Reaction of Alkyl, Alkenyl, and Aryl Halides with Metals

This is among the earliest mention of reactions of alkyl halides with sodium or potassium, and, indeed, it follows fairly closely the announcement, by Humphry Davy (in the Bakerian lecture in 1807) of the production of sodium (Na) and potassium (K) by electrolysis of their alkali salts. 

In that vein, it has been subsequently shown (by reactions which most probably involve a series of one electron transfer processes) that halogens can be stripped from almost any alkyl, alkenyl, or aryl halide by treatment with lithium (Li) or sodium (Na) in the presence of a proton donor such as (commonly used) 2-methyl-2-propanol (r-butanol [(CH3)3C-OH], r-BuOH). A solvent such as oxacyclopentane (THE) helps to moderate the reaction. Rearrangements of the carbon skeleton are common. Because it usually lacks selectivity, the method is only rarely used (unless the goal is destruction of the halogenated compound). 

It is also possible to reductively dehalogenate more reactive alkyl halides with either zinc (Zn) dust or a copper-zinc alloy (Cu-Zn) in the presence of an alcohol and protic acid, but this method also suffers from lack of specificity and is also frequently attended by skeletal rearrangement. 

The more selective methods of reduction, which also minimize rearrangement reactions of the reacting alkyl halide (substrate), generally involve isolation (and thus at least partial purification) of an organometallic intermediate. Using one or more of these methods has enabled alkyl, alkenyl, and aryl halides, in the presence of an appropriate (usualy ethereal or hydrocarbon) solvent, to react with a variety of metals to produce organometallic compounds. 

Regardless of the actual structure of the Grignard reagent, which may depend on temperature, solvent, and particular substrate and even source of magnesium metal, it is generally written in the form R-Mg-X (or Ar-Mg-X) with the justification that it appears to behave that way.  

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