Unlike Grignard and organolithium reagents, organometallic compounds of the most electropositive metals (Na, K, etc.) react rapidly with haloalkanes. As a result, attempts to convert RX into RNa or RK by reaction with the corresponding metal lead to alkanes by a reaction called Wurtz coupling.
2 RX + 2 Na → R-R + 2 NaX
which is the result of
R-X + 2 Na → R-Na + NaX
followed rapidly by
R-Na + R-X → R-R + NaX
When it was still in use, the Wurtz coupling reaction was employed mainly for the preparation of alkanes by the coupling of two identical alkyl groups (e.g., equation 1 below). Suggest a reason why Wurtz coupling might not be a useful method for coupling two different alkyl group! (equation 2).
2 CH3CH2CH2C1 + 2 Na → CH3CH2CH2CH2CH2CH3 + 2 NaCl.....................(1)
CH3CH2C1 + CH3CH2CH2C1 + 2 Na → CH3CH2CH2CH2CH3 + 2 NaCl............(2)