Wurtz reaction

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The Wurtz reaction, named after Charles-Adolphe Wurtz, is a coupling reaction in organic chemistry, organometallic chemistry and recently inorganic main group polymers, whereby two alkyl halides are reacted with sodium to form a new alkane:

2R–X + 2Na → R–R + 2Na+X

Other metals have also been used to effect the Wurtz coupling, among them silver, zinc, iron, activated copper, indium and a mixture of manganese and copper chloride.[1] The related reaction dealing with alkyl halides is called the Wurtz-Fittig reaction.This can be explained by the formation of free radical intermediate and its subsequent disproportionation to give alkene.

Mechanism[edit]

The reaction consists of a halogen-metal exchange involving the radical species R• (in a similar fashion to the formation of a Grignard reagent and then the carbon–carbon bond formation in a nucleophilic substitution reaction.)

One electron from the metal is transferred to the halogen to produce a metal halide and an alkyl radical.

R–X + M → R• + M+X

The alkyl radical then accepts an electron from another metal atom to form an alkyl anion and the metal becomes cationic. This intermediate has been isolated in several cases.

R• + M → RM+

The nucleophilic carbon of the alkyl anion then displaces the halide in an SN2 reaction, forming a new carbon-carbon covalent bond.

RM+ + R–X → R–R + M+X

Examples and reaction conditions[edit]

A typical reactions would be when methyl iodide or ethyl chloride are reacted with powdered sodium metal in anhydrous ether. An alkane containing double the number of carbon atoms is formed, i.e. methyl iodide gives ethane, and ethyl chloride gives n-butane.

                         CH3-Cl + 2Na + Cl-CH3 ------>CH3-CH3 + 2NaCl 

The solvent, ether in this case, must be anhydrous (free of moisture) because the alkyl anions are so basic (the pKa of the alkyl proton is 48–50) that they readily deprotonate water to hydroxide ion, forming alkanes, and reducing the yield of the desired product.

Due to several limitations (see below) this reaction is very seldom used, especially since alkanes can be obtained from variety of natural sources, such as crude oil or by transformations of fatty acids. However, Wurtz coupling is useful in closing small, especially three-membered, rings: bicyclobutane was prepared this way using 1-bromo-3-chlorocyclobutane and sodium, giving 95% yield of the product:

Wurtz-reaction Bicyclobutane synthesis.svg

Limitations[edit]

The Wurtz reaction is limited 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 as the difference in the boiling points of the products is very low in these cases, making distillation unusable. 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.

References[edit]

  1. ^ March Advanced Organic Chemistry 5th edition p. 535