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) \(\mathrm{Cl}_{2}, \mathrm{H}_{2} \mathrm{O}\)

Short answer

(c) SOCl_2 is the correct reagent.

Step-by-step solution

Understand the Problem

We need to identify the chemical reagent that can convert benzoic acid into benzoyl chloride. Benzoyl chloride is an acyl chloride and typically formed from the corresponding carboxylic acid through a chlorination process.

Review Chemical Reagents

Four different methods are provided: (a) Chlorine and light (Cl_2, hv), (b) Sulfuryl chloride (SO_2Cl_2), (c) Thionyl chloride (SOCl_2), (d) Chlorine and water (Cl_2, H_2O). We need to determine which reagent is suitable for converting a carboxylic acid into an acyl chloride.

Assess Each Reagent for Acyl Chloride Formation

(a) Cl_2, hv is used for halogenation reactions and not suitable for acyl chloride formation. (b) SO_2Cl_2 is more often used for chlorine substitutions in arenes rather than forming acyl chlorides from carboxylic acids. (c) Thionyl chloride (SOCl_2) is a primary reagent for converting carboxylic acids to acyl chlorides. (d) Cl_2, H_2O is not appropriate for acyl chloride formation as it leads to oxidation or chlorination.

Select the Correct Reagent

From the analysis, SOCl_2 (thionyl chloride) is the reagent that is commonly used to convert carboxylic acids into acyl chlorides by replacing the hydroxyl group with a chlorine atom.

Key concepts

Acyl Chloride Formation

Acyl chlorides are one of the most reactive derivatives of carboxylic acids and play a vital role in organic synthesis. The conversion of a carboxylic acid into an acyl chloride involves replacing the hydroxyl group (-OH) in the carboxylic acid with a chlorine atom. This transformation is essential because acyl chlorides serve as intermediates in the synthesis of other functional groups, like esters and amides.

The process hinges on the use of a chlorinating agent, which introduces the chlorine atom into the molecule. Given the high reactivity of acyl chlorides, they can readily participate in nucleophilic acyl substitution reactions. To ensure the successful formation of the acyl chloride, the choice of chlorinating agent is crucial. The agent must be strong enough to facilitate the replacement but not so strong that it leads to unwanted side reactions. In the context of benzoyl chloride preparation, thionyl chloride ( SOCl_2 ) excels at this task, effectively yielding the desired acyl chloride from benzoic acid.

Thionyl Chloride

Thionyl chloride, denoted as ( SOCl_2 ), is a widely used reagent in organic chemistry, especially known for its effectiveness in converting carboxylic acids to acyl chlorides. The reaction with thionyl chloride is not only efficient but also convenient, as it typically produces gaseous byproducts—sulfur dioxide ( SO_2 ) and hydrogen chloride ( HCl )—which are easily removed from the reaction mixture.

Here's how the reaction unfolds:

• The carboxylic acid, like benzoic acid, reacts with thionyl chloride.
• The hydroxyl group in the acid is replaced by a chlorine atom from thionyl chloride.
• The byproducts ( SO_2 ) and ( HCl ) are released as gases, simplifying the purification of the benzoyl chloride product.

Thionyl chloride's reliability and gaseous byproducts that escape the reaction environment make it an ideal choice for preparing acyl chlorides, especially on a laboratory scale.

It is crucial, however, to conduct this reaction in a well-ventilated area due to the release of ( HCl ) and ( SO_2 ), both of which are corrosive and harmful if inhaled. Proper safety precautions, such as using a fume hood, should always be observed.

Benzoic Acid Conversion

Benzoic acid is a simple aromatic carboxylic acid, and its conversion to benzoyl chloride offers many synthetic possibilities. This conversion is key for creating benzoyl derivatives that can further react to form a variety of compounds.

To achieve this transformation, benzoic acid is treated with a chlorinating agent like thionyl chloride ( SOCl_2 ), triggering a substitution reaction where the -OH group is replaced by a chlorine atom. The resulting benzoyl chloride is a versatile chemical, which can be used to:

• Create esters by reacting with alcohols.
• Synthesize amides when combined with amines.
• Participate in various organic synthesis paths, opening the door to more complex chemical architectures.

Each of these reactions leverages the reactive acyl chloride group, enabling the formation of a vast array of organic compounds. Thus, understanding this conversion process is not only a cornerstone topic in organic chemistry but also quintessential for those looking to explore the extensive field of chemical synthesis.