Muscling In Organometallics

Grignard reagents (organomagnesium compounds) are extremely useful in many organic reactions. These materials are relatively easy to prepare, but they re very sensitive to trace amounts of moisture and air and decompose if either is present. 

The preparation of a Grigncird reagent begins with magnesium metal and dry ether (in most cases, either diethyl ether or THF, tetrahydrofuran). The ether cleans the surface of the metal and takes the reagent into solution for reaction. (If either the ether or the reaction vessel contains moisture, the yield is poor.) The magnesium then reacts with either an alkyl halide or an aryl halide. The ease of reactivity decreases in the order R1 RBr RCl. Iodides may react too rapidly, but chlorides may react too slowly. Thus bromides are usually the best. The general reaction is 

Pay particular attention to the bond polarity (6 5 ). The carbon atom has a partially negative charge (and therefore is a nucleophile) and the magnesium atom has a partially positive charge (so is an electrophile). 

Grignard reagents behave both as bases and as nucleophiles. The basicity leads to the requirement that water be rigorously excluded from the reaction mixture both in the preparation and in the use of the Grignard reagent. 

Basicity refers to the ability of a Grignard reagent to react with proton donors, including weak donors like water. The carbanion is the conjugate base of a very weak acid (K = 1(H° - 10 °). This process is 

Part IV Advanced Topics (Every Student s Nightmare) 

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