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Introduction to Substitution by Metals

The alkali metals (like Li, Na, K etc.) and the alkaline earth metals (like Mg and Ca, together with Zn) are good reducing agents, the former being stronger than the latter. Sodium, for instance, reduces elemental chlorine to chloride anion (sodium is oxidized to its cation), as do the other metals under changing conditions. In a identical way these same metals reduce the carbon-halogen bonds of alkyl halides. The halogen is converted to halide anion and the carbon bonds to the metal (the carbon has carbanionic character). Halide reactivity get increases in the order: Cl < Br < I. The following equations demonstrate these reactions for the generally used metals magnesium and lithium (R may be hydrogen or alkyl groups in any combination). The alkyl magnesium halides explained in the second reaction are called Grignard Reagents after the French chemist, Victor Grignard, who exposed them. The another metals mentioned above react in an identical manner, but the two displayed here are the most extensively used. Even though the formulas displayed here for the alkyl Grignard  and lithium reagents reflect the Substitution by Metals of the reactions and are extensively used in the chemical literature, they do not precisely show the structural nature of these significant substances. Mixtures of polymeric and another associated and complexed species are in equilibrium under the conditions generally employed for their preparation.

R3C-X   +   2Li   -->  R3C-Li   +   LiX     An Alkyl Lithium Reagent

R3C-X   +   Mg   -->  R3C-MgX     A Grignard Reagent

The metals considered to here are insoluble in most organic solvents, therefore these reactions are clearly heterogeneous, that is take place on the metal surface. The conditions essential to get a successful reaction are critical. 
First, the metal must be clean and finely separated so as to provide the largest possible surface area for reaction. 
Second, a appropriate solvent must be employed. For alkyl lithium formation pentane, ethyl or hexane ether may be employed; but ethyl ether or THF are important for Grignard reagent formation. 
Third, because these organometallic compounds are extremely reactive, contaminants like water, alcohols and oxygen must be avoided.

These reactions are clearly substitution reactions, but they cannot be categorized as nucleophilic substitutions, like were the earlier reactions of alkyl halides. Since the functional carbon atom has been reduced, the polarity of the resulting functional group is inverted (an initially electrophilic carbon becomes nucleophilic). This alteration, displayed below, makes alkyl Grignard and lithium reagents unique and helpful reactants in synthesis.

2144_Substitution by Metals Homework Help.jpg

Other significant reaction exhibited by these organometallic reagents is metal exchange. In the first illustartion below, methyl lithium reacts with cuprous iodide to give a lithium dimethylcopper reagent, which is considered as aGilman reagent. Another alkyl lithiums give identical Gilman reagents. A helpful application of these reagents is their capability to couple with alkyl, aryl and vinyl iodides, as displayed in the second equation. Afterwards we shall find that Gilman reagents also show helpful carbon-carbon bond forming reactions with conjugated enones and with acyl chlorides.

2 CH3Li   +   CuI   -->  (CH3)2CuLi   +   LiI     Formation of a Gilman Reagent

(C3H7)2CuLi   +   C6H5I   -->  C6H5-C3H7   +   LiI   +   C3H7Cu     A Coupling Reaction

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