Wurtz reaction

Mechanism

The reaction proceeds by an initial metal–halogen exchange, which is described with the following idealized stoichiometry: : R−X + 2 M → RM + MX This step may involve the intermediacy of radical species R·. The conversion resembles the formation of a Grignard reagent. The RM intermediates have been isolated in several cases. The radical is susceptible to diverse reactions. The organometallic intermediate (RM) next reacts with the alkyl halide (RX) forming a new carbon–carbon covalent bond. : RM + RX → R−R + MX The process resembles an SN2 reaction, but the mechanism is probably complex.

Examples and reaction conditions

The reaction is intolerant of many functional groups which would be attacked by sodium. For similar reasons, the reaction is conducted in unreactive polar aprotic solvents such as ether, dimethylformamide(DMF) or tetrahydrofuran (THF). In efforts to improve the reaction yields, other metals have also been tested to effect the Wurtz-like couplings: silver, zinc, iron, activated copper, indium, as well as mixture of manganese and copper chloride.

Wurtz coupling is useful in closing small, especially three-membered, rings. In the cases of 1,3-, 1,4-, 1,5-, and 1,6- dihalides, Wurtz-reaction conditions lead to formation of cyclic products, although yields are variable. Under Wurtz conditions, vicinal dihalides yield alkenes, whereas geminal dihalides convert to alkynes. 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.

Extensions to main group compounds

Although the Wurtz reaction is only of limited value in organic synthesis, analogous couplings are useful for coupling main group halides. Hexamethyldisilane arises efficiently by treatment of trimethylsilyl chloride with sodium: : (Me = CH3 Tetraphenyldiphosphine is prepared analogously: : (Ph = C6H5) Triphenylarsine can also be prepared by combining arsenic trichloride, chlorobenzene, and sodium: :

When applied to main group dihalides, rings and polymers result. Polysilanes and polystannanes are produced in this way :

References

References

1. March ''Advanced Organic Chemistry'' 4th edition p. 535.
2. Gary M. Lampman, C. Aumiller. (1971). "Bicyclo[1.1.0]Butane". Organic Syntheses.
3. (1950). "Tetraphenylarsonium Chloride Hydrochloride". Organic Syntheses.
4. (2016). "Polystannanes: Processible molecular metals with defined chemical structures". Chemical Society Reviews.