Problem 123 - A Complete Resource Book in Chemistry for JEE Main

Question

m-Chlorobenzaldehyde on reaction with conc. KOH at room temperature gives (a) potassium m-chlorobenzoate and \(\mathrm{m}\)-hydroxybenzaldehyde (b) m-hydroxybenzaldehyde and m-chlorobenzyl alcohol (c) m-chlorobenzyl alcohol and m-hydroxybenzyl alcohol (d) potassium m-chlorobenzoate and m-chlorobenzyl alcohol

Step-by-Step Solution

Answer

The answer is (d) potassium m-chlorobenzoate and m-chlorobenzyl alcohol.

Step 1: Recognize the Reaction Type

This reaction is a chemical conversion involving m-Chlorobenzaldehyde and concentrated KOH. The use of KOH indicates a possible Cannizzaro reaction, which typically occurs with aldehydes that do not have -hydrogen atoms, leading to the disproportionality of aldehydes into alcohols and acids.

Step 2: Analyze the Substrate Structure

Examine the structure of m-Chlorobenzaldehyde. It lacks -hydrogen atoms due to the presence of the aromatic ring attached to the aldehyde group. This makes it a candidate for the Cannizzaro reaction, where one molecule will oxidize while the other reduces.

Step 3: Determine Possible Products

In a Cannizzaro reaction, typically, one molecule of aldehyde is reduced to alcohol while the other is oxidized to carboxylic acid. If one considrs m-Chlorobenzaldehyde, one likely outcome is the formation of m-chlorobenzyl alcohol and potassium m-chlorobenzoate.

Step 4: Match the Products with Given Options

Based on the expected products from a Cannizzaro reaction, identify the option that reflects both the reduction and oxidation changes. The reduction product (alcohol) is m-chlorobenzyl alcohol, and the oxidation product (acid now as salt) is potassium m-chlorobenzoate, matching option (d).

Key Concepts

m-Chlorobenzaldehyde

m-Chlorobenzaldehyde is an interesting compound in the world of chemistry especially when discussing reactions involving aromatic aldehydes. This compound features a benzene ring (aromatic ring) with a chlorine atom (Cl) substituting the hydrogen atom at the meta-position (third carbon from the aldehyde group) and an aldehyde group (CHO).
This specific placement significantly influences its reactivity.

• Due to the absence of alpha-hydrogen atoms (hydrogens connected to the carbon adjacent to the carbonyl group), it undergoes a unique class of reactions known as Cannizzaro reactions.
• These reactions are known for occurring in certain aldehydes, typically those lacking these hydrogens — like m-Chlorobenzaldehyde.

The presence of the electronegative chlorine atom also serves to influence the electronic properties of the compound, pulling electron density through the benzene ring.
This is foundational in understanding its behavior in reactions.

KOH reaction

The Cannizzaro reaction with potassium hydroxide (KOH) is an important method for converting certain aldehydes, like m-Chlorobenzaldehyde, into alcohols and carboxylic acid salts.
KOH plays a critical role in facilitating this reaction:

• Being a strong base, KOH provides the hydroxide ions (OH-) necessary to start the reaction.
• These hydroxide ions attack the carbonyl carbon of the aldehyde, starting the transformation process.

In the Cannizzaro reaction, two molecules of aldehyde react differently: one of them is oxidized to an acid while the other is reduced to an alcohol.
For m-Chlorobenzaldehyde, the products include m-chlorobenzyl alcohol (the reduced form) and potassium m-chlorobenzoate (the salt form of the oxidized acid).
This showcases a rare display of simultaneous oxidation and reduction in a single reaction sequence.

Aromatic aldehydes

Aromatic aldehydes, such as m-Chlorobenzaldehyde, are unique due to their structure comprising a benzene ring with an aldehyde group attached.
The structural arrangement impacts the type of reactions they undergo. The aromatic ring tends to stabilize the molecule, affecting the reactivity of the aldehyde group in these compounds:

• Due to resonance, the electron cloud in the benzene ring may enhance or impede certain chemical reactions.
• For m-Chlorobenzaldehyde, the absence of alpha-hydrogen atoms makes it eligible for the Cannizzaro reaction, which is characteristic for such compounds without these specific hydrogens.

These compounds are crucial in synthetic organic chemistry due to their potential to form a variety of end products based on their reactivity.
The interaction of their aromatic nature coupled with the aldehyde functionality provides a pathway to a range of chemical transformations.

oxidation-reduction reactions

Oxidation-reduction reactions, commonly known as redox reactions, involve the transfer of electrons between two chemical substances.
In the context of the Cannizzaro reaction, these play a pivotal role. Here's how they work in the case of m-Chlorobenzaldehyde:

• One molecule of the aldehyde undergoes oxidation — losing electrons — to become a carboxylic acid (or its salt).
• Simultaneously, another molecule is reduced — gaining electrons — to form an alcohol.

The beauty of the Cannizzaro reaction lies in this simultaneous dual transformation within a single reaction medium.
This makes it an efficient way to convert certain aldehydes lacking alpha-hydrogens, like m-Chlorobenzaldehyde, into useful chemical products.
This kind of reaction is not just a fundamental chemical process but also a great learning example of balanced redox processes, showcasing how oxidation and reduction can occur concurrently and complementarily.