ORGANIC LECTURE NOTES
TOPIC : GRIGNARD REAGENT
Contents
Introduction of
Organometallic
compound
Preparations
Reactivity of G.R.
Chemical Reactions
Examples
LECTURE
GRIGNARD REAGENT
1. Introduction of Organometallic compounds
Organometallic compounds are the organic compounds in which a metal atom is directly attached to carbon
atom through covalent bond or ionic bond. For example
C–M or e.g. R-MgX, R2CuLi, R2Zn
(Where C is a carbon atom of an organic molecule and M is a metal atom)
If the metal atom is attached to oxygen, nitrogen. sulphur, etc., then such an organic compound is not
regarded as an organometallic compound. The following structural formula do not belong to the family of
organometallic compounds.
RONa (Sodium alkoxide). CH3COONa (Sodium acetate), CH3COOAg (Silver acetate), RSK (Potassium
mercaptide) RNHK (N-Alkylpotassamide), (CH3COO)4Pb (Lead tetraacetate), etc.
Note : It should be noted that (CH3)4Si (Tetramethylsilane, TMS) is also not an organometallic compound
because silicon is a nonmetal.
Most important examples of organometallic compounds are Grignard's reagents. In Grignard's reagent, the
carbon and magnesium atoms are bonded with each other through polar covalent bond and magnesium atom
is attached to halogen by ionic bond.
(Functional part of a Grignard's reagent molecule)
In organometallic compounds, the metal atom can be bonded to carbon atom of a hydrocarbon radical
(Saturated, unsaturated, aliphatic, alicyclic or aromatic) or carbon atom of a heterocyclic radical. Some
examples are given below.
1. Saturated Aliphatic Grignard's reagent
R – MgX (Alkylmagnesium halide)
CH3 – MgI (Methylmagnesium iodide)
2. Unsaturated Aliphatic Grignard's reagent
(i) Alkenyl Grignard's reagent
CH2 = CH – CH2 – MgX (Allylmagnesium halide)
(ii) Alkynyl Grignard's reagent
CH C – CH2 – MgX (Propargylmagnesium halide)
3. Alicyclic Grignard's reagent
(Cyclohexylmagnesium halide)
4. Aromatic Grignard's reagent
(Phenylmagnesium halide)
C6H5CH2MgCl (Benzylmagnesium halide)
2. Preparation
Dry and pure
RX + Mg Ether
RMgX
Ether is used as a solvent because it is a Lewis base that donates its lone pair of electrons to electron-
deficient magnesium atom, therefore providing stability to the Grignard's reagent by completing the octet on
magnesium atom.
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Process : To an etherial solution of alkyl halide Mg metal is added at very low temp. (0 – 5°C). A vigorous
reaction takes place , and a solution of G.R. is obtained. It cannot be evaporated to get it in solid state. The
reaction will be explosive. It is stable only in solution state.
Reactivity order with respect to X (For preparation of RMgX)
R–X:
Iodides forms organometallic compounds at the fastest rate.
Structural stability of G.R.
If the alkyl part has more stable negative charge, then RMgX is more stable. It will be less reactive
e.g. CH3 – CH2 – MgX ; CH2 = CH – MgX ; CH3C C – MgX
Reactivity order : 1>2>3
Stability order : 1<2<3
3. Reactivity of Grignard Reagent :
It has been found out by estimation that there is 35% ionic character in carbon-magnesium bond of Grignard's
reagent. Therefore, there is a tendency of forming carbanion by heterolysis of this polar coordinate bond as
follows.
+
The carbanion (a nucleophile) formed as shown above, attacks the positively, charged electrophilic centre of
other compound. Therefore. It can be said that if a Grignard reagent is regarded as the substrate, then
electrophilic displaces MgX, i.e. electrophilic substitution (SE) reaction takes place.
R – MgX R – E (SE Product)
If Grignard reagent is regarded as the attacking reagent, then the nucleophilic carbanion of Grignard's re-
agent will attack the other compound taken as substrate.
+
3.1 Reactivity order of Grignard's reagent
On having same hydrocarbon radical, the order of reactivity of Grignard's reagents will be as follows :
RMgI > RMgBr > RMgCl
3.2. Reactivity order with respect to active H
Acidic H > – CHO > > > >R–X
Except X (halogen) all other functional groups must be absent in the alkyl group otherwise. G.R. will be
destroyed by internal reactions.
[– NO2, – CN must also be absent]
4. Chemical Reaction
4.1 Reaction with acidic Hydrogen (H)
Compounds having reactive hydrogen or halogen atom give substitution products on reacting with Grignard's
reagent.
Z – H + R – MgX R – H + Z – MgX
(where Z is an atom or group having high electron attracting capacity)
Z – X + R – MgX Z – R + MgXX
(Where X and X are halogen atoms)
Both the above are examples of nucleophilic substitution reaction of Z – H and Z – X (not of the Grignard's
reagents).
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Examples :
R – MgX + H – Z R – H + ZMgX
CH3 – MgX + H – OH CH3 – H + Mg(OH)X
C2H5 – MgX + H – OR C2H5 – H + Mg(OR)X
C3H7 – MgX + H – OC6H5 C3H7 – H + Mg(OC6H5)X
C2H5 – MgX + H – NH2 C2H5 – H + Mg(NH2)X
CH3 – MgX + H – NHR CH3 – H + Mg(NHR)X
C2H5 – MgX + H – NR2 C2H5 – H + Mg(NR2)X
CH3 – MgX + H – NHC6H5 CH3 – H + Mg(NHC6H5)X
C2H5 – MgX + H – SR C2H5 – H + Mg(SR)X
CH3 – MgX + H – C N CH3 – H + Mg(CN)X
C2H5 – MgX + H – C CH C2H5 – H + HC C – MgX (Ethynylmagnesium halide)
CH3 – MgX + H – C CR CH3 – H + R – C C – MgX (Alkynylmagnesium halide)
Methane gas is released on reacting methylmagnesium iodide with a compound containing reactive
hydrogen atom. The reaction is used for estimation of reactive hydrogen atoms present in a molecule. This
method is called zerewitinoff method of estimation of reactive hydrogen atoms.
4.2 Reaction with alkyl halide : Coupling between a Grignard reagent and alkyl halide containing a
reactive halogen atom can be effected directly ; this reaction is probably SN2.
The yield is very good if R2 is allyl, t-butyl, or benzyl this reaction also works well for -halogenoethers.
(ii) With alkyl halide (coupling)
R – MgX + X – R R – R + MgXX
Ex. R – MgX + X CH2 CH CH2 R – CH2 – CH = CH2 + MgXX
Allyl halide
4.3 Reactions involving addition on polar Bond
Grignard's reagents form adducts by addition on the following types of pi bonds.
, , and C N , – N = O,
|
R – MgX + R C OMgX
|
Adduct
Example : With >C = O
ether
(1)
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H H
| |
HOH
(a) R – MgX + R C OMgX R C OH
| |
H H
1 Alcohol
R
|
(b) R – MgX + R C OH
|
H
2 Alcohol
R R
| |
(c) R – MgX + R C OMgX R C OH
| |
R R
3 Alcohol
4.4 With (Acid halide)
R 'MgX R 'MgX
H3 O
With
R ''MgX R ''MgX
H3 O + R'OMgX
4.5 With Epoxides
R CH2 CH2 R CH2 CH2
| |
OMgX OH
1 Alcohol
having two more C
CH3
|
SN2
Ex. R CH2 CH OH
(2)
CH 3
|
SN2
R CH 2 C OH
|
CH 3
( 3 )
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4.6 With Oxygen
R – MgX + O = O R – O – O – MgX
R – O – O – MgX + R – MgX 2R – O – MgX
R – O – MgX + HOH R – O – H + Mg(OH)X
Primary, secondary and tertiary alcohols can be obtained by above reaction.
Phenol is obtained on hydrolysis of the product obtained by reaction of arylmagnesium bromide with oxygen.
C6H5MgBr + O = O C6H5O – OMgBr
C6H5O – OMgBr + C6H5MgBr 2C6H5 – OMgBr
C6H5 – OMgBr + H2O C6H5 – OH + Mg(OH)Br
Other phenols can be prepared by taking any aryl (Ar) group in place of phenyl group.
OO Ar MgBr
Ar – MgBr Ar – O – OMgBr 2Ar – O – MgBr
HOH
Ar – OMgBr ArOH
For example, on taking p-toluenemagnesium bromide p-cresol is formed in the end.
(i) OO
(p)CH3 – C6H4 – MgBr ( (p ) CH3 C 6H4 OH
ii ) HOH p Cresol
p-Toluenemagnesium bromide
4.7 With CO2 (Synthesis of carboxylic acids)
A carboxylic acid is formed on hydrolysis of the adduct formed by passing carbon dioxide in the ethereal
solution of a Grignard's reagent.
HOH
R MgX C O R C OMgX R C OH
|| Mg(OH) X
|| ||
O O O
4.8 With R – C N
(1) rds H O
2
NH3 +
Applications :
G.R. is used to prepare alcohols from those alkyl halides / aryl halides which do not give normal SN reactions
Ex. (i)
(ii)
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H H H H
| | | |
Ex. R C OEt R C OEt R C R R CR
| || | | |
MgX O OMgX OMgX OH
Tertiary alcohols are obtained on hydrolysis of the product obtained by taking excess of Grignard's reagent
and adding an ester of a higher homologue of formic acid.
R R R R
| | | |
R C OEt R C OEt RCR R C R
| || | | |
MgX O OMgX OMgX OH
Various alcohols can be prepared by changing R in the above synthesis.
Di Grignard Reagent :
(a) C C C C C C–C–C–C=C
| |
X X
(1, 2)
(b) C C C C C
| |
X X
(1, 3)
(c) C C C C C
| |
X X
(1, 4)
(d) C C C C C
| |
X X
(1, 5)
Ex. Imp. Que. (JEE-95)
(1) HCl / R O
2 2 Ethyl acetate
C = C – C – C = C
( 2 ) Mg (3 )
Sol. Mg
CCCCC
| |
Cl Cl
HO
2
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