Which alkanes cannot be prepared by Wurtz reaction.

Hint: We know that the Wurtz reaction is one of the reactions which produces simple higher alkane of the two equivalents derived from alkyl halide in presence of dry ether is a coupling reaction which produces alkane with an increased carbon along with sodium salt of alkyl halide.

Complete answer:
In the Wurtz reaction, alkyl halides are combined to give us a higher alkane and a metal halide compound. Here, the simplest alkane present, which has only one carbon atom, will not be formed through the Wurtz reaction. We know that Wurtz reaction is used in organic chemistry and it includes the conversion of alkyl halides to alkanes in presence of sodium metal. This reaction is called a coupling reaction. In this reaction, we take dry ether medium and in which sodium metal is reacted with two alkyl halides to give us a higher alkane and a sodium and halogen containing compound.
The alkyl anion nucleophile displaces the halide resulting in formation of a higher alkane. As we can see from the above mechanism, Wurtz reaction involves formation of higher alkanes, even if we take the simplest alkyl halide. As methane has only one carbon atom, it cannot be formed during the Wurtz reaction on the other hand ethane, butane and propane can be formed as they are higher alkanes.
Methane \[\left( C{{H}_{4}} \right)\] contains only one atom, therefore the Wurtz reaction, which needs two organic compound molecules, can't be ready. The Wurtz reaction will prepare symmetrical alkanes with Associate in Nursing equal range of carbon atoms. For the preparation of alkanes with odd numbers of carbon atoms owing to the formation of facet products, the Wurtz reaction isn't favored. Additionally, to heptane, as an example, by starting with \[1-\] bromopropane and \[1-\] bromobutane, dissolvent and hydrocarbon are the side products.
Alkyl halides are treated with metal in a very dry ethereal (moisture-free) solution for the process of upper alkanes during this reaction and also are used for the preparation of upper alkanes containing an excellent number of atoms of carbon. The Wurtz-Fittig reaction is taken into account the corresponding reaction involved with aryl halides.

Note:
Remember that the Wurtz reaction is preferred for the preparation of alkanes with even numbers of carbon atoms. It is not used for the preparation of alkanes with an odd number of carbon atoms due to the formation of side products. So only the alkanes with minimum required carbons can be formed from the Wurtz reaction.