Ch4 : ROH + HX

Chapter 4: Alcohols and Alkyl Halides

Reaction of Alcohols with Hydrogen Halides

Reaction type: Nucleophilic Substitution (S_N1 or S_N2)

Summary

When treated with HBr or HCl alcohols typically undergo a nucleophilic substitution reaction to generate an alkyl halide and water.
Alcohol relative reactivity order : 3^o > 2^o > 1^o > methyl.
Hydrogen halide reactivity order : HI > HBr > HCl > HF (paralleling acidity order).
Reaction usually proceeds via an S_N1 mechanism which proceeds via a carbocation intermediate, that can also undergo rearrangement.
Methanol and primary alcohols will proceed via an S_N2 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.

S_N1 MECHANISM FOR REACTION OF ALCOHOLS WITH HBr

Step 1:
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.

Step 2:
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)

Step 3:
Attack of the nucleophilic bromide ion on the electrophilic carbocation creates the alkyl bromide.