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Grignard reagent

From Academic Kids

A Grignard Reagent is an alkyl- or aryl- magnesium halide. It is important in the synthesis of carbon-carbon bonds. Grignard reagents undergo reactions with compounds containing carbonyl groups. These reactions create alcohols.

Victor Grignard, of the University of Lyons, won the 1912 Nobel Prize in Chemistry for his discovery of Grignard reagents.

Grignard reagents are formed by reacting alkyl or aryl halides with organomagnesium metal, conferring a negative charge on the terminal carbon, a rare occurrence. Bromides are most often used, as they work the fastest and are readily available among halides (iodide and chloride are also used, while fluoride is generally unreactive towards organomagnesium compounds. The Grignard reaction is exothermic and because of a oxide layer present on the magnesium, the start of the reaction is sometimes delayed. A crystal of iodine is often introduced to initiate the reaction.

Contents

Nucleophilic addition reactions

In reactions involving Grignard reagents, it is important to ensure that no water is present, which would otherwise cause the reagent to rapidly decompose. Thus, most Grignard reactions occur in solvents such as anhydrous ether or tetrahydrofuran, because the oxygen of these solvents stabilizes the magnesium reagent. The reagent may also react with oxygen present in the atmosphere, inserting an oxygen atom between the carbon base and the magnesium halide group. Thus, many of these reactions are carried out in nitrogen or argon atmospheres.

Missing image
Grignard-Reaction_Mechanism.png
Grignard reaction mechanism between acetone and Grignard reagent—a bromide in this case. Notice that water is required to form the alcohol.

Grignard reagents react with formaldehyde to form primary alcohols, with other aldehydes to form secondary alcohols, and with ketones to form tertiary alcohols. Quenching a Grignard with dry ice yields the carboxylic acid [1] (http://www.orgsyn.org/orgsyn/prep.asp?prep=cv1p0361). The Bouveault aldehyde synthesis is a one-pot chemical reaction that converts a primary alkyl halide to an aldehyde one carbon longer.

Coupling reaction

A Grignard reagent can also be involved in a coupling reaction. For example [1] nonylmagnesium bromide (CAS registry number 39691-62-8) reacts with a aryl chloride to a nonyl benzoic acid (CAS registry number 38289-46-2). An iron catalyst is used and not a expensive palladium catalyst. Acac stands for acetyl acetonate (CAS registry number 14024-18-01).


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4nonylbenzoicacid2.gif
image:4nonylbenzoicacid2.gif



See also

references

[1] 4-Nonylbenzoic Acid A. Frstner, A. Leitner, G. Seidel. Org. Synth. 2004, 81, 33-42.online article (http://www.orgsyn.org/orgsyn/prep.asp?prep=v81p0033) from Organic Synthesesde:Grignard-Reaktion ja:グリニャール試薬

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