In organic chemistry, the Wurtz reaction, named after Charles Adolphe Wurtz, is a coupling reaction in which two 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, such as 1,6-dibromohexane + 2 Na →
cyclohexane + 2 NaBr.
A related reaction, which combines alkyl halides with aryl halides is called the Wurtz–Fittig reaction. Despite its very modest utility, the Wurtz reaction is widely cited as representative of reductive coupling.[March Advanced Organic Chemistry 4th edition p. 535.]
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
free 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 S
N2 reaction, but the mechanism is probably complex.
Examples and reaction conditions
The reaction is intolerant of many
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,
Rieke metal 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:
Tetraphenyldiphosphine is prepared analogously:
Similar couplings have been applied to many main group halides. When applied to main group dihalides, rings and polymers result. and are produced in this way
See also
Further reading
