14.4.1 Grignard reagentAnimportant class of organo metallic compound discovered by victor Grignard in1900 is alkyl magnesium halidereferred as Grignard reagent.Victor Grignard was awarded nobel prize in 1912for this discovery.NomenclatureThe general formula forGrignard reagent is written as R – MgXwhereR = alkyl or alkyl group, X = Cl, Br or IGrignardreagent are named by simply adding the name of the metal to that of the organicgroup bonded to the metalExampleCH3 Mg I – Methylmagnesium iodideCH3 CH2 Mg Br – Ethyl magnesium bromideThe carbon – magnesium bond in Grignardreagent is covalent but highly polar.
The carbon atom is more electro negativethan magnesium. Hence the carbon atom has partial negative charge and themagnesium atom has partial positive charge R MgX14.4.2.PreparationWhen a solution ofalkyl halide in ether is allowed to stand over magnesium pieces,the metalgradually dissolves and alkyl magnesium halide (Grignard reagent ) is formed.All the reagents used should bepure and dryExample 14.
4.3. Synthetic usesof Grignard reagent Grignard reagent are synthetically veryuseful compounds. These reagents are converted to various organic compoundslike alcohols, carboxylic acids, aldehydes and ketenes.
The alkyl group beingelectron rich act as a carbanion or a nucleophile. They would attack polarisedmolecules at a point of low electrondensity. The following reactions illustrate the synthetic uses of Grignardreagent (1)Preparation of Primary alcohol Formaldehyde reacts with Grignard reagent togive addition products which on hydrolysis yield primary alcohol. 2)Preparation of Secondary alcohol Aldehydes otherthan formaldehyde, reacts with Grignard reagent to give addition product which on hydrolysis yield secondary alcohol.
3)Preparation of Tertiary alcohol Ketone reacts withGrignard reagent to give an addition product which on hydrolysis yield tertiaryalcohols. Example (4)Preparation of Aldehyde Ethylformate reactswith Grignard reagent to form aldehyde. However, with excess of Grignardreagent it forms secondary alcohol.Example (5)Preparation of Ketone Acid chloride reactswith Grignard reagent to form ketenes. However, with excess of Grignard reagentit forms tertiary alcohol.Example