ORGANOMETALLIC COMPOUNDS

GRIGNARD REAGENT

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DEEPSHIKHA

CHEMISTRY DEPARTMENT

Organometallic Compounds are chemical compounds which contain at least one bond between a metallic element and a carbon atom belonging to an organic molecule. Even metalloid elements such as silicon, tin, and boron are known to form organometallic compounds which are used in some industrial chemical reactions.

 Grignard reagents are formed by the reaction of magnesium metal with alkyl or alkenyl halides. They’re extremely good nucleophiles, reacting with electrophiles such as carbonyl compounds (aldehydes, ketones, esters, carbon dioxide, etc) and epoxides. They’re also very strong bases and will react with acidic hydrogens (such as alcohols, water, and carboxylic acids).

Grignard reagent----RMgX

A Grignard reagent has a very polar carbon–magnesium bond in which the carbon atom has a partial negative charge and the metal a partial positive charge.

The polarity of the carbon–magnesium bond is opposite that of the carbon–halogen bond of haloalkanes. Because the carbon atom in a Grignard reagent has a partial negative charge, it resembles a carbanion, and it reacts with electrophilic centers such as the carbonyl carbon atom of aldehydes, ketones, and esters. .

Formation Of Grignard Reagents

Grignard reagents are made through the addition of magnesium metal to alkyl or alkenyl halides. The halide can be Cl, Br, or I (not F). It’s slightly easier to make Grignards from the iodides and bromides, however. Note what’s happening here – the magnesium is “inserting” itself between the carbon and the halide

Preparation of Grignard reagent

Grignard reagents are produced from the heated combination of halogenoalkane and magnesium in the presence of diethyl ether (ethoxyethane). The reaction should be kept dry to avoid the resulting Grignard reagent from reacting with water.

The reactions involving Grignard reagents, as sources of nucleophiles, are usually referred to as Grignard reactions.

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Chemical reactions

The Grignard reagents are highly basic and can react with protic compounds like water, acids, alcohols, 1-alkynes etc., by giving corresponding alkanes.

 Ethylmagnesium bromide liberates ethane gas when treated with water.

Grignard Reagents Are Strong Bases – Protonation

 Grignard reagents are essentially the conjugate bases of alkanes, they’re also extremely strong bases. This means that sometimes acid-base reactions can compete with their nucleophilic addition reactions. One common situation where this crops up is when Grignard reagents are added to carboxylic acids. I

One of the most common uses of Grignard reagents is in their reaction with aldehydes and ketones to form alcohols. In the first step, the Grignard forms the carbon-carbon bond. This results in an alkoxide (the conjugate base of an alcohol). To form the alcohol, it’s necessary to add acid at the end of the reaction 

Reaction with Aldehydes and Ketones

Reaction with Aldehydes to form secondary alcohol

Reaction with ketones to form tertiary alcohols

Grignard Reagent add twice To Esters And Acid Halides

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Grignard Reagent add twice To Esters And Acid Halides

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Another important reaction of Grignard reagents is that they will add to epoxides to form carbon-carbon bonds. One thing to keep in mind here is that the tendency is for them to add to the less substituted end of the epoxide – that is, the less sterically hindered end.

Addition Of Grignard Reagents To Epoxides

Reaction Of Grignard Reagents With Carbon Dioxide 

Grignard reagents also add to carbon dioxide (CO2) to form carboxylates, in a reaction similar to their reactions with ketones and aldehydes. The carboxylates are converted to carboxylic acids after addition of acid 

Reaction with sulphur dioxide

Ethanedithionic acid

Reaction with Carbon disulphide

Methanesulphinic acid 

Reaction with nitriles

Reaction with HCN

The Grignard reagents are also used to prepare nitriles by reacting them with cyanogen or cyanogen chloride.

Amines can be prepared by reacting these reagents with Chloramine, NH₂Cl.

The alkyl iodides can be prepared via Grignard reagents. The alkylmagnesium chlorides or bromides are treated with Iodine to get corresponding alkyl iodides.

A Wurtz like coupling reaction is also possible when the Grignard reagent is treated with an alkyl halide to furnish an alkane. Indeed it is a side reaction that may be possible during the preparation of Grignard reagent. This reaction is catalyzed by Cuprous (Cu^I) ions.

 Dialkyl cadmium compounds are formed when the Grignard reagents are made to react with cadmium chloride.

The dialkyl cadmium compounds furnish ketones upon reacting with acid halides.

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