3.5      describe the substitution reaction of methane with bromine to form bromomethane in the presence of UV light.

This reaction only occurs if there is ultraviolet (UV) light present (e.g., sunlight).  It does not occur in the dark.  It is called a substitution reaction because one of the H atoms on methane is swapped for a Br atom from the bromine (like substituting players in a rugby or hockey match).  The other product is hydrogen bromide (HBr), which is an acidic gas like hydrogen chloride (HCl).

The brown coloured bromine loses its colour as it reacts.  The products are colourless.  The equation for the reaction is shown below.

CH4(g) + Br2(l) → CH3Br (l) + HBr(g)

The picture shows a space-fill model of bromomethane. Can you identify the atoms?

Warning:         Don’t get this confused with the bromine water test to distinguish between alkanes and alkenes!