||Chapter 4: Alcohols and Alkyl Halides
Reaction of Alcohols
with Hydrogen Halides
Reaction type: Nucleophilic Substitution (SN1
- 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.
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.