The Grignard Reaction is the addition of an organomagnesium halide (Grignard reagent) to a ketone or aldehyde, to form a tertiary or secondary alcohol, respectively. The reaction with formaldehyde leads to a primary alcohol.

Grignard Reagents are also used in the following important reactions: The addition of an excess of a Grignard reagent to an ester or lactone gives a tertiary alcohol in which two alkyl groups are the same, and the addition of a Grignard reagent to a nitrile produces an unsymmetrical ketone via a metalloimine intermediate. (Some more reactions are depicted below)

So far, we have built a small repertoire of reactions that can be used to convert one functional group to another. We have briefly discussed converting alkenes to alkanes; alkanes to alkyl halides; alkyl halides to alcohols; alcohols to ethers, aldehydes, or ketones; and aldehydes to carboxylic acids. We have also shown how carboxylic acids can be converted into esters and amides. We have yet to encounter a reaction, however, that addresses a basic question: How do we make CC bonds? One answer resulted from the work that Francois Auguste Victor Grignard started as part of his Ph.D. research at the turn of the century.

Grignard noted that alkyl halides react with magnesium metal in diethyl ether (Et₂O) to form compounds that contain a metal-carbon bond. Methyl bromide, for example, forms methylmagnesium bromide.

Because carbon is considerably more electronegative than magnesium, the metal-carbon bond in this compound has a significant amount of ionic character. Grignard reagents such as CH₃MgBr are best thought of as hybrids of ionic and covalent Lewis structures.

The Synthesis of Triphenylmethanol via a Grignard Reagent

In this experiment, the organometallic regent, phenyl magnesium bromide, a Grignard Reagent, was reacted with benzophenone and hydrolyzed to synthesize triphenylmethanol. The final product, triphenylmethanol, was weighed and the melting point and percent yield were determined. The limiting reagent in the reaction was benzophenone. The melting point measured was between 153 degrees Celsius and 157 degrees Celsius. The literature value for the melting point of the product is 164.2 degrees Celsius. The percent yield determined was fifty-nine percent. Although, the melting point is close to the literature value, it can be determined that there are impurities in the product. These impurities could be a result of the byproduct biphenyl, which occurs when two phenyl radicals combine during the synthesis of the Grignard Reagent, or because of the presence of water in any of the reaction tubes, which destroys the Grignard Reagent.

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Ravi Kesarwani

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