Addition reactions to alkenes

Alkenes react with hydrogen halides and halogens by addition.

This results in two parts of the reagent becoming bonded to the original doubly bonded carbons.

Thus dihaloalkanes in which the halogen atoms are on adjacent carbon atoms can be prepared by reaction of halogens with alkenes.

Show/hide Examples of Br₂ addtion

Reaction 1:

Reaction 2:

The position of the bromine atoms in the products depends on the position of the double bond in the reactant.

Monohaloalkanes can be prepared by reaction of alkenes with HX (HCl or HBr). This reaction can result in two products if the alkene is not symmetrical (same two groups attached to both doubly bonded carbon atoms).
 
Show/hide Examples of HCl addition
In reaction of HCl with symmetrical alkenes a single haloalkane is produced.

Reaction 3:

CH₃CH=CHCH₃

This alkene is symmetrical because both doubly bonded carbons are also bonded to CH₃ and H.
 

Reaction 4:

CH₃CH₂CH=CHCH₂CH₃

This alkene is symmetrical because both doubly bonded carbons are also bonded to CH₃CH₂ and H.