Haloalkanes and other compounds with the halogen atom bonded to either sp3-hybridized or sp2-hybridized carbon atoms (aryl and vinyl halides) react with magnesium metal to yield organomagnesium halides called Grignard reagents. Grignard reagents are usually prepared in diethyl ether (CH3CH2O─CH2CH3). An ether solvent is essential for the reaction. The French chemist Victor Grignard discovered this reaction in 1900, and it has been studied and used extensively ever since.
Grignard reagents form easily from 1°, 2°, and 3° alkyl halides, although their reactivities differ. Aryl and vinyl halides react somewhat more slowly, and the cyclic ether tetrahydrofuran (THF) is required to prepare Grignard reagents of these compounds. The higher boiling point of the cyclic ether provides more vigorous reaction conditions, but the rate of the reaction also increases because THF solvates the Grignard reagent better than diethyl ether.
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Molecular model of a complex of methyl-magnesium chloride, a Grignard reagent, in which two molecules of tetrahydrofuran, THF, are bound to magnesium. The model is based on the crystal structure.
The order of reactivity of the halogens in haloalkanes is I > Br > CI > > F. Organofluorides are so unreactive that they are never used to prepare Grignard reagents. Organohalogen compounds containing bromine and chlorine are readily available, and are commonly used to prepare Grignard reagents. Grignard reagents are used synthetically to form new carbon–carbon bonds. A Grignard reagent has a very polar carbon–magnesium bond in which the carbon atom has a partial negative charge and the metal a partial positive charge.
The polarity of the carbon–magnesium bond is opposite that of the carbon–halogen bond of haloalkanes. Because the carbon atom in a Grignard reagent has a partial negative charge, it resembles a carbanion, and it reacts with electrophilic centers such as the carbonyl carbon atom of aldehydes, ketones, and esters. We will discuss this chemistry extensively in later chapters.
Grignard reagents react rapidly with acidic hydrogen atoms in molecules such as alcohols and water. When a Grignard reagent reacts with water, a proton replaces the halogen, and the product is an alkane. The Grignard reagent therefore provides a pathway for converting a haloalkane to an alkane in two steps.