Reactions of organolithium and Grignard reagents reflect the nucleophilic (and basic) character of the functional carbon in these compounds. This change, shown below, makes alkyl lithium and Grignard reagents unique and useful reactants in synthesis. Because the functional carbon atom has been reduced, the polarity of the resulting functional group is inverted (an originally electrophilic carbon becomes nucleophilic). These reactions are obviously substitution reactions, but they cannot be classified as nucleophilic substitutions, as were the earlier reactions of alkyl halides. Third, since these organometallic compounds are very reactive, contaminants such as water, alcohols and oxygen must be avoided. For alkyl lithium formation pentane, hexane or ethyl ether may be used but ethyl ether or THF are essential for Grignard reagent formation. The conditions necessary to achieve a successful reaction are critical.įirst, the metal must be clean and finely divided so as to provide the largest possible surface area for reaction. The metals referred to here are insoluble in most organic solvents, hence these reactions are clearly heterogeneous, i.e. R 3C- X + Mg - > R 3C- Mg X A Grignard Reagent Halide reactivity increases in the order: Cl R 3C- Li + Li X An Alkyl Lithium Reagent The halogen is converted to halide anion, and the carbon bonds to the metal (the carbon has carbanionic character). In a similar fashion these same metals reduce the carbon-halogen bonds of alkyl halides. Sodium, for example, reduces elemental chlorine to chloride anion (sodium is oxidized to its cation), as do the other metals under varying conditions. The alkali metals (Li, Na, K etc.) and the alkaline earth metals (Mg and Ca, together with Zn) are good reducing agents, the former being stronger than the latter. Reactions of Alkyl Halides with Reducing Metals
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