||Chapter 8: Nucleophilic Substitution
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
Hydrogen halide reactivity order : HI > HBr > HCl > HF (paralleling acidity
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.
| SN1 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 carbcation
creates the alkyl bromide.