||Chapter 15: Alcohols, Diols
Reaction of Alcohols
with Hydrogen Halides
(review of Chapter 4)
Reaction type: Nucleophilic Substitution
(SN1 or SN2)
- When treated with HBr or HCl
alcohols typically undergo a nucleophilic substitution reaction to generate
an alkyl halide and water.
- Alcohol relative reactivity order
: 3o > 2o > 1o > methyl.
- Hydrogen halide reactivity order
: HI > HBr > HCl > HF (paralleling acidity order).
- Reaction usually proceeds via
an SN1 mechanism which proceeds
via a carbocation intermediate, that can
also undergo rearrangement.
- Methanol and primary alcohols
will proceed via an SN2 mechanism
since these have highly unfavourable carbocations.
- The reaction of alcohols with
HCl in the presence of ZnCl2 (catalyst) forms the basis of the
Lucas test for alcohols.
MECHANISM FOR REACTION OF ALCOHOLS WITH HBr
An acid/base reaction. Protonation of the alcoholic oxygen to make a
better leaving group. This step is very fast and reversible. The
lone pairs on the oxygen make it a Lewis base.
Cleavage of the C-O bond allows the loss of the good leaving
group, a neutral water molecule, to give a carbocation intermediate.
This is the rate determining step (bond breaking is endothermic)
Attack of the nucleophilic bromide ion on the electrophilic carbocation
creates the alkyl bromide.
||© Dr. Ian Hunt, Department of Chemistry, University