Problem 101 - A Complete Resource Book in Chemistry for JEE Main

Question

The reaction of benzaldehdye with alkali gives (a) benzene + benzyl alcohol (b) phenol + benzene (c) benzene \(+\) benzyl alcohol (d) benzyl alcohol + sodium benzoate

Step-by-Step Solution

Answer

The reaction produces benzyl alcohol and sodium benzoate, so the correct choice is (d).

Step 1: Understand the Reaction

The reaction involving benzaldehyde and an alkali is known as the Cannizzaro reaction. In this reaction, an aldehyde without an alpha hydrogen undergoes self-oxidation and self-reduction in the presence of a strong base such as sodium hydroxide (NaOH).

Step 2: Identify the Reaction Mechanism

In the Cannizzaro reaction, one molecule of benzaldehyde is reduced to benzyl alcohol and the other molecule is oxidized to benzoic acid. Since a strong alkali is present, the benzoic acid forms a salt, such as sodium benzoate.

Step 3: Analyze the Products

The products of the Cannizzaro reaction with benzaldehyde and alkali thus are benzyl alcohol and sodium benzoate. These are formed because one molecule of benzaldehyde is converted to alcohol, and the other to a carboxylate salt.

Step 4: Match with the Answer Options

Comparing the products—benzyl alcohol and sodium benzoate—with the given options, we find that option (d) correctly describes the outcome of the Cannizzaro reaction under the provided conditions.

Key Concepts

Benzaldehyde

Benzaldehyde is a simple aromatic compound that consists of a benzene ring attached to an aldehyde functional group. It is often recognized by its almond-like aroma. This chemical plays a crucial role in organic reactions, one of which is the Cannizzaro reaction.

Benzaldehyde itself does not have an alpha hydrogen, which is necessary for a cross-aldol reaction. Instead, when it reacts with a strong base, like sodium hydroxide, it can undergo the Cannizzaro reaction.

• Structure: C₆H₅CHO
• Aroma: Almond-like
• Reactivity: Lacks alpha hydrogen

It's important to recognize how benzaldehyde's molecular structure makes it a perfect candidate for this unique reaction, leading to a dual transformation into both an alcohol and a carboxylic acid salt.

Benzyl Alcohol

Benzyl alcohol is a primary alcohol derived from benzaldehyde through a reduction process in the Cannizzaro reaction. It appears as a colorless liquid and possesses a mild pleasant aroma. Benzyl alcohol is commonly used in the food industry, cosmetics, and as a solvent.

In the context of the Cannizzaro reaction:

• Benzaldehyde is reduced to form benzyl alcohol.
• It involves the transfer of electrons that result in the gain of hydrogen atoms.
• This reduction process is crucial for the balance in the reaction.

Through this reaction, benzaldehyde is reduced on one side, countering the simultaneous oxidation happening with another benzaldehyde molecule.

Sodium Benzoate

Sodium benzoate is a salt formed when benzoic acid ions, produced from benzaldehyde oxidation, react with sodium ions from the base used in the Cannizzaro reaction. Sodium benzoate is notably used as a preservative in foods due to its ability to inhibit microbial growth.

In the Cannizzaro reaction:

• Oxidation of benzaldehyde gives benzoic acid.
• Benzoic acid reacts with sodium hydroxide to form sodium benzoate.
• Sodium benzoate has the chemical formula C₆H₅COONa.

This transformation showcases how the oxidation half of the reaction complements the formation of benzyl alcohol by providing a balanced chemical equation.

Oxidation and Reduction

Oxidation and reduction, together known as redox reactions, are chemical processes where oxidation refers to the loss of electrons, while reduction refers to the gain of electrons. In a typical redox reaction, these processes happen simultaneously.

In the Cannizzaro reaction:

• The oxidation process converts one benzaldehyde molecule into benzoic acid.
• Simultaneous reduction turns another benzaldehyde into benzyl alcohol.
• This creates a self-redox reaction where one element serves both to oxidize and reduce.

The Cannizzaro reaction perfectly exemplifies how an organic compound without alpha hydrogen can undergo redox reactions without external oxidizing/reducing agents, relying solely on the compound's intrinsic reactivity and the presence of a strong base.