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Grignard reagent
A Grignard reagent is prepared by the reaction of magnesium metal with an organohalogen compound in an ether solvent.
An ether solvent is necessary for the formation of the Grignard reagent. It is thought that the unshared electrons of the oxygen coordinate with the Mg to stabilize the organometallic compound. Also, the alkyl portion of the ether provides a hydrocarbon-like solvent for the organic part of the Grignard reagent. Both these factors result in Grignard reagents being soluble in ethers.
Diethyl ether is the usual solvent because it is inexpensive. Other ethers, such as tetrahydrofuran, can also be used.
Grignard reagents are extremely strong bases that react with acids, alcohols, amines, water, and even such weak acids as alkynes with a terminal triple bond (RC≡CH, pKa 26). In these reactions, the R of RMgX removes a proton from the acidic compound and becomes an alkane (RH).
Because acid-base reactions are very rapid compared to other organic reactions, a compound with a removable H+ quickly destroys a Grignard reagent. Therefore in carrying out a Grignard reaction, we must use scrupulously dry glassware, solvents, and reagents. In addition, the reagents must not contain a group with a reactive hydrogen unless an alkane is the desired product.
In the reaction with a carbonyl compound, the organic portion of a Grignard reagent acts as a strong nucleophile and attacks the partially positive carbonyl carbon to yield a magnesium alkoxide.
Subsequent treatment of the alkoxide with aqueous acid yields the
alcohol itself.
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