Wurtz reaction is limited for the synthesis of symmetrical alkanes (alkanes with an even number of carbon atoms) In the reaction, two similar alkyl halides are taken as reactants and an alkane, containing double the number of carbon atoms, are formed.
Example:

$C{H}_3-Br+2Na+Br—C{H}_3\stackrel{Dry ether}{\to }C{H}_3—C{H}_3+2Na Br$
$Bromomerthane Ethane$

Wurtz reaction cannot be used for the preparation of unsymmetrical alkanes because if two dissimilar alkyl halides are taken as the reactants, 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. For example, the reaction of bromomethane and iodoethane gives a mixture of alkanes. 

$$\begin{gathered} C{H}_3—I+2Na+I—C{H}_{2}C{H}_3 \\ Iodomethane \downarrow Dry ether \\ C{H}_3-C{H}_{2}C{H}_3+2Nal \\ C{H}_3-I+2Na+I-C{H}_3 \stackrel{Dry ether\hspace{0.17em}}{\to }2Nal+C{H}_2-C{H}_3 \\ ethane \\ C{H}_{3}C{H}_2-I+2Na+I-C{H}_{2}C{H}_3\stackrel{Dry ether}{\to }2NaI+C{H}_{2}C{H}_3-C{H}_{2}C{H}_3 \\ Bu\tan e \end{gathered}$$

The boiling points of alkanes (obtained in the mixture) are very close. Hence, it becomes difficult to separate them