Wurtz reaction is limited for the synthesis of symmetrical alkanes (alkanes having an even number of carbon atoms) In the wurtz reaction, two similar alkyl halides are taken as reactants and an alkane, containing double the number of carbon atoms, are formed as the product, as shown in the reaction below : -

Wurtz reaction cannot be used for the preparation of unsymmetrical alkanes because if two dissimilar alkyl halides are taken as the reacting species, then a mixture of alkanes is obtained as the products. Since the reaction involves free radical species, a side reaction also occurs to produce an alkene. This could be easily understood by the the reaction of bromomethane and iodoethane giving a mixture of alkanes as shown below : -

The alkanes that are obtained in the mixture have very close boiling point making their separation rather more difficult.

Hence, Wurtz reaction is not preferred for the preparation of alkanes containing odd number of carbon atoms.