Final answer:
The mechanism for converting secondary alcohols to alkyl halides using SOCl₂ generally results in retention of stereochemistry due to the formation of a sulfite intermediate, different from elimination reactions which do not involve substitution of the hydroxyl group.
Step-by-step explanation:
The stereochemistry of alkyl halide formation from secondary alcohols using thionyl chloride (SOCl₂) generally results in retention of stereochemistry. This transformation does not proceed via the formation of a planar carbocation intermediate as seen in typical SN1 reactions, which would lead to racemization. Instead, the reaction likely proceeds through an inversion of configuration at the carbon bearing the hydroxyl group. However, under certain conditions with SOCl₂, it is possible to observe retention due to the formation of a chlorosulfite intermediate where the chloride ion displaces the sulfite group, thus retaining the original configuration.
Comparatively, in an elimination reaction like the dehydration of alcohols to form alkenes, the reaction involves protonation of the hydroxyl group and subsequent loss of water to form a carbocation. This carbocation then undergoes elimination to form a double bond. In this case, there is no substitution of the hydroxyl group, rather, it's an elimination of water. As a result, there is no issue of retention of stereochemistry.
The SN1 mechanism refers to a substitution reaction where the leaving group departs before the nucleophilic attack occurs, resulting in a mixture of enantiomers in the product. For secondary alcohols to undergo this type of reaction with SOCl2 (thionyl chloride), the alcohol is first reacted with sulfuric acid (H2SO4) to form an alkyl chloride. The sulfuric acid protonates the alcohol, making it a better leaving group. Then, the alkyl chloride reacts with thionyl chloride to form an alkyl chloride and SO2 gas. The stereochemistry is retained in this reaction because the nucleophilic attack occurs after the leaving group has departed.