Wurtz reaction is used for preparation of alkanes. During this reaction, an alkyl halide with half the number of carbon atoms than the desired alkane is made to react with sodium metal in acetone. This leads to the formation of the desired alkane.
e.g., If the desired alkane is ethane, methyl iodide is taken.
H3C−I+2Na+I−CH3 →

H3C−CH3

Ethane

+2NaI
While this method is highly successful for producing alkanes with even number of carbon atoms but it gives a mixture of alkanes when odd numbered alkanes are to be formed.
This happens because two different alkyl halides not only react with each other but also with themselves.
This can be better illustrated with an example - propane is prepared by taking methyl iodide and ethyl iodide.
H3C−I+2Na+I−C2H5

acetone

→

CH3−CH2−CH3

Propane

+2NaI
The side reactions that take place in the reaction mixture are :
H3C−I+2Na+I−CH3

acetone

→

C2H5−C2H5

Butane

+2NaI
Thus, one gets a mixture of propane, ethane and butane which need to be separated in order to get the desired product.
That is why Wurtz reaction is not preferred for preparation of alkanes containing odd number of carbon atoms.