See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Concept: When a secondary alcohol reacts with thionyl chloride (SOCl2) in the absence of a base (like pyridine), the reaction proceeds through an internal nucleophilic substitution mechanism called S_Ni (substitution nucleophilic internal). Step 1 - Formation of chlorosulfite ester: The OH group of the alcohol reacts with SOCl2 to form an alkyl chlorosulfite intermediate (R-O-S(=O)-Cl), releasing HCl. The configuration at the stereocenter is retained at this stage. Step 2 - S_Ni mechanism: The chlorosulfite ester undergoes decomposition via a cyclic transition state in which the chlorine atom (from the -O-SOCl group) is delivered to the same face (front-side) of the carbon from which the -OSOCl group departs, through a concerted intramolecular process. This results in retention of configuration at the stereocenter. Step 3 - Products: The reaction releases SO2 and regenerates HCl (or it was already released in step 1), giving the alkyl chloride with the same configuration as the starting alcohol. Observation from the image: Both the starting alcohol and the product chloride show H on a wedge (back) and OH/Cl on a wedge (front) at the same carbon, confirming retention of configuration. Why other options fail: (b) S_N2 would give inversion of configuration (Walden inversion), not retention. (c) S_E2 is an elimination mechanism, not substitution. (d) S_N1 would give a racemic mixture via a planar carbocation intermediate, not retention. The S_Ni mechanism uniquely explains retention of stereochemistry in the reaction of alcohols with SOCl2 in the absence of an external base. Therefore, the correct answer is A.