24.3 Photochemical Bromination of an Alkane

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

Demonstration:

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.

Observations:

    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