14.4.1 are synthetically very useful compounds. These reagents are

14.4.1 Grignard reagent

An
important class of organo metallic compound discovered by victor Grignard in
1900 is alkyl magnesium halide
referred as Grignard reagent.Victor Grignard was awarded nobel prize in 1912
for this discovery.

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Nomenclature

The general formula for
Grignard reagent is written as    R – Mg
X

where
R = alkyl or alkyl group, X = Cl, Br or I

Grignard
reagent are named by simply adding the name of the metal to that of the organic
group bonded to the metal

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  

 R       
MgX

14.4.2.
Preparation

When a solution of
alkyl halide in ether is allowed to stand over magnesium pieces,the metal
gradually dissolves and alkyl magnesium halide (Grignard reagent  ) is formed.All the reagents used should be
pure and dry

Example

 

 

 

 

 

 

14.4.3. Synthetic uses
of Grignard reagent 

Grignard reagent are synthetically very
useful compounds. These reagents are converted to various organic compounds
like alcohols, carboxylic acids, aldehydes and ketenes. The alkyl group being
electron rich act as a carbanion or a nucleophile. They would attack polarised
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 yield primary alcohol.

 

 

2)
Preparation of Secondary alcohol

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

 

 

 

 

 

3)
Preparation of Tertiary alcohol

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

Example

 

 

(4)
Preparation of Aldehyde

      Ethylformate 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 ketenes. However, with excess of Grignard reagent
it forms tertiary alcohol.

Example