Wurtz reaction

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 carbon-carbon bond:

2R-X + 2Na → R-R + 2Na⁺X⁻

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Mechanism

This reaction involves the radical species R•.

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

R-X + Na → R• + Na⁺X⁻

The alkyl radical then accepts an electron from another sodium atom to form an alkyl anion and the sodium becomes cationic.

R• + Na → R⁻Na⁺

The alkyl anion then displaces the halide in an S_N2 reaction, forming a new carbon-carbon covalent bond.

R⁻Na⁺ + R-X → R-R + Na⁺X⁻

Example

Here methyl iodide and ethyl chloride are reacted with powdered sodium metal in anhydrous ether. Alkanes containing double the number of carbon atoms are formed, i.e. methyl iodide gives ethane, and ethyl chloride gives n-butane.

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 alcohols, and reducing the yield of the desired product.

Limitations

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. Since the reaction involves free radical species, a side reaction also occurs to produce an alkene. This side-reaction becomes more significant when the alkyl halides are bulky at the halogen-attached carbon. This is because the activation energy required for the S_N2 reaction in the second step becomes significantly high, so the alternate elimination mechanism is favored.

The Corey-House-Posner-Whitesides reaction is an alternative route to alkane synthesis that overcomes some of the limitations of the Wurtz reaction.

References

• Wurtz, Ann. Chim. Phys. [3] 44, 275 (1855); Ann. 96, 364 (1855).
• Organic-chemistry.org
• Organic Chemistry, by Morrison and Boyd
• Organic Chemistry, by Graham Solomons and Criag Fryhle, Wiley Publications