Problem 22 - Chemistry in 30 days

Question

\(m\)-Chlorobenzaldehyde on reaction with conc. KOH at room temperature gives (a) Potassium \(m\)-chlorobenzene and \(m\)-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 correct choice is (d), potassium m-chlorobenzoate and m-chlorobenzyl alcohol.

Step 1: Identify the Reaction Type

The reaction involved here is the Cannizzaro reaction, which is typical for aromatic aldehydes with no hydrogen on the alpha carbon. It involves disproportionation.

Step 2: Reaction Mechanism

In the Cannizzaro reaction, two molecules of the aldehyde react with strong base (in this case, KOH), with one molecule being reduced to an alcohol and the other being oxidized to a salt of the corresponding acid.

Step 3: Analyze the Substituent Effect

Since the reaction is with KOH at room temperature, and considering it is a typical Cannizzaro reaction, the "m" (meta) position of the chloro substituent does not significantly impede the reaction.

Step 4: Predict the Products

Given that chloro substituent is ortho/para directing but inert in this scenario, m-chlorobenzaldehyde will yield m-chlorobenzyl alcohol (from reduction) and potassium m-chlorobenzoate (from oxidation).

Step 5: Finalize the Choice

Based on the predicted products, the correct answer is that the reaction forms potassium m-chlorobenzoate and m-chlorobenzyl alcohol, which corresponds to option (d).

Key Concepts

m-Chlorobenzaldehyde

m-Chlorobenzaldehyde is a derivative of benzaldehyde where a chlorine atom is attached at the meta position of the benzene ring.
This compound is an aromatic aldehyde, meaning it features both an aldehyde group (-CHO) and a benzene ring.
The presence of the chloro group in m-Chlorobenzaldehyde influences its chemical properties and reactions.

• The chlorine atom affects the distribution of electrons around the benzene ring.
• This impact generally does not hinder reactions, such as the Cannizzaro reaction, due to the nature of the substituent and reaction conditions.

This positions m-Chlorobenzaldehyde well for participating in these kinds of base-induced reactions at room temperature.

Reaction Mechanisms

A reaction mechanism provides a detailed pathway showing how reactants transform into products. In organic chemistry, understanding the mechanism is crucial for predicting outcomes.
The Cannizzaro reaction, employed by m-Chlorobenzaldehyde, specifically involves nucleophilic attack of a strong base (such as KOH) on the aldehyde.
Here's a brief overview of the Cannizzaro reaction mechanism:

• Initially, hydroxide ions from KOH attack one of the carbonyl carbon atoms in the aldehyde.
• This provokes a rearrangement that leads one aldehyde molecule to a carboxylic acid derivative, which is typically a salt under these conditions.
• Simultaneously, the other aldehyde molecule is reduced to form alcohol.

Effectively, two molecules of m-Chlorobenzaldehyde undergo disproportionation, generating both an alcohol and a salt.

Disproportionation

Disproportionation is a specific type of redox reaction where a single compound undergoes simultaneous oxidation and reduction to form different products.
The Cannizzaro reaction exemplifies disproportionation, especially with m-Chlorobenzaldehyde, where it splits into alcohol and acid salt.

• The original valence state of two aldehyde groups differs in the final products.
• In the case of m-Chlorobenzaldehyde, different processes turn them into the alcohol and the salt of a carboxylic acid.

Thus, disproportionation guides the dual-path production of products, ensuring efficiency in transforming starting materials.

Aromatic Aldehydes

Aromatic aldehydes are aldehyde compounds that include a benzene ring, providing unique properties and reaction capabilities.
These compounds, such as m-Chlorobenzaldehyde, are crucial in organic synthesis and reactions, including the Cannizzaro reaction.
Characteristics of aromatic aldehydes include:

• The aroma emitted due to their volatile nature.
• Sector-specific reactions due to substitutions on the benzene ring.

With these attributes, aromatic aldehydes form an essential groundwork in the synthesis of more complex molecules, often participating in key reactions that transform and enhance raw materials.