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Concept
Réaction de Wurtz
Résumé
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.
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.
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 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.
Although the Wurtz reaction is only of limited value in organic synthesis, analogous couplings are useful for coupling main group halides.
Source officielle
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