These are reactions where some atoms or groups of atoms are eliminated from the organic molecule. It is the inverse of addition reactions.
They are of great importance to the chemical industry in the production of polyethylene, which is the raw material for obtaining plastics. The main elimination reactions are:
- hydrogen elimination (dehydrogenation)
- Halogen elimination (de-halogenation)
- halide elimination
- elimination of water (alcohol dehydration)
From an alkane, it is possible to obtain a heat catalyzed alkene. Example:
Vicinal dihalides reacting with an alcohol catalyzed zinc form alkenes. Example:
Halides such as HCl, HBr and HI may be eliminated from a base-catalyzed alkyl halide, which may be KOH and an alcohol.
Intramolecular dehydration of alcohol catalyzed by concentrated sulfuric acid and heat (170 ° C) occurs with the elimination of water and alkene.
Another dehydration that can occur is the intermolecular dehydration of two alcohols, forming ether and eliminating water. The reaction should be catalyzed by concentrated sulfuric acid and heat (140 ° C).
1 molecule alcohol = intramolecular dehydration = alkene
2 molecules alcohol = intermolecular dehydration = ether