||Chapter 8: Nucleophilic Substitution
Reaction of Alcohols with other Halogenating agents (SOCl2,
(review of chapter 4)
Reaction type: Nucleophilic Substitution (SN1 or SN2)
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 1o and 2o ROH (via SN2
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
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.
SN2 MECHANISM FOR REACTION
OF ALCOHOLS WITH SOCl2
The oxygen of the alcohol reacts as a nucleophile on the electrophilic
S atom (look at the electronegtive groups attached to the S).
The added base removes the proton from the O.
The intermediate collapses to reform the S=O and a chloride ion is lost
as a leaving group.
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