NCEA Level 3 Organic Chemistry Reaction Schemes Practice Test

Replacing the hydroxyl group on an alcohol with chloride is how you make a haloalkane. The reagents thionyl chloride (SOCl2), phosphorus trichloride (PCl3), or phosphorus pentachloride (PCl5) do this effectively by turning the poor leaving group OH into a much better leaving group and supplying chloride to complete the substitution. Mechanistically, SOCl2 reacts with ROH to form a chlorosulfite intermediate, which then undergoes chloride attack to give RCl while SO2 and HCl are expelled. With PCl3 or PCl5, the alcohol is converted to ROCl (or undergoes a sequence that ultimately yields RCl), with byproducts such as HCl and phosphate species. In all cases, chloride becomes the nucleophile that displaces the improved leaving group, giving the haloalkane. Performing the reaction under reflux provides the necessary energy to drive the substitution to completion and helps manage the evolved gases or byproducts, ensuring the process proceeds efficiently.