Chapter 8: Nucleophilic Substitution

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 (via S_N2 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.

SN2 MECHANISM FOR REACTION OF ALCOHOLS WITH SOCl2

Step 1: The oxygen of the alcohol reacts as a nucleophile on the electrophilic S atom (look at the electronegtive groups attached to the S).      Step 2: The added base removes the proton from the O.     Step 3: The intermediate collapses to reform the S=O and a chloride ion is lost as a leaving group.         Step 4: An SN2 reaction of the chloride ion reacts as a nucleophile on the electrophilic C from the alcohol to displace SO2 (the leaving group) and release another chloride ion while forming the alkyl chloride

Related Reactions

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

© Dr. Ian Hunt, Department of Chemistry