Question

Why are bromo-alkanes and iodo-alkanes preferred in Wurtz reactions?​

Answer 1

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The general equation of Wurtz Reaction can be written as:

2R–X + 2Na → R–R + 2Na+X−

Lets see the basic mechanism of this reaction.

The reaction consists of a halogen-metal exchange involving the radical species (R•).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− ……(1)

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 → R−M+ …….(2)

The nucleophilic carbon of the alkyl anion then displaces the halide forming a new carbon-carbon covalent bond.

R−M+ + R–X → R–R + M+X− ……(3)

If we use fluorine then in eq(1) the free radical will not be formed as fluorine is highly electronegative. now, in eq(3) the MX so formed will be highly unstable if we use any halide other then bromine and iodine as the size of M(say sodium) is very small and halogens other then Cl or Br will not form stable molecule.