Problem 124 - A Complete Resource Book in Chemistry for JEE Main

Question

A mixture of benzaldehyde and formaldehyde on heating with aqueous \(\mathrm{NaOH}\) solution gives (a) benzyl alcohol and sodium formate (b) sodium benzoate and methyl alcohol (c) sodium benzoate and sodium formate (d) benzyl alcohol and methyl alcohol

Step-by-Step Solution

Answer

The mixture of benzaldehyde and formaldehyde with \(\mathrm{NaOH}\) yields benzyl alcohol and sodium formate (option a).

Step 1: Understanding the Problem

We are given a mixture of benzaldehyde and formaldehyde and are asked about the products formed upon heating this mixture with aqueous \(\mathrm{NaOH}\). This is a classic example of the Cannizzaro reaction.

Step 2: Recognizing the Reaction Type

The Cannizzaro reaction is a redox reaction where one molecule of an aldehyde is oxidized to the corresponding carboxylic acid while another molecule of the aldehyde is reduced to an alcohol in the presence of a strong base like \(\mathrm{NaOH}\). However, formaldehyde reacts uniquely by being both oxidized and reduced.

Step 3: Analyzing the Behavior of Formaldehyde

Formaldehyde, when subjected to the Cannizzaro reaction, yields methanol and sodium formate since it can act in both oxidation and reduction processes.

Step 4: Analyzing the Behavior of Benzaldehyde

Benzaldehyde usually undergoes the Cannizzaro reaction when there's no \(\alpha\)-hydrogen, forming benzyl alcohol and sodium benzoate as the products.

Step 5: Identifying Product Formation

From the reaction, benzaldehyde leads to the production of benzyl alcohol and sodium formate (from formaldehyde), as formaldehyde prefers to participate by creating sodium formate and methanol.

Step 6: Concluding the Products

Thus, the reaction of benzaldehyde with formaldehyde in the presence of \(\mathrm{NaOH}\) primarily results in benzyl alcohol and sodium formate.

Key Concepts

Benzaldehyde

Benzaldehyde is an organic compound, commonly recognized for its characteristic almond-like fragrance. It is a type of aromatic aldehyde and serves as a fundamental building block in organic synthesis.
Benzaldehyde is unique because it lacks an alpha hydrogen, which makes it suitable for participating in the Cannizzaro reaction. When benzaldehyde is treated with a strong base like sodium hydroxide (\( \mathrm{NaOH} \)), it undergoes a redox process. During this reaction, one molecule of benzaldehyde is oxidized to form a carboxylic acid (which can further become sodium benzoate), while another molecule is reduced to produce benzyl alcohol.
Benzaldehyde's absence of the alpha hydrogen is crucial since it prevents it from participating in aldol reactions, directing its reactivity towards the Cannizzaro reaction once oxidizing and reducing occurs due to the influence of \( \mathrm{NaOH} \).

Formaldehyde

Formaldehyde is the simplest form of aldehyde, with the chemical formula \( \mathrm{HCHO} \). It holds a special place in the Cannizzaro reaction due to its readiness to be simultaneously oxidized and reduced.
In the presence of a strong base such as \( \mathrm{NaOH} \), formaldehyde can behave in a dual manner. This dual reaction means that it can convert into both a formate salt and an alcohol, specifically sodium formate and methanol, respectively, during the Cannizzaro reaction.
Formaldehyde is integral to the reaction because its unique behavior allows it to participate fully, contributing to the formation of different products without needing additional aldehyde molecules with structural complexities. Thus, formaldehyde stands out among aldehydes for its versatility and efficiency in the Cannizzaro reaction.

Sodium Formate

Sodium formate is a byproduct formed during the Cannizzaro reaction of formaldehyde. This compound is the sodium salt of formic acid and appears prominently when formaldehyde undergoes oxidation in the presence of a strong base like \( \mathrm{NaOH} \).
In the Cannizzaro reaction, formaldehyde is uniquely capable of undergoing both oxidation and reduction. When oxidized, formaldehyde is transformed into formic acid, which subsequently forms sodium formate by reacting with the sodium ions from \( \mathrm{NaOH} \).
Sodium formate's formation is significant as it highlights the oxidative transformation element in the Cannizzaro reaction, providing an excellent demonstration of chemical change sequences that occur with simple aldehydes like formaldehyde.

Benzyl Alcohol

Benzyl alcohol is one of the primary products formed when benzaldehyde undergoes the Cannizzaro reaction. This simple aromatic alcohol appears when benzaldehyde is reduced during the reaction process.
The transformation into benzyl alcohol occurs because, in the presence of a strong base like \( \mathrm{NaOH} \), one benzaldehyde molecule acts as a reducing agent. This reduction leads to the conversion of an aldehyde group into an alcohol group. Benzyl alcohol, with its pleasant aroma and use as a common solvent in various applications, helps to illustrate the practical side of the Cannizzaro reaction.
The formation of benzyl alcohol in this particular setup underscores the reactivity of benzaldehyde in the absence of \( \alpha \)-hydrogens, making it a prime example of redox chemistry dynamics at work in organic synthesis.