Chemistry · Ch 10 — Halogen Derivatives
Reaction with active metals
Reaction with active metals
Active metals such as sodium, magnesium and cadmium readily combine with alkyl chlorides, bromides and iodides to form compounds containing a carbon-metal bond, known as organometallic compounds. (a) Reaction with magnesium: an alkyl halide treated with magnesium metal in dry ether gives an alkyl magnesium halide, R-Mg-X, called the Grignard reagent (R-X + Mg, dry ether -> R-Mg-X); the C-Mg bond is polar covalent with carbon pulling electron density from the more electropositive magnesium, so the Grignard reagent's carbon behaves as a nucleophile, reacting readily with water or any compound bearing a hydrogen on an electronegative atom (for example R-Mg-X + CH3OH -> R-H + Mg(X)(OCH3), and R-Mg-X + NH3 -> R-H + Mg(X)(NH2)) -- which is exactly why any reaction involving a Grignard reagent must be carried out under strictly anhydrous conditions, since even trace moisture would destroy the reagent before it could be used for its intended purpose. Vinyl and aryl halides, not just alkyl halides, can also form Grignard reagents. (b) Wurtz reaction: alkyl halides react with metallic sodium in dry ether to form a higher alkane with double the carbon count of the starting alkyl halide, plus sodium halide (2 R-X + 2 Na, dry ether -> R-R + 2 NaX); using a mixture of two different alkyl halides instead gives a statistic …
Worked out. When an alkyl halide is treated with magnesium metal in dry ether as solvent, it gives an alkyl magnesium halide, R-Mg-X, known as the Grignard reagent (R-X + Mg, dry ether -> R-Mg-X). The carbon-magnesium bond in the Grignard reagent is a polar covalent bond, with carbon pulling electron density away from the more electropositive magnesium, so carbon carries a partial negative charge and the reagent behaves as a nucleophile at that carbon. Grignard reagents are highly reactive and react readily with water or with any compound containing a hydrogen attached to an electronegative element -- for example R-Mg-X + CH3OH -> R-H (a hydrocarbon) + Mg(X)(OCH3), and R-Mg-X + NH3 -> R-H + Mg(X)(NH2) -- which is why reactions involving a Grignard reagent must always be carried out under strictly anhydrous (moisture-free) conditions. Vinyl and aryl halides can also form Grignard reagents, not just alkyl halides. Victor Grignard receiv …
Worked out. Alkyl halides react with metallic sodium in dry ether as solvent to form a higher alkane containing double the number of carbon atoms present in the alkyl halide, plus sodium halide; this reaction is called the Wurtz reaction: 2 R-X + 2 Na (dry ether) -> R-R + 2 NaX -- for example 2 C2H5Br + 2 Na (dry ether) -> CH3-CH2-CH2-CH3 (butane) + 2 NaBr. When a mixture of two DIFFERENT alkyl halides is used instead of one alkyl halide on its own, all three possible alkanes are formed together (a statistical mixture), which makes the Wurtz reaction a poor choice for deliberately making an unsymmetric …