Wurtz Coupling

In organic chemistry, the Wurtz reaction, named after Charles Adolphe Wurtz, is a coupling reaction whereby two alkyl halides are treated with sodium metal to form a higher alkane.
: 2 R−X + 2 Na → R−R + 2 NaX
The reaction is of little value except for intramolecular versions. A related reaction, which combines alkyl halides with aryl halides is called the Wurtz–Fittig reaction.

Mechanism

The reaction proceeds by an initial metal–halogen exchange, which involves this idealized stoichiometry:
: R−X + 2 M → RM + MX
This step involves the intermediacy of radical species R·. This step resembles the formation of a Grignard reagent. These RM intermediates have been isolated in several cases.

The organometallic intermediate next reacts with the alkyl halide forming a new carbon–carbon covalent bond.
: RM + RX → R−R + MX
The process resembles an S_N2 reaction, but the process is probably complex, which may explain the inefficiency of the reaction.

Examples and reaction conditions

The reaction is intolerant of a range of functional groups which would be attacked by sodium. For similar reasons, the reaction is conducted in unreactive solvents such as ethers. In efforts to improve the reaction, other metals have also been evaluated to effect the Wurtz-like couplings among them silver, zinc, iron, activated copper, indium and a mixture of manganese and copper chloride.

Wurtz coupling is useful in closing small, especially three-membered, rings. Bicyclobutane was prepared this way from 1-bromo-3-chlorocyclobutane in 95% yield. The reaction is conducted in refluxing dioxane, at which temperature, the sodium is liquid. This reaction has poor yield which is a consequence of multiple product formation. In the case of (1,3), (1,4), (1,5), (1,6) dihalides, it leads to formation of cyclic products. In vicinal dihalides, it forms alkenes, whereas in geminal dihalides, it forms alkynes.

:

Extensions to main group compounds

Although the Wurtz reaction is only of limited value in organic chemistry, analogous couplings are useful for coupling main group halides. Hexamethyldisilane arises efficiently by treatment of trimethylsilyl chloride with sodium:
:
Tetraphenyldiphosphine is prepared analogously:
:

Similar couplings have been applied to many main group halides. When applied to main group dihalides, rings and polymers result. Polysilanes are produced in this way
:

Limitations

The Wurtz reaction is seldom used because of side reactions.March Advanced Organic Chemistry 7th edition p. 512 It has limited use to the synthesis of symmetric alkanes. If two dissimilar alkyl halides are taken as reactants, then the product is a mixture of alkanes that is often difficult to separate by fractional distillation as the differences between the boiling points of the products are typically very low. Methane can not be obtained by this method. This type of reaction fails in case of tertiary halides. Also, since the reaction involves free radical species, a side reaction occurs to produce an alkene. This side reaction becomes more significant when the alkyl halides are bulky at the halogen-attached carbon atom.

See also

* Wurtz–Fittig reaction
* Ullmann reaction

Further reading

*
*
Organic-chemistry.org
*''Organic Chemistry'', by Morrison and Boyd
*''Organic Chemistry'', by Graham Solomons and Craig Fryhle, Wiley Publications

References

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Condensation reactions
Carbon-carbon bond forming reactions
Name reactions