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Chapter: 11th Chemistry : Haloalkanes and Haloarenes

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Organo metallic Compounds

Organo metallic compounds are organic compounds in which there is a direct carbon –metal bond

Organo metallic Compounds

 

Organo metallic compounds are organic compounds in which there is a direct carbon –metal bond

Example

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


 

Preparation

 

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

Example


 

Uses of Grignard reagent

 

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.

 

(1) Preparation of primary alcohol

Formaldehyde reacts with Grignard reagent to give addition products which on hydrolysis yields primary alcohol.


 

2) Preparation of secondary alcohol

Aldehydes other than formaldehyde, react with Grignard reagent to give addition product which on hydrolysis yields secondary alcohol.


 

3) Preparation of Tertiary alcohol

Ketone reacts with Grignard reagent to give an addition product which on hydrolysis yields tertiary alcohols.

Example


 

(4) Preparation of aldehyde

Ethyl formate reacts with Grignard reagent to form aldehyde. However, with excess of Grignard reagent it forms secondary alcohol.

Example


 

(5) Preparation of ketone

Acid chloride reacts with Grignard reagent to form ketones. However, with excess of Grignard reagent it forms tertiary alcohol.

Example


 

6) Preparation of carboxylic acids

Solid carbon dioxide reacts with Grignard reagent to form addition product which on hydrolysis yields carboxylic acids.

For Example


 

7) Preparation of esters

Ethylchloroformate reacts with Grignard reagent to form esters.

Example


 

8) Preparation of higher ethers

Lower halogenated ether reacts with Grignard reagent to form higher ethers.

Example


 

9) Preparation of alkyl cyanide

Grignard reagent reacts with cyanogen chloride to from alkyl cyanide

Example


 

10) Preparation of Alkanes

Compounds like water, alcohols and amines which contain active hydrogen atom react with Grignard reagents to form alkanes.

Example


 

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