Chlorobenzene is not formed in the reaction of phenol with thionyl chloride because the nucleophilic substitution reaction required to substitute the hydroxyl group in phenol with a chlorine atom does not readily occur. The reason for this is that the carbon-oxygen bond in phenol has partial double bond character due to resonance. This double bond character makes the C-O bond strong and less reactive towards nucleophilic substitution reactions, which typically involve the cleavage of a single C−O bond. Hence, the assertion (A) is correct. The reason (R) provided is also correct, and it is indeed the correct explanation for why phenol does not react in the proposed manner. The carbon-oxygen bond's partial double bond character reduces the tendency of the C-O bond to participate in substitution reactions required for chlorobenzene formation. Thus, the correct answer is: Option A: Both (A) and (R) are correct and (R) is the correct explanation of (A).
