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Explanation

n-Butane is a symmetrical molecule and hence a pure sample of n-butane is obtained when ethyl iodide is heated with Na metal in dry ether.
`underset"Ethyl iodide "(CH_3CH_2I)+ 2Na + ICH_2CH_3 underset"Heat"overset"Dry ether"to underset"n-Butane"(CH_3CH_2CH_2CH_3)+ 2NaI`
But isobutane is an unsymmetrical molecule. To prepare isobutane we need two alkyl halides, i.e., methyl iodide and isopropyl iodide. Whenever a mixture of two alkyl halides is heated with Na metal , actually three alkanes are formed . This is due to the reason that two alkyl halides in addition to reacting with each other also react amongst themselves giving a mixture of three alkanes as shown below :
`underset"Isopropyl iodide "(CH_3-oversetoverset(CH_3)|CH-I)+2Na+underset"Methyl idodide"(I-CH_3) underset"Heat"overset"Dry ether"to underset"Isobutane"(CH_3-oversetoverset(CH_3)|CH-CH_3+2NaI`
`underset"Isopropyl iodide "(CH_3-oversetoverset(CH_3)|CH-I)+2Na+I-oversetoverset(CH_3)|CH-CH_3 underset"Heat"overset"Dry ether"to underset"2,3-Dimethylbutane"(CH_3-oversetoverset(CH_3)|CH-oversetoverset(CH_3)|CH-CH_3)+2NaI`
`underset"Methyl iodide"(CH_3-I)+2Na+I-CH_3 underset"Heat"overset"Dry ether "to underset"Ethane"(CH_3-CH_3)+2NaI`
Since a mixture of three alkanes is formed which is difficult to separate, therefore, a pure sample of isobutane cannot be prepared by Wurtz reaction .