Why is Wurtz reaction not preferred for the preparation of alkanes containing an odd number of carbon atoms? Illustrate your answer by taking one example.

The Wurtz reaction is primarily used for the synthesis of symmetrical alkanes,which contain an even number of carbon atoms. In this reaction,two molecules of an alkyl halide react with $2Na$ in the presence of dry ether to form an alkane with double the number of carbon atoms.
Example: $CH_3-Br + 2Na + Br-CH_3 \xrightarrow{\text{Dry ether}} CH_3-CH_3 + 2NaBr$
It is not preferred for preparing alkanes with an odd number of carbon atoms because using two different alkyl halides (e.g.,$R-X$ and $R'-X$) results in a mixture of three different alkanes ($R-R$,$R'-R'$,and $R-R'$). Since these alkanes have similar boiling points,it is extremely difficult to separate them by fractional distillation.
For example,the reaction of $CH_3-I$ and $CH_3CH_2-I$ produces a mixture:
$CH_3-I + 2Na + I-CH_2CH_3 \xrightarrow{\text{Dry ether}} CH_3-CH_2CH_3 + 2NaI$
$CH_3-I + 2Na + I-CH_3 \xrightarrow{\text{Dry ether}} CH_3-CH_3 + 2NaI$
$CH_3CH_2-I + 2Na + I-CH_2CH_3 \xrightarrow{\text{Dry ether}} CH_3CH_2-CH_2CH_3 + 2NaI$