Chapter 5: Structure and Preparation of Alkenes. Elimination Reactions

Summary

• When heated with strong acids catalysts (most commonly H₂SO₄, H₃PO₄), alcohols typically undergo a 1,2-elimination reactions to generate an alkene and water.
• Also known as dehydration since it involves the removal of a molecule of water.
• Alcohol relative reactivity order : 3^o > 2^o > 1^o
• Regioselectivity : major product is usually the more highly substituted alkene (alkene stability) Zaitsev's Rule.
• Stereoselectivity : trans- > cis- again controlled by stability
• Reaction usually proceeds via an E1 mechanism which proceeds via a carbocation intermediate, that can often undergo rearrangement.
• Primary alcohols will proceed via an E2 mechanism since the primary carbocation is highly unfavourable.
• Other common strong acids such as HCl, HBr or HI are less suitable catalysts as nucleophilic substitution reactions will probably interfere.

• Dehydrohalogenation of Alkyl Halides
• Reaction of Alcohols with Hydrogen Halides

E1 MECHANISM FOR ALCOHOLS
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: An acid/base reaction. Deprotonation by a base (a water molecule) from a C atom adjacent to the carbocation center leads to the creation of the C=C

© Dr. Ian Hunt, Department of Chemistry