Which one of the following undergoes reaction with 50% sodium hydroxide solution to give the corresp

Q: Why doesn't butanal undergo this reaction?

Butanal has $\alpha$-hydrogen atoms. In the presence of a strong base like NaOH, the base abstracts the acidic $\alpha$-hydrogen to form an enolate ion, leading to Aldol condensation rather than the Cannizzaro reaction.

Q: What exactly is a disproportionation reaction?

A disproportionation reaction is a specific type of redox reaction where a single substance is simultaneously oxidized and reduced to form two different products. In this case, the aldehyde is reduced to an alcohol and oxidized to a carboxylic acid salt.

Q: Why do we need a concentrated (50%) NaOH solution?

A concentrated strong base is required to ensure the nucleophilic attack of the hydroxide ion on the carbonyl carbon, which is the first step of the Cannizzaro mechanism, rather than just acting as a base to abstract protons (which is favored in dilute conditions if $\alpha$-hydrogens were present).

Q: Can ketones undergo the Cannizzaro reaction?

Generally, no. Ketones do not undergo the Cannizzaro reaction because they lack the hydrogen atom attached to the carbonyl carbon, which is essential for the hydride transfer step in the reaction mechanism.

Q: What happens if we use formaldehyde instead of benzaldehyde?

Formaldehyde ($HCHO$) also lacks $\alpha$-hydrogens and will undergo the Cannizzaro reaction to produce methanol ($CH_3OH$) and sodium formate ($HCOONa$).

Question

Which one of the following undergoes reaction with 50% sodium hydroxide solution to give the corresponding alcohol and acid?

Options:

(A) Phenol

(B) Benzaldehyde

(C) Butanal

(D) Benzoic acid

Sigma Percentile · Accepted Answer

### The Magic of Disproportionation: Unraveling the Cannizzaro Reaction

Imagine you are a detective analyzing a chemical crime scene. You are given a mysterious compound 'A' that, when thrown into a vat of concentrated $50\%$ sodium hydroxide, magically splits its identity. Half of it transforms into an alcohol, and the other half morphs into a carboxylic acid salt. This isn't just any random chemical event; it is the signature of one of the most elegant transformations in organic chemistry.

### Decoding the Clues

The reaction profile we are looking at is a classic **disproportionation reaction**. Disproportionation occurs when a single reactant is simultaneously oxidized and reduced to form two different products. In the realm of organic chemistry, when an aldehyde is treated with a strong, concentrated base and yields an alcohol (the reduced product) and a carboxylic acid salt (the oxidized product), we are witnessing the famous **Cannizzaro reaction**.

### The Cannizzaro Condition

However, there is a strict bouncer at the door of the Cannizzaro club: **the aldehyde must have absolutely zero $\alpha$-hydrogens**.

Why is this so critical? The $\alpha$-carbon is the carbon atom directly attached to the carbonyl group. If there are hydrogen atoms attached to this $\alpha$-carbon, they are unusually acidic. A strong base like $NaOH$ will take the path of least resistance and simply pluck off that acidic proton, creating an enolate ion. This leads the reaction down a completely different path known as the **Aldol condensation**.

To force the base to attack the carbonyl carbon directly (which initiates the Cannizzaro mechanism), we must remove the temptation of $\alpha$-hydrogens entirely.

### Evaluating the Suspects

Let's line up our suspects from the options provided:

1.  **Phenol:** This is an aromatic alcohol, not an aldehyde. It will simply form sodium phenoxide with $NaOH$. It's out.
2.  **Butanal ($CH_3CH_2CH_2CHO$):** This is an aldehyde, but if we look at the carbon right next to the $CHO$ group, it has two plump $\alpha$-hydrogens. It will happily undergo Aldol condensation. It's out.
3.  **Benzoic acid:** This is already a carboxylic acid. It will just undergo a simple acid-base neutralization to form sodium benzoate. It's out.
4.  **Benzaldehyde ($C_6H_5CHO$):** Let's look closely at benzaldehyde. The carbonyl group is attached directly to the benzene ring. That specific carbon in the ring already has four bonds (two to the adjacent ring carbons, one double bond within the ring, and one to the carbonyl carbon). There is no room for a hydrogen atom. **It has zero $\alpha$-hydrogens!**

### The Final Verdict

Benzaldehyde fits the profile perfectly. When two molecules of benzaldehyde are treated with concentrated $NaOH$, the Cannizzaro reaction proceeds flawlessly:

$$2C_6H_5CHO + NaOH \rightarrow C_6H_5CH_2OH + C_6H_5COONa$$

One molecule is reduced to **benzyl alcohol** ($C_6H_5CH_2OH$), and the other is oxidized to **sodium benzoate** ($C_6H_5COONa$). The mystery is solved, and benzaldehyde is our culprit!

Animated Solution for Chemistry - Aldehydes and Ketones: Which one of the following undergoes reaction with 50% sodium hydroxide solution to give the corresponding alcohol and acid?

Select Answer:

Visualized Solution

$A + NaOH \to Alcohol + Acid$

$The Cannizzaro Reaction$

$Condition for Cannizzaro$

$Analyzing the Options$

$The Final Reaction$

$The Way Forward$

The Sigma Insight: Cannizzaro Reaction

The Magic of Disproportionation

Decoding the Clues

The Cannizzaro Condition

Evaluating the Suspects

The Final Verdict