Benzoyl chloride is prepared from benzoic acid by:

A. Cl2 and hν

B. SO2Cl2

C. SOCl2

D. Cl2 and H2O

Benzoyl chloride, an important compound in organic chemistry, can be synthesized from benzoic acid through a specific reaction. The correct method for this transformation is option C: SOCl2, or thionyl chloride. Let’s break down why this is the right choice and explore the process in detail.

The Reaction Mechanism

When benzoic acid reacts with thionyl chloride, it undergoes a substitution reaction where the hydroxyl group (-OH) of the carboxylic acid is replaced by a chlorine atom. This reaction is favored because thionyl chloride is a good chlorinating agent.

Step-by-Step Process

• Activation of Benzoic Acid: The carboxylic acid group of benzoic acid is activated by the thionyl chloride. The sulfur atom in SOCl2 forms a bond with the oxygen of the -COOH group, leading to the formation of an intermediate.
• Formation of Benzoyl Chloride: As the reaction proceeds, the -OH group is eliminated as a molecule of sulfur dioxide (SO2) and hydrogen chloride (HCl) is released. This results in the formation of benzoyl chloride.

Why Other Options Are Not Suitable

Let’s briefly discuss why the other options do not lead to the formation of benzoyl chloride:

• A. Cl2 and hν: This option suggests using chlorine gas and light, which typically leads to free radical halogenation rather than the selective substitution needed to convert benzoic acid to benzoyl chloride.
• B. SO2Cl2: While this compound can also chlorinate, it is less commonly used for this specific transformation compared to SOCl2.
• D. Cl2 and H2O: This combination would not yield benzoyl chloride; instead, it could lead to hydrolysis or other unwanted reactions.

Applications of Benzoyl Chloride

Benzoyl chloride is a versatile reagent in organic synthesis. It is often used to produce benzoyl peroxide, which is a common ingredient in acne treatment products, and it serves as an acylating agent in various chemical reactions.

In summary, the preparation of benzoyl chloride from benzoic acid is efficiently achieved using thionyl chloride (SOCl2). This method is favored for its effectiveness and the clean reaction pathway it provides, making it a staple in organic synthesis.