Chapter 4: Alcohols and Alkyl Halides

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

Reaction type: Nucleophilic Substitution (usually 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 (i.e. SN2)
• In each case a base is used to "mop-up" the acidic by-product.
• Common bases are triethylamine, Et₃N, or pyridine, C₅H₅N.
• 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.
• The mechanism for the reaction of thionyl chloride with an alcohol is shown below:

SN2 MECHANISM FOR REACTION OF ALCOHOLS WITH SOCl2

Step 1: The oxygen of the alcohol reacts as a nucleophile on the electrophilic S aotm (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

© Dr. Ian Hunt, Department of Chemistry