Chapter 15: Alcohols, Diols and Thiols

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

Reaction type: Nucleophilic Substitution (S_N1 or S_N2)

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
• 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.
• The advantage of these reagents is in that the reaction is not under the strongly acidic conditions like using HCl or HBr.

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

© Dr. Ian Hunt, Department of Chemistry