Hydrocarbons and Bromine

Both alkanes and alkenes will react with bromine water (a bromine solution). However, there are stark differences between these reactions:

Why do we see a reaction with the alkene, but not with the alkane?

Alkanes - Substitution

The reaction between alkanes and bromine (or any other halogen such as chlorine or iodine) is called a substitution reaction. This is because a hydrogen atom is removed and replaced by a bromine atom. We call alkanes saturated because there is nowhere for any more atoms to be added until another atom is first removed.

This reaction requires UV light, and is very slow, which is why we did not see anything happen in the video (above). There is also a (gaseous) acidic product, which could be tested for as it turns damp blue litmus paper red:

NOTE: The same reaction occurs with chlorine, creating HCl vapour.

Alkenes - Addition

The reaction between alkenes and bromine is called an addition reaction. This is because no atoms are removed to accommodate the bromine atoms. Instead, the C=C (double) bond is broken. We call alkenes unsaturated because they can have other atoms added to them, without first having to remove another atom.

The reaction is fast, and does not require UV light. It also does not create any other products, so any fumes given off would have no effect upon damp blue litmus paper.

Alkane or Alkene?

As UV light is required for the alkane reaction to occur, and is much slower, this is a quick test to differentiate alkanes from alkenes. After adding bromine, place the test tube(s) in a dark cupboard for a few minutes. Only the alkene will have decolourised the bromine.