Chapter 8: Nucleophilic Substitution

Reaction of Alcohols with Hydrogen Halides
(review of chapter 4)

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.

SN1 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 carbcation creates the alkyl bromide.

© Dr. Ian Hunt, Department of Chemistry