Question

The following compound which cannot be prepared by Wurtz reaction is:
[A] Methane
[B] Ethane
[C] Butane
[D] Propane

Answer 1

Hint:
In 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 Wurtz reaction and will be the correct answer.

Complete step by step answer:
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.
We can write the reaction as-
     \[2R-X+2Na\to R-R+2N{{a}^{+}}{{X}^{-}}\]
The mechanism of the reaction includes a radical species which is involved in the halogen-metal exchange with the carbon-carbon bond forming. It follows a nucleophilic substitution pathway.
     \[R-X+M\to R\centerdot +{{M}^{+}}{{X}^{-}}\]
The metal atom transfers one of its electrons to the halide, forming a metal halide and the alkyl radical is left behind as shown in the reaction above.
     \[R\centerdot +M\to {{R}^{-}}{{M}^{+}}\]
The alkyl radical accepts an electron from another metal and forms an alkyl anion as we can see in the above reaction.
     \[{{R}^{-}}{{M}^{+}}+R-X\to R-R+{{M}^{+}}{{X}^{-}}\]
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 containing 2, 4 and 3 carbon atoms respectively.

Therefore, the correct answer is option [A] Methane.

Note:
The Wurtz reaction cannot prepare methane because it contains only one carbon atom. As Wurtz reaction includes addition of the alkyl groups forming a higher alkane, the formation of smallest alkane possible through Wurtz mechanism is ethane as it has at least two carbon atoms and it can be formed by heating methyl halides and sodium metal.