24.3 Photochemical Bromination of an Alkane

Chemical Concepts Demonstrated: light-catalyzed reactions, thermolytic and photolytic reactions, free-radical reactions, reactivity of alkanes


  • The two crystallizing dishes contain pentane and pentene and are placed on the overhead projector.
  • Bromine in dichloromethane is added to both of the dishes.


    The pentene changes to colorless very quickly.  The pentane changes after exposure to the overhead's light.

Explanation (including important chemical equations):

    The light-catalyzed reaction between an alkane and bromine can be stated as follows:

    Chain initiation:        Br2 + hv --> 2 Br.

    Chain propogation:  Br. + R-H --> R-Br + H.

                                   H. + Br2 --> H-Br + Br.

    Chain termination:    2 Br. --> Br2

                                   2 H. --> H2

                                   H. + Br. --> HBr