When a mixture of $n$ -butyl bromide and ethyl bromide is treated with sodium metal in the presence of dry ether, then ethane cannot be formed because reaction follows Wurtz-Fittig reaction. It is a type of coupling reaction.
$\underset{\text{Octane}}{C _{4} H _{9}- C _{4}} H _{9}+ \underset{\text{Hexane}}{C _{4} H _{9}- C _{2}} H _{5}+ \underset{\text{Butane}}{C _{2} H _{5}- C _{2}} H _{5}$