Institution: বঙ্গবন্ধু শেখ মুজিবুর রহমান বিজ্ঞান ও প্রযুক্তি বিশ্ববিদ্যালয় (BSMRSTU) Exam: A ইউনিট || বঙ্গবন্ধু শেখ মুজিবুর রহমান বিজ্ঞান ও প্রযুক্তি বিশ্ববিদ্যালয় || 2011
Topic: Chemistry (রসায়ন), Subject: General Science (সাধারণ বিজ্ঞান)

Best Answer Explained in Below-:

The question revolves around the reaction of benzoic acid (C6H5COOH) with phosphorus pentachloride (PCl5). Phosphorus pentachloride is a potent chlorinating agent known to convert carboxylic acids into acyl chlorides.

When benzoic acid reacts with PCl5, the hydroxyl group (-OH) of the carboxylic acid is replaced by a chlorine atom from PCl5, forming an acyl chloride (specifically, benzoyl chloride). The overall reaction can be summarized as follows:

\[
C_6H_5COOH + PCl_5 \rightarrow C_6H_5COCl + POCl_3 + HCl
\]

Here, C6H5COCl is benzoyl chloride, which corresponds to option A (C6H5COCl) in the provided choices.

The other options can be dismissed for the following reasons:
- Option B (ClC6H4COOH) suggests a chlorinated benzoic acid, which is not a product of the reaction.
- Option C (ClC6COOH) does not represent a valid acyl chloride from benzoic acid.
- Option D (C6H5CH2Cl) would imply a different type of chlorination involving the benzyl position, not the formation of an acyl chloride.

Thus, the correct answer is A) C6H5COCl, confirming that the reaction yields benzoyl chloride from benzoic acid and phosphorus pentachloride.