Preparation: RX + Mg → RMgX (dry ether or THF, anhydrous conditions)

Reactivity of RX with Mg: RI > RBr > RCl > RF (weaker C-X bond reacts faster) Aryl and vinyl halides also form Grignard reagents (PhBr + Mg → PhMgBr).

Key requirement: Absolutely anhydrous conditions. Any protic source (H2O, ROH, RCOOH, NH3, terminal alkynes) destroys the Grignard: RMgX + H2O → RH + Mg(OH)X.

Functional group incompatibility: Cannot prepare Grignard from molecules containing -OH, -NH2, -COOH, -SH (they have acidic H's that would react).