonyl Group

Both acetone (a ketone) and benzaldehyde (an aldehyde) contain a carbonyl group ($\ce{C=O}$). However, they differ in reactivity due to the groups attached to that carbonyl.

• Benzaldehyde has a phenyl ring, which stabilizes the carbonyl carbon via resonance, making it less reactive towards nucleophilic addition than a typical aldehyde.
• Acetone has two electron-donating methyl groups, which make it less reactive than formaldehyde, but it still undergoes typical ketone reactions.

The key is to know which reagents react with both types of carbonyl compounds, and which react with only one (or neither).

Mistake #1: Assuming Fehling's Solution reacts with both

The Mistake: Students often remember that Fehling's solution is a test for aldehydes and assume it will react with benzaldehyde (an aldehyde) and also with acetone (a ketone) because both have a carbonyl group.

Why it's wrong: Fehling's solution (containing $\ce{Cu^{2+}}$ in a basic tartrate complex) is an oxidizing agent. It only oxidizes aliphatic aldehydes. It does not oxidize ketones (like acetone) or aromatic aldehydes (like benzaldehyde). Benzaldehyde is an aromatic aldehyde and is not oxidized by Fehling's solution.

How to avoid: Memorize the specific limitations of common tests.

• Fehling's test: Positive for aliphatic aldehydes only. Negative for ketones and aromatic aldehydes.
• Tollens' test: Positive for all aldehydes (aliphatic and aromatic). Negative for ketones.

Since Fehling's solution reacts with neither acetone nor benzaldehyde, it is the correct answer to the question.

Mistake #2: Forgetting that Phenylhydrazine reacts with both

The Mistake: Students might think phenylhydrazine is too bulky to react with a hindered ketone like acetone, or that it only reacts with aldehydes.

Why it's wrong: Phenylhydrazine ($\ce{C6H5NHNH2}$) is a nucleophile that attacks the electrophilic carbonyl carbon. It undergoes a condensation reaction (forming a hydrazone) with both aldehydes and ketones. Acetone and benzaldehyde both react readily with it.

How to avoid: Remember that hydrazine derivatives (phenylhydrazine, 2,4-dinitrophenylhydrazine) are general reagents for all carbonyl compounds. They are used to test for the presence of any aldehyde or ketone.

Mistake #3: Confusing Sodium Hydrogen Sulphite's reactivity

The Mistake: Students might think sodium hydrogen sulphite ($\ce{NaHSO3}$) only reacts with aldehydes, or that it doesn't react with aromatic aldehydes.

Why it's wrong: Sodium hydrogen sulphite adds to the carbonyl group of both aldehydes and many ketones (including acetone) to form a crystalline bisulphite addition product. Benzaldehyde also reacts, though the equilibrium is less favorable than with aliphatic aldehydes. The reaction is still observable. …