Problem 67 - A Complete Resource Book in Chemistry for JEE Main

Question

Benzoyl chloride is prepared from benzoic acid by (a) \(\mathrm{Cl}_{2}, \mathrm{hv}\) (b) \(\mathrm{SO}_{2} \mathrm{Cl}_{2}\) (c) \(\mathrm{SOCl}_{2}\) (d) \(\mathrm{Cl}_{2}, \mathrm{H}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

Use SOCl2 to convert benzoic acid to benzoyl chloride.

Step 1: Identify the Target Conversion

The goal is to convert benzoic acid to benzoyl chloride. Recognize that benzoic acid (C6H5COOH) needs to be converted to benzoyl chloride (C6H5COCl), which involves replacing the hydroxyl group (-OH) in benzoic acid with a chlorine atom (Cl).

Step 2: Understand Possible Reaction Mechanisms

Investigate the typical reactions for converting carboxylic acids to acyl chlorides. Acyl chlorides can be synthesized using reagents like thionyl chloride (SOCl2) or phosphorus pentachloride (which is not an option here but provides similar chemistry to SOCl2).

Step 3: Analyze Each Reagent

Evaluate each reagent: (a) Chlorine gas with light (hv) is commonly used for chlorination but not for converting carboxylic acids to acyl chlorides. (b) SO2Cl2 is generally used for chlorination of hydrocarbons, not acylation. (c) SOCl2 reacts with carboxylic acids to produce acyl chlorides effectively, releasing SO2 and HCl as byproducts. (d) Chlorine and water could form hydrochloric acid but don't facilitate the conversion to benzoyl chloride specifically.

Step 4: Choose the Correct Reagent

Based on analysis in Step 3, the most suitable reagent for converting benzoic acid (C6H5COOH) to benzoyl chloride (C6H5COCl) is thionyl chloride (SOCl2), as this reagent specifically facilitates the transformation from -OH to -Cl in carboxylic acids.

Key Concepts

Benzoic Acid Conversion

When it comes to chemical transformations, converting benzoic acid into benzoyl chloride is a process involving precise changes at the molecular level. The starting compound, benzoic acid (C₆H₅COOH), contains a carboxyl group with an -OH attached. To transform benzoic acid into benzoyl chloride (C₆H₅COCl), you must replace this -OH group with a chlorine atom.
This change is not just a simple substitution but involves breaking and forming covalent bonds, which require specific chemical conditions and reagents. The choice of reagent is crucial because it dictates the efficiency and effectiveness of the conversion. Understanding this conversion lays the foundation for comprehending more complex organic syntheses and related transformations in organic chemistry.

Thionyl Chloride Reaction

Thionyl chloride (SOCl₂) is a highly effective reagent frequently used in the conversion of carboxylic acids into acyl chlorides. It operates through a mechanism that enables the replacement of the hydroxyl group in acids with a chlorine atom, thus forming an acyl chloride.
During the reaction between benzoic acid and thionyl chloride, the following occurs:

• The hydroxyl group from the acid is replaced by a chlorine atom.
• The reaction proceeds with the release of two by-products—sulfur dioxide (SO₂) and hydrochloric acid (HCl).

This reaction is preferred in laboratory settings because it is straightforward and yields minimal side-products, which simplifies purification processes. Furthermore, it is rapid and generally runs at room temperature, making it both practical and efficient for synthetic applications.

Acyl Chlorides Formation

The formation of acyl chlorides is a fundamental reaction in organic chemistry, as these compounds serve as pivotal intermediates in the synthesis of various derivatives. Acyl chlorides, like benzoyl chloride, are characterized by their reactive nature, mainly due to the presence of the highly electrophilic carbonyl group linked to a chlorine atom. This setup predisposes acyl chlorides to participate in nucleophilic acyl substitution reactions.
Acyl chlorides' utility stems from their versatility in subsequent synthetic steps, where they can easily form:

• Esters through reactions with alcohols.
• Amides when reacted with primary or secondary amines.

Understanding the formation and reactivity of acyl chlorides is crucial for students and chemists who aim to create a variety of functional groups, enabling the synthesis of complex organic molecules in chemical laboratories across the globe.