Grignard Reagents

Grignard reagents contain a strongly polarised carbon-magnesium bond in which carbon carries a large partial negative charge and behaves as a powerful carbanion-like base. Because of this, even a trace of water or any compound with an acidic hydrogen instantly protonates the carbon, so the reagent must be made and used under strictly anhydrous conditions. When moisture reaches ethylmagnesium bromide, the ethyl carbanion abstracts a proton from water to give ethane, while the magnesium ends up as the basic salt magnesium hydroxybromide, Mg(OH)Br. Ethanol with magnesium oxide is wrong because the carbon is protonated rather than oxygenated; no C-O bond forms. Ethene with magnesium bromide is incorrect since protonation gives the saturated alkane, not an alkene. Diethyl ether with magnesium metal is wrong as the solvent is not regenerated and magnesium is not reduced back. The same destructive protonation happens with alcohols, amines, terminal alkynes and even carboxylic acids, which is why all reagents and apparatus must be rigorously dried before a Grignard reaction is attempted. This sensitivity is a direct consequence of the carbon carrying so much negative charge that it behaves as the conjugate base of a very weak acid, the parent alkane. This matches the NCERT caution about keeping Grignard reactions moisture-free. Sanity check: an ethyl group plus one proton gives C2H6, ethane, confirming the product.