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Grignard Reaction

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The Grignard reaction is one of the most useful and widely applied reactions both in inorganic and organic chemistry. The significance lies in the facile generation of a carbanionic group, which then may be introduced into a great variety of molecules. Therefore, Grignard compounds usually are not isolated, but are generated and allowed to react further with the substrate. In certain cases, however, these organometallics have been isolated. 

Generally Grignard reaction is conducted in donor solvents. When ether is not expedient, other solvents may be used, but higher temperatures are required. The organomagnesium halides are known as grignard reagents. These were discovered by Victor grignard and so named grignard reagents. In view of the tremendous synthetic potantials of grignard reagents, Victor grignard was awarded Nobel Prize in 1912.

Definition

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"Any addition of a grignard reagent (RMgX) to a unsaturated carbon, especially the carbonyl containing compound is known as a grignard reaction."

In other words, The grignard reagents react with carbonyl compounds (aldehydes and ketones) to give alcohols. This reaction is known as grignard reaction. A grignard reagent is represented as RMgX. Its exact structure is believed that a grignard reagent exists as a coordination complex with ether as shown below.

Grignard Reagent RMgX

The structure of phenyl magnesium bromide as given below has been established by x-ray diffraction studies.

Phenyl Magnesium Bromide

Grignard Synthesis

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The grignard reagents are represented as R-Mg-X,

Where
  • R = alkyl, alkynyl or aryl group and
  • X = Cl, Br, I
Grignard reagents acts as nucleophiles and attack unsaturated carbon especially the carbon of a carbonyl group. The grignard reagents are synthesized by the action of magnesium on alkyl halide in anhydrous ether.

R-X + Mg $\xrightarrow[reflux]{ether}$ RMgX (grignard reagent)
  • R = CH3 or C2H5
  • X = I
For a given halogen, the reactivity of an alkyl group is CH3 > C2H5 > C3H7 or in other words with the increase in carbon atoms, the formation of grignard reagent becomes difficult. Also for an alkyl halide, the ease of formation of grignard reagent is of the order RI > RBr > RCl.

Preparation of Grignard Reagent

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The grignard reagent is one of the most important and versatile in organic chemistry. The grignard reagent is synthesized by the reaction of an alkyl halide with elemental magnesium in the presence of anhydrous ether.

Grignard Reagent Synthesis

A grignard reagent is formally the magnesium salt, R3C-+MgX, of a carbon acid, R3C-H and is thus a carbon anion, or carbanion. But because hydrocarbons are such weak acids, carbon anions are very strong bases. Grignard reagents must therefore be protected from atmospheric moisture to prevent their being protonated and destroyed in an acid-base reaction.

Grignard Reaction Mechanism

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The grignard reactions is the addition of an organomagnesium halide to a ketone or aldehyde, to former a tertiary or secondary alcohol respectively.

Reaction of Grignard

The mechanism proceeds through the mechanism of nucleophilic addition so that according to electron transfer mechanism the sterically hindered substrates may react.

Grignard Reaction Mechanism

Ketone is formed through an intermediate enolate formation by using grignard reagent which acts as a base. The formed transition state is six-membered which is cyclic is reduced so that hydride is formed from $\beta$-carbon.

Mechanism of Grignard Reaction

Reactions of Grignard Reagents

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  • Reaction with acid chlorides of carboxylic acid form ketones.
Grignard Reaction
  • Formation of 3o alcohols.
Formation of Alcohols
  • Reaction with nitriles.
Reaction with Nitriles
  • Reaction with CO2.
Reaction with Carbon Dioxide
  • Reaction with oxiranes.
Reaction with Oxiranes