Wurtz reaction or Wurtz coupling reaction is generally defined for alkyl halides, where alkyl halides are converted into a suitable alkane by nucleophilic substitution mechanism. The reaction generally involves metal with a solvent, especially a nonpolar one, so that reaction between metal and solvent does not occur. The solvent used in this reaction could be dry ether, Xylene, Dioxane, THF (Tetrahydrofuran), etc. This reaction is generally useful for the synthesis of an even number of carbon-containing compounds. However, an odd number of carbon-containing products also possible in this reaction but at the expense of the purity of the desired product. A mixture of products (alkane) is formed by taking two different alkyl halides in this reaction. The reaction also suggested the formation of C-C bonds and used only primary halides. Other halides such as secondary or tertiary halides undergo disproportionation or elimination reactions.
This reaction is also known as a coupling reaction because of the coupling of two alkyl halides. If the same alkyl halides are using for the synthesis of alkanes, the reaction is called as Wurtz homo-coupling reaction; otherwise, if different alkyl halides are using, the reaction is called Wurtz hetero-coupling reaction. Similarly, if an alkyl halide reacts with an aryl halide (benzene derivative), the reaction is termed as Wurtz-fittig reaction.
Mechanism of the reaction
It has to be noted that the above reaction mechanism follows doubtful mechanism in many research, some research suggested that the mechanism of the reaction follows the above mechanism or others suggested free radical mechanism.
However, the important point here is the formation of carbanion which acts as a nucleophile. Another important point is the rate of removing the halide ion from the target RX. As X = Cl, Br, I, therefore it is worth noting that the removal of halide ions depends on their size. Since I ion has the largest size from their respective elements, which ultimately weakened the bond of R-X and easily removed by the incoming nucleophile and follow SN2 mechanism.
Therefore the rate of removal of ions would be I > Br > Cl in this reaction.
Some important questions to understand the Wurtz reaction.
Limitations of Wurtz Reactions:
1. Solvent must be anhydrous and nonpolar for the reaction otherwise moisture reacts with the highly reactive metals.
2. Methane cannot be formed by this method.
3. Even number of carbon can be synthesized by this method. However, odd number of carbon-containing alkanes can also be synthesized. But the yield of the desired compound would be low due to side reactions.
4. Tertiary alkyl halides cannot be used in this reaction due to the steric effect which converts alkane into an alkene.
5. Alkenyl halides will not participate in the reaction due to less availability of anion (Carboanion) as a nucleophile.
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