Metal–halogen exchange
chemical reaction
In organometallic chemistry, metal–halogen exchange is a fundamental reaction that converts an organic halide into an organometallic product. The reaction commonly involves the use of electropositive metals (Li, Na, Mg) and organochlorides, bromides, and iodides. Particularly well-developed is the use of metal–halogen exchange for the preparation of organolithium compounds.
Lithium–halogen exchange
Two kinds of lithium–halogen exchange can be considered: reactions involving organolithium compounds and reactions involving lithium metal. Commercial organolithium compounds are produced by the heterogeneous (slurry) reaction of lithium with organic bromides and chlorides:
2 Li + R−X → LiX + R−Li
Often the lithium halide remains in the soluble product.
Most of this article is about the homogeneous (one-phase) reaction of preformed organolithium compounds:
R−Li + R′−X → R−X + R′−Li
Butyllithium is commonly used. Gilman and Wittig independently discovered this method in the late 1930s. It is not a salt metathesis reaction, as no salt is produced.
Lithium–halogen exchange is frequently used to prepare vinyl-, aryl- and primary alkyllithium reagents. Vinyl halides usually undergo lithium–halogen exchange with retention of the stereochemistry of the double bond.
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