Wurtz Reaction

Wurtz Reaction

An ethereal solution of an alkyl halide (preferably the bromide or iodide) is treated with sodium, e.g.,

R¹X + R²X + 2Na →  R¹ – R² + 2NaX

when we do not wish to specify a particular alkyl group, we use the symbol R. When we deal with two unspecified alkyl groups which may, or may not, be the same, we can indicate this by R¹ and R² : also, when dealing with compounds containing a halogen atom, and we do not wish to specify the halogen, we can indicate the presence of the unspecified halogen atom by means of X.

Consideration of the equation given above shows that in addition to the desired alkane R¹ – R², there will also be present the alkanes R¹ – R² and R² – R². Unsaturated hydrocarbons are also obtained. Obviously, then, the best yield of an alkane will be obtained when R¹ and R² are the same, i.e., when the alkane contains an even number of carbon atoms and is symmetrical. It has been found that the Wurts reaction gives good yields only for ‘even carbon’ alkanes of high molecular weight, and that the reaction generally fails with tertiary alkyl halides.

Sodium is used in the Wurts reaction. Other metals, however, in a finely divided state, may also be used, e.g., Ag, Cu.

Two mechanisms have been suggested for the Wurts reaction.

• The intermediate formation of an organo-metallic compound, e.g., the formation of n-butane from ethyl bromide.

C₂H₅ –Br +2Na^.  →  C₂H₅^- Na⁺ + NaBr

C₂H₅^- Na⁺  +  C₂H₅Br → C₂H₅ – C₂H₅ + NaBr

• The international formation of free redicals, e.g.,

C₂H₅ –Br +2Na^.  →  C₂H₅ + NaBr

C₂H₅  +  C₂H₅ → C₂H₅ – C₂H₅

One of the properties of free radicals is disproportionation, i.e., intermolecular hydrogenation, one molecule acquiring hydrogen at the expense of the other, e.g.,

C₂H₅  +  C₂H₅ → C₂H₆ – C₂H₄

This would account for the presence of ethane and ethylene in the products. According to Morton, ethane and ethylene may be produced as follows :

This is a bimolecular elimination mechanism. On the other hand, the mechanism of alkane formation is generally accepted as usually being a bimolecular nucleophilic substitution:

The free-radical mechanism, however, has been shown to operate in some cases (Bryce-Smith, 1963).