In elimination reactions the elements of a small molecule are "eliminated" from an organic molecule.
Elimination of H–OH (H2O) from alcohols or H–X (HBr or HCl) from haloalkanes gives rise to alkenes.
Elimination occurs on heating a haloalkane with a strong base dissolved in alcohol.
The double bond in the product is between the carbon bearing X (Br or Cl) in the reactant and an adjacent carbon that bore a hydrogen.
A single alkene results if
Two or three alkenes result if
the groups bonded to the carbon bearing X are not the same, the number of alkenes possible is equal to the number of different groups.
CH3CH2 and CH3 group
Two different groups
Two products (but-1-ene and but-2-ene)
Elimination reactions are the reverse of addition reactions. The product organic molecule is more unsaturated than the reactant organic molecule.
Elimination of H–OH (H2O) from alcohols or H–X (HBr or HCl) from haloalkanes gives rise to alkenes.
Elimination occurs on heating a haloalkane with a strong base dissolved in alcohol.
 | strong base (KOH in alcohol) |  + X– + H–OH |
The double bond in the product is between the carbon bearing X (Br or Cl) in the reactant and an adjacent carbon that bore a hydrogen.
A single alkene results if
has only one carbon bearing H adjacent to the carbon bearing X
CH3CH2CH2Br
One product (propene)
CH3CH2CH2Br
One product (propene)
two groups bonded to the carbon bearing X (Cl, Br) are the same.
Two CH3 groups
One product (propene).
Two CH3 groups
One product (propene).
Two or three alkenes result if
the groups bonded to the carbon bearing X are not the same, the number of alkenes possible is equal to the number of different groups.
CH3CH2 and CH3 group
Two different groups
Two products (but-1-ene and but-2-ene)