Chemistry · Ch 14 — Haloalkanes and Haloarenes
Preparation
Preparation
A solution of an alkyl halide in ether, left standing over pieces of magnesium metal, gradually dissolves the metal and forms the alkyl magnesium halide (Grignard reagent) directly, with no separate catalyst or initiator needed. Every reagent used -- the ether solvent, the alkyl halide, and the magnesium turnings themselves -- must be pure and rigorously dry, since the reagent that forms is exceptionally sensitive to moisture (see the Evaluate Yourself box below). Example: iodomethane + Mg (dry ether) gives methyl magnesium iodide (CH3MgI). The same procedure, using an aryl halide instead of an al …
Worked out. Book's practice box (no printed solution): why must even traces of moisture be avoided in preparing/using a Grignard reagent? RMgX is both a very strong base and a very strong nucleophile/carbanion-equivalent; the moment it meets any source of 'active hydrogen' -- water, an alcohol, an amine -- it is instantly protonated to the simple alkane R-H (exactly the Preparation-of-Alkanes reaction of section 14.4.2), destroying the reagent before it can do the intended synthetic chemistry, and any water present would similarly quench the Grignard reagent needed for a later step (own solution, not printed in …