Question

Benzoyl chloride can be prepared from benzoic acid by

• A. \(SO_2Cl_2\)
• B. \(Cl_2/\text{Light}\)
• C. \(SOCl_2\)
• D. \(Cl_2+H_2O\)

Hint:

Carboxylic acid to acid chloride: \[ RCOOH \xrightarrow{SOCl_2} RCOCl \] Thionyl chloride is commonly preferred because gaseous by-products escape.

Answer 1

The Correct Option is C

Concept:
Acid chlorides are prepared from carboxylic acids by replacing the \(-OH\) group of \(-COOH\) with chlorine. The common reagent used for this conversion is thionyl chloride: \[ SOCl_2 \]

Step 1: Write the starting compound.
Benzoic acid is: \[ C_6H_5COOH \] Benzoyl chloride is: \[ C_6H_5COCl \] So the conversion is: \[ C_6H_5COOH \rightarrow C_6H_5COCl \]

Step 2: Use thionyl chloride.
The reaction is: \[ C_6H_5COOH+SOCl_2 \rightarrow C_6H_5COCl+SO_2+HCl \]

Step 3: Why \(SOCl_2\) is preferred.
The by-products are: \[ SO_2 \] and \[ HCl \] Both are gases and escape from the reaction mixture. This drives the reaction forward.

Step 4: Check the options.
Option (A) \(SO_2Cl_2\) is not the standard reagent for making acid chloride from acid.
Option (B) \(Cl_2/\text{Light}\) is used for free radical chlorination, not this conversion.
Option (C) \(SOCl_2\) is correct.
Option (D) \(Cl_2+H_2O\) does not convert benzoic acid to benzoyl chloride. Hence, the correct answer is: \[ \boxed{(C)\ SOCl_2} \]