Problem 49 - A Complete Resource Book in Chemistry for JEE Main

Question

Benzoyl chloride is prepared from benzoic acid by (a) \(\mathrm{Cl}_{2}\), 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

Option (c) \(\text{SOCl}_2\) is used to prepare benzoyl chloride from benzoic acid.

Step 1: Identify Reactants and Process

Benzoyl chloride can be prepared from benzoic acid by replacing the -OH group of benzoic acid with a -Cl group. The common reagent for this conversion is a chlorinating agent that can effectively replace -OH with -Cl.

Step 2: Examine Reaction Options

Evaluate each of the provided options:(a) \(\text{Cl}_{2}, hv\) is used for radical chlorination, which is not suitable for this conversion.(b) \(\text{SO}_{2} \text{Cl}_{2}\) actually converts alcohols to chlorides but requires light or heat.(c) \(\text{SOCl}_{2}\) is typically used for converting carboxylic acids to acyl chlorides by directly replacing the hydroxyl group with a chlorine atom.(d) \(\text{Cl}_{2}, \text{H}_{2} \text{O}\) does not efficiently produce benzoyl chloride from benzoic acid.

Step 3: Select Correct Reagent

Based on the analysis of options, option \(\text{c: SOCl}_{2}\) is the correct choice since it is a standard reagent used to convert carboxylic acids to acyl chlorides, including benzoic acid to benzoyl chloride.

Key Concepts

Chlorinating Agents

Chlorinating agents are chemical compounds that contain chlorine and are capable of introducing chlorine into other substances. They are essential in various chemical processes, especially in organic synthesis. These agents work by replacing hydrogen or hydroxyl groups in compounds with chlorine.

Some common chlorinating agents include:

• Thionyl chloride (\( \text{SOCl}_2 \)): Widely used to convert carboxylic acids to acyl chlorides. It replaces the -OH group with a -Cl group through nucleophilic substitution.
• Phosphorus pentachloride (\( \text{PCl}_5 \)): Another agent that can achieve similar transformations, often used in laboratories for chlorination purposes.
• Sulfuryl chloride (\( \text{SO}_2 \text{Cl}_2 \)): Converts alcohols to chlorides but requires input of energy in the form of light or heat to facilitate the reaction.

While each agent has its unique properties and applications, the choice of a chlorinating agent depends on the specific chemistry involved, and the nature of the reactants.

Benzoic Acid

Benzoic acid is a simple aromatic carboxylic acid, often encountered as a white crystalline solid. Its structure is composed of a benzene ring attached to a carboxyl group (COOH).

The carboxylic group in benzoic acid makes it a prime candidate for various chemical reactions:

• Acid-base reactions: Benzoic acid can donate a hydrogen ion (H⁺) easily, making it acidic in nature.
• Esterification: Reacting with alcohols, benzoic acid forms esters and water, in a classic condensation reaction.

In the context of synthesizing benzoyl chloride, the carboxylic group is replaced by a chlorine atom through the use of a chlorinating agent like \( \text{SOCl}_2 \). This transformation effectively transforms benzoic acid into benzoyl chloride by substituting the hydroxyl group with chlorine.

Acyl Chlorides

Acyl chlorides, also known as acid chlorides, are a group of organic compounds derived from carboxylic acids. In these compounds, the hydroxyl group from the carboxyl group is replaced by a chloride ion.

Key characteristics of acyl chlorides include:

• Reactivity: Acyl chlorides are highly reactive due to the presence of the electronegative chlorine, which makes them excellent acylating agents.
• Applications: They are commonly used in the synthesis of esters, amides, and other derivatives.

When benzoic acid is converted to benzoyl chloride, the resulting compound is an acyl chloride. The process involves the reaction of the carboxylic acid with a chlorinating agent, allowing for facile substitution of -OH with -Cl. Benzoyl chloride plays a significant role in the field of organic synthesis, serving as a valuable intermediate for more complex chemical constructions.