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In the formation of a Grignard reagent from a compound possessing both bromine and chlorine, magnesium preferentially inserts into the C–Br bond over the C–Cl bond. For example, the formation of a Grignard reagent from 1-bromo-2-chloroethane proceeds with good selectivity:[1]

Grignard formation from 1-bromo-2-chloroethane and subsequent transmetallation to tin

How can this be explained? Can it be rationalised using a reaction mechanism?

  1. Simon, P. Y.-R.; Oreal, H.; Audran, G.; Schulz, M.; Joly, J.-P.; Siri, D.; Siri, A. Proteasome Inhibiting β-Lactam Prodrugs Useful for the Treatment of Cancer and Neurodegenerative Disorders. World Patent WO 2018/115497 A1, June 28, 2018.
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    $\begingroup$ Simply put the C-Br bond is weaker than C-Cl, so oxidative addition occurs much faster. Oxidative addition is treated in more depth in any textbook on organometallic chemistry (the usual context is transition metals but it applies equally well to Mg). $\endgroup$ – orthocresol♦ Apr 4 '17 at 17:44

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