Chemistry - Halogen Derivative Question with Solution | TestHub

Question Explanation: Identify reactions that correctly depict the stereochemical outcome of nucleophilic substitution.

Concept: Stereochemistry in nucleophilic substitution ${\mathrm{SN}}^2,{\mathrm{SN}}_{\mathrm{i}}$ reactions. Solution:

Reactions 1 & 2: Alcohol → Alkyl Iodide ( $\mathrm{P}/{\mathrm{I}}_2$ )

The reaction of an alcohol ${\mathrm{PI}}_3$ to produce an alkyl iodide is an example of an ${\mathrm{SN}}^2$ reaction. ${\mathrm{SN}}_{}^2$ reactions proceed with complete inversion of configuration (Walden inversion) at the chiral center. In the reactant: OH and ${\mathrm{CH}}_3$ groups are shown trans to the ring, with -OH on the ${\mathrm{C}}_1$ atom and ${\mathrm{CH}}_3$ on the ${\mathrm{C}}_2$ atom. Correct product stereochemistry (Inversion): The -I group must be cis to the ${\mathrm{CH}}_3$ group on ${\mathrm{C}}_2$.

Option 1: Shows retention (trans ⟶ trans). (Incorrect)

Option 2: Shows inversion (trans → cis). (Correct)

Reactions 3 & 4: Alcohol → Alkyl Chloride ( $-{\mathrm{SOCl}}_2/$ Pyridine)

The reaction of an alcohol with thionyl chloride in the presence of pyridine is a classic method to prepare alkyl chlorides. This condition promotes an ${\mathrm{SN}}_{}^2$ mechanism on the reactive intermediate, leading to complete inversion of configuration.

Option 3: Shows retention (Retention of $\mathrm{R}\to \mathrm{R}$ or $\mathrm{S}\to \mathrm{S}$ configuration). (Incorrect)

Option 4: Shows inversion (Inversion of configuration, $\mathrm{R}\to \mathrm{S}$ or $\mathrm{S}\to \mathrm{R}$ ). (Correct) Final Answer: B, D