Organometallic Compounds || Grignard Reagent

Organometallic Compounds

Grignard Reagent / Alkyl magnesium halide (R-Mg-X)

Method of Preparation

The Grignard reagents are prepared by the action of activated magnesium on organic halides in presence of diethyl ether or THF in anhydrous conditions.
R — X + Mg ---THF---> R — Mg — X

Important Reactions

Reaction with compounds having acidic hydrogen

The Grignard reagents are highly basic and can react with compounds having acidic hydrogen like water, acids, alcohols, 1-alkynes etc. and gives corresponding alkanes.
R — Mg — X + H₂O → R — H + Mg(OH)X
R — Mg — X + R — OH → R — H + Mg(OR)X
R — Mg — X + H — C ≡ C — CH₃ → R — H + CH₃ — C ≡ C — Mg — X

Reaction with Carbonyl compounds

Grignard reagent reacts with carbonyl compounds followed by hydrolysis to form alcohol.
a. With Formaldehyde gives Primary alcohol.
R — Mg — X + HCHO → R — CH₂ — OH
b. With Acetaldehyde gives Secondary alcohol.
R — Mg — X + CH₃CHO → R₂CH — OH
c. With Ketone gives Tertiary alcohol.
R — Mg — X + CH₃ — CO — CH₃ → R₃C — OH

Reaction with Esters and Acid halides

Grignard reagent reacts with Esters and Acid halides followed by hydrolysis to form tertiary alcohol.
Two moles of Grignard reagent reacts with esters, give tertiary alcohols.
CH₃ — COO — C₂H₅ + 2R — Mg — X ---H₃O⁺---> R₂CH₃C — OH
Two moles of Grignard reagent reacts with acid halide, give tertiary alcohols.
CH₃ — CO — Cl + 2R — Mg — X ---H₃O⁺---> R₂CH₃C — OH

Reaction with CO₂ and CS₂

Grignard reagent reacts with CO₂ and CS₂ followed by hydrolysis to form carboxylic acid and dithionic acid respectively.
R — Mg — X + CO₂ ---H₃O⁺---> RCOOH
R — Mg — X + CS₂ ---H₃O⁺---> RCSSH

Reaction with SO₂ and SO₃

Grignard reagent reacts with SO₂ and SO₃ followed by hydrolysis to form sulphinic acid and sulphonic acid respectively.
R — Mg — X + SO₂ ---H₃O⁺---> RSOOH
R — Mg — X + SO₃ ---H₃O⁺---> RSO₂OH

Reaction with R — C ≡ N and H — C ≡ N

Grignard reagent reacts with R — C ≡ N and H — C ≡ N followed by hydrolysis to form ketones and aldehyde respectively.
R — Mg — X + R — C ≡ N ---H₃O⁺---> R₂C=O
R — Mg — X + H — C ≡ N ---H₃O⁺---> R — CHO

Reaction with Epoxide (cyclic ether)

Grignard reagent reacts with Epoxide followed by hydrolysis form alcohol.
R — Mg — X + (CH₂)₂O ---H₃O⁺---> R — CH₂ — CH₂ — OH

Prepare the following compounds from suitable Grignard Reagents
Tertiarybutyl Alcohol
Propionic Acid
Secondarybutyl Alcohol

Tertiarybutyl Alcohol can be prepared by the reaction of acetone with methylmagnesium bromide followed by hydrolysis.
Propionic acid can be prepared by the reaction of carbondioxide with ethylmagnesium bromide followed by hydrolysis.
Secondarybutyl Alcohol can be prepared by the reaction of acetaldehyde with ethylmagnesium bromide followed by hydrolysis.