Chapter 8: Nucleophilic Substitution

Reaction of Alcohols with other Halogenating agents (SOCl₂, PX₃)
(review of chapter 4)

Reaction type: Nucleophilic Substitution (SN1 or SN2)

Summary

• Alcohols can also be converted to alkyl chlorides using thionyl chloride, SOCl₂, or phosphorous trichloride, PCl₃.
• Alkyl bromides can be prepared in a similar reaction using PBr₃.
• Used mostly for 1^o and 2^o ROH (via SN2 mechanism)
• In each case a base is used to "mop-up" the acidic by-product.
• Common bases are triethylamine, Et₃N, or pyridine, C₆H₅N.
• In each case the -OH reacts first as a nucleophile, attacking the electrophilic center of the halogenating agent.
• A displaced halide ion then completes the substitution displacing the leaving group.
• Note that it is not -OH that leaves, but a much better leaving group (e.g. SO₂)
• The advantage of these reagents is in that the reaction is not under the strongly acidic conditions like using HCl or HBr.

• Preparation and Reaction of Tosylates
• Reaction of Alcohols with Hydrogen Halides