Substitution and addition reactions with halogens

The alkenes are much more reactive than the alkanes by virtue of the double bonds present. The pi bond can react to form two new signui bonds, leaving the original C-C sigma bond intact. These double bonds make the molecules vulnerable to attack by certain reagents, e g. bromine, which can add across the double bond. 

The difference between alkanes and alkenes is the fact that the alkanes react in such a way that the hydrogen atoms are progressively substituted by other atoms, e.g. Q, and the reaction gives more than one product. The 

For simplicity we will look at the reaction of methane and chlorine. 

The reaction is quite gentle if carried out in diffused light. Notice how the number of H atoms goes down in the organic compound and the number of Cl atoms goes up until there is no more H to replace. Notice also that every time a hydrogen is replaced, a molecule of HCl is formed  

If bromine water is used (Br2/H20) the product also contains the colourless 

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