Problem 200 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following undergoes reaction with \(50 \%\) sodium hydroxide solution to give the corresponding alcohol and acid? (a) phenol (b) benzaldehyde (c) butanal (d) benzoic acid

Step-by-Step Solution

Answer

Benzaldehyde (b) undergoes the Cannizzaro reaction with 50% NaOH to give alcohol and acid.

Step 1: Understanding the Chemical Entities

First, identify and understand each of the chemical entities listed in the options: (a) phenol is an aromatic alcohol; (b) benzaldehyde is an aromatic aldehyde; (c) butanal is an aliphatic aldehyde; (d) benzoic acid is an aromatic carboxylic acid.

Step 2: Recalling Cannizzaro Reaction

The Cannizzaro reaction is a chemical reaction where an aldehyde having no alpha-hydrogen undergoes disproportionation in the presence of a strong base like sodium hydroxide to form an alcohol and a carboxylic acid. Both benzaldehyde and butanal are aldehydes, but benzaldehyde has no alpha-hydrogen, making it suitable for Cannizzaro reaction.

Step 3: Analyzing the Reaction Suitability

Evaluate each option: Phenol and benzoic acid cannot undergo Cannizzaro reaction because they are not aldehydes. Butanal does not undergo Cannizzaro reaction because it has alpha-hydrogens, making it undergo aldol condensation instead. Benzaldehyde can undergo Cannizzaro reaction because it lacks alpha-hydrogens.

Step 4: Determining the Correct Option

The correct option that undergoes a reaction with 50% sodium hydroxide solution to give the corresponding alcohol and acid is benzaldehyde (b) via the Cannizzaro reaction.

Key Concepts

Aldehydes Without Alpha-Hydrogens

Aldehydes come in different types, but one classification involves the presence or absence of alpha-hydrogens. Alpha-hydrogens are the hydrogen atoms that are attached to the carbon atom directly adjacent to the aldehyde group. When we talk about aldehydes without alpha-hydrogens, we're referring to compounds like benzaldehyde, where this characteristic is absent. These aldehydes are unique because they do not have the hydrogen necessary for certain reactions, such as aldol condensation. Instead, they can react in other ways. When an aldehyde lacks alpha-hydrogens, it often means it is more likely to undergo reactions such as the Cannizzaro reaction. This lack of alpha-hydrogens prevents it from forming enolates or engaging in self-condensation, leading instead to disproportionation when in the presence of a strong base.

Disproportionation Reaction

A disproportionation reaction is a type of redox reaction where one molecule is simultaneously oxidized and reduced, resulting in two different products. In the context of aldehydes with no alpha-hydrogens, like benzaldehyde, disproportionation is a key reaction involving these compounds. During the Cannizzaro reaction, which is a classic example of disproportionation, two molecules of an aldehyde without alpha-hydrogens react in the presence of a strong base, such as sodium hydroxide. One aldehyde molecule is reduced to an alcohol, and the other is oxidized to a carboxylic acid. This process is significant because it produces two valuable products from one reactant without the need for additional reagents or numerous reaction steps. It showcases how disproportionation can occur under the right conditions, with benzaldehyde serving as the perfect candidate due to its structural properties.

Strong Base Reactions

Strong bases, like sodium hydroxide, are pivotal in driving certain types of reactions, especially in organic chemistry. In the Cannizzaro reaction, a strong base provides the environment necessary for the disproportionation of aldehyde molecules without alpha-hydrogens. When benzaldehyde, lacking alpha-hydrogens, is treated with a 50% sodium hydroxide solution, the base facilitates the transfer of a hydride ion from one aldehyde molecule to another. This hydride transfer is what enables the simultaneous oxidation of one molecule into a carboxylic acid and the reduction of another into an alcohol. The role of the strong base is crucial—it supplies the hydroxide ions that attack the carbonyl group of the aldehyde, initiating the disproportionation process. Without a strong base like NaOH, the unique redox reaction that characterizes the Cannizzaro mechanism wouldn't occur as efficiently, demonstrating the importance of the reaction conditions.