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Chemistry LibreTexts

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.
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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:       Br+ hv --> 2 Br.

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

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

Chain termination:    2 Br--> Br2

                              2 H--> H2

                              H+ Br--> HBr

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