||Chapter 4: Alcohols and Alkyl Halides
Reaction of Alcohols with other Halogenating agents
Reaction type: Nucleophilic Substitution (usually 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
- Used mostly for 1o and 2o 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,
- 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
The oxygen of the alcohol reacts as a nucleophile on the electrophilic
S aotm (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