Organo metallic Compounds
Organo metallic compounds are organic compounds in which there is a direct carbon –metal bond
CH3 Mg I - Methyl magnesium iodide
CH3 CH2 Mg Br - Ethyl magnesium bromide
The carbon - magnesium bond in Grignard reagent is covalent but highly polar. The carbon atom is more electro negative than magnesium. Hence, the carbon atom has partial negative charge and the magnesium atom has partial positive charge
When a solution of alkyl halide in ether is allowed to stand over pieces of magnesium metal, the metal gradually dissolves and alkyl magnesium halide (Grignard reagent) is formed. All the reagents used should be pure and dry
Grignard reagents are synthetically very useful compounds. These reagents are converted to various organic compounds like alcohols, carboxylic acids, aldehydes and ketones. The alkyl group being electron rich acts as a carbanion or a nucleophile. They would attack polarized molecules at a point of low electron density. The following reactions illustrate the synthetic uses of Grignard reagent.
Formaldehyde reacts with Grignard reagent to give addition products which on hydrolysis yields primary alcohol.
Aldehydes other than formaldehyde, react with Grignard reagent to give addition product which on hydrolysis yields secondary alcohol.
Ketone reacts with Grignard reagent to give an addition product which on hydrolysis yields tertiary alcohols.
Ethyl formate reacts with Grignard reagent to form aldehyde. However, with excess of Grignard reagent it forms secondary alcohol.
Acid chloride reacts with Grignard reagent to form ketones. However, with excess of Grignard reagent it forms tertiary alcohol.
Solid carbon dioxide reacts with Grignard reagent to form addition product which on hydrolysis yields carboxylic acids.
Ethylchloroformate reacts with Grignard reagent to form esters.
Lower halogenated ether reacts with Grignard reagent to form higher ethers.
Grignard reagent reacts with cyanogen chloride to from alkyl cyanide
Compounds like water, alcohols and amines which contain active hydrogen atom react with Grignard reagents to form alkanes.
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