The reaction mechanism for the conversion of an alcohol to a halide using thionyl chloride SOCl2 involves the following steps:1. Nucleophilic attack of the oxygen atom in the alcohol on the sulfur atom in thionyl chloride: The oxygen atom in the alcohol acts as a nucleophile and attacks the electrophilic sulfur atom in SOCl2. This leads to the formation of a tetrahedral intermediate with the oxygen atom bonded to the sulfur atom and a chloride ion leaving.2. Elimination of a chloride ion: In the tetrahedral intermediate, one of the chloride ions leaves as a leaving group, resulting in the formation of a chlorosulfite intermediate RO-SOCl .3. Intramolecular nucleophilic attack by the chloride ion: The chloride ion that left in the previous step acts as a nucleophile and attacks the carbon atom bonded to the oxygen atom in the chlorosulfite intermediate. This leads to the cleavage of the carbon-oxygen bond and the formation of the alkyl halide R-Cl and a byproduct, sulfur dioxide SO2 .The stereochemistry of the product formed depends on the stereochemistry of the starting alcohol and the reaction conditions. If the starting alcohol is chiral and the reaction proceeds via an SN2 mechanism, the product will have an inverted stereochemistry at the chiral center. However, if the reaction proceeds via an SN1 mechanism, the product will be a racemic mixture due to the formation of a carbocation intermediate, which can be attacked by the nucleophile from either side. In general, primary and secondary alcohols tend to undergo SN2 reactions, while tertiary alcohols undergo SN1 reactions.