Generation, Structure, Stability and Reactivity of Carbanions | Chemistry Optional Notes for UPSC PDF Download

 Page 1


 
____________________________________________________________________________________________________ 
 
 
  
 
 
 
Subject Chemistry 
Paper No and Title Paper 5: Organic Chemistry-II (Reaction Mechanism-1) 
Module No and Title Module 7: Generation, structure, stability and reactivity of 
carbanions 
  
 
 
 
Page 2


 
____________________________________________________________________________________________________ 
 
 
  
 
 
 
Subject Chemistry 
Paper No and Title Paper 5: Organic Chemistry-II (Reaction Mechanism-1) 
Module No and Title Module 7: Generation, structure, stability and reactivity of 
carbanions 
  
 
 
 
 
____________________________________________________________________________________________________ 
 
 
  
 
 
 
 
TABLE OF CONTENTS  
1. Learning Outcomes 
2.Introduction 
3. Generation of a carbanion 
3.1 Proton abstraction 
 3.2 Decarboxylation 
3.3 Addition of nucleophile to alkene 
3.4 Formation of organometallic compounds 
3.6 Ylides 
3.7 Dithiane 
3.8 Carbanions of weak CH acids 
3.9 Chiral carbanion 
 
4. Features of carbanion 
5. Stability of carbanion 
5.1 Inductive effect 
5.2 Extent of conjugation of the anion 
5.3 Hybridization of the charge-bearing atom 
5.4 Aromaticity 
6. Reactivity of carbanion 
6.1 Displacement reaction  
6.2 Elimination reaction  
6.3 Condensation reaction 
6.4 Addition reaction  
6.5 Rearrangement reaction 
6.6Anionic polymerisation reaction  
6.7 Wittig reaction 
7. Summary 
  
Page 3


 
____________________________________________________________________________________________________ 
 
 
  
 
 
 
Subject Chemistry 
Paper No and Title Paper 5: Organic Chemistry-II (Reaction Mechanism-1) 
Module No and Title Module 7: Generation, structure, stability and reactivity of 
carbanions 
  
 
 
 
 
____________________________________________________________________________________________________ 
 
 
  
 
 
 
 
TABLE OF CONTENTS  
1. Learning Outcomes 
2.Introduction 
3. Generation of a carbanion 
3.1 Proton abstraction 
 3.2 Decarboxylation 
3.3 Addition of nucleophile to alkene 
3.4 Formation of organometallic compounds 
3.6 Ylides 
3.7 Dithiane 
3.8 Carbanions of weak CH acids 
3.9 Chiral carbanion 
 
4. Features of carbanion 
5. Stability of carbanion 
5.1 Inductive effect 
5.2 Extent of conjugation of the anion 
5.3 Hybridization of the charge-bearing atom 
5.4 Aromaticity 
6. Reactivity of carbanion 
6.1 Displacement reaction  
6.2 Elimination reaction  
6.3 Condensation reaction 
6.4 Addition reaction  
6.5 Rearrangement reaction 
6.6Anionic polymerisation reaction  
6.7 Wittig reaction 
7. Summary 
  
 
____________________________________________________________________________________________________ 
 
 
  
 
 
1. Learning Outcomes 
After studying this module, you shall be able to 
• Know the various types of carbanions 
• Learn the features of carbanions 
• Learn the stability of carbanion 
• Identify different types of reaction involved in the formation of carbanions 
• Learn the reactivity of carbanions 
2. Introduction 
Heterolytic cleavage of a bond, where carbon retains both the shared pair of electrons results into 
the formation of a carbanion (i.e, carbon atom having negative charge).In these species, carbon 
atom carrying negative charge has eight electrons in the valence shell- six from three covalent 
bonds and two from lone pair of electrons. 
 
The basic form of carbanion is methide ion (CH
3 
-
) also commonly known as methyl carbanion. It 
is carbanion of methane (CH
4
) formed by loss of a proton (hydrogen ion, H
+
). 
. 
Nuceophilic carbon species are required to form new carbon- carbon bonds. Carbanions are 
known to be good nucleophiles.Thus, carbanions are important in chemical synthesis as 
intermediates and thus they are used in the preparation of other substances.Carbanions are used 
for making important industrial products such as plastics. 
Stable carbanions do exist. In 1984 Olmstead synthesised lithiumcrown ethersaltof the 
triphenylmethyl carbanion. It was obtained by adding n-butyllithium to triphenylmethane in THF 
at low temperatures followed by addition of 12-crown-4. 
Page 4


 
____________________________________________________________________________________________________ 
 
 
  
 
 
 
Subject Chemistry 
Paper No and Title Paper 5: Organic Chemistry-II (Reaction Mechanism-1) 
Module No and Title Module 7: Generation, structure, stability and reactivity of 
carbanions 
  
 
 
 
 
____________________________________________________________________________________________________ 
 
 
  
 
 
 
 
TABLE OF CONTENTS  
1. Learning Outcomes 
2.Introduction 
3. Generation of a carbanion 
3.1 Proton abstraction 
 3.2 Decarboxylation 
3.3 Addition of nucleophile to alkene 
3.4 Formation of organometallic compounds 
3.6 Ylides 
3.7 Dithiane 
3.8 Carbanions of weak CH acids 
3.9 Chiral carbanion 
 
4. Features of carbanion 
5. Stability of carbanion 
5.1 Inductive effect 
5.2 Extent of conjugation of the anion 
5.3 Hybridization of the charge-bearing atom 
5.4 Aromaticity 
6. Reactivity of carbanion 
6.1 Displacement reaction  
6.2 Elimination reaction  
6.3 Condensation reaction 
6.4 Addition reaction  
6.5 Rearrangement reaction 
6.6Anionic polymerisation reaction  
6.7 Wittig reaction 
7. Summary 
  
 
____________________________________________________________________________________________________ 
 
 
  
 
 
1. Learning Outcomes 
After studying this module, you shall be able to 
• Know the various types of carbanions 
• Learn the features of carbanions 
• Learn the stability of carbanion 
• Identify different types of reaction involved in the formation of carbanions 
• Learn the reactivity of carbanions 
2. Introduction 
Heterolytic cleavage of a bond, where carbon retains both the shared pair of electrons results into 
the formation of a carbanion (i.e, carbon atom having negative charge).In these species, carbon 
atom carrying negative charge has eight electrons in the valence shell- six from three covalent 
bonds and two from lone pair of electrons. 
 
The basic form of carbanion is methide ion (CH
3 
-
) also commonly known as methyl carbanion. It 
is carbanion of methane (CH
4
) formed by loss of a proton (hydrogen ion, H
+
). 
. 
Nuceophilic carbon species are required to form new carbon- carbon bonds. Carbanions are 
known to be good nucleophiles.Thus, carbanions are important in chemical synthesis as 
intermediates and thus they are used in the preparation of other substances.Carbanions are used 
for making important industrial products such as plastics. 
Stable carbanions do exist. In 1984 Olmstead synthesised lithiumcrown ethersaltof the 
triphenylmethyl carbanion. It was obtained by adding n-butyllithium to triphenylmethane in THF 
at low temperatures followed by addition of 12-crown-4. 
 
____________________________________________________________________________________________________ 
 
 
  
 
 
 
3. Generation of Carbanion 
Carbanion are generated as intermediate in various organic reactions. Some of the methods for 
the generation of carbanion are: 
• Proton abstraction 
• Decarboxylation 
• Addition of nucleophile to alkene 
• Formation of organometallic compounds 
 
3.1 Proton abstraction 
When proton is abstracted from a carbon centre then the resulting anion is called a carbanion. 
 
The acidic hydrogen of an organic substrate can be abstracted by an appropriate base. For 
example carbanion generated from carbonyl compounds. 
Here, are some examples showing generation of carbanion by abstraction of the acidic proton 
using a base (OH
-
, NH
2
-
, RO
-
). 
 
Page 5


 
____________________________________________________________________________________________________ 
 
 
  
 
 
 
Subject Chemistry 
Paper No and Title Paper 5: Organic Chemistry-II (Reaction Mechanism-1) 
Module No and Title Module 7: Generation, structure, stability and reactivity of 
carbanions 
  
 
 
 
 
____________________________________________________________________________________________________ 
 
 
  
 
 
 
 
TABLE OF CONTENTS  
1. Learning Outcomes 
2.Introduction 
3. Generation of a carbanion 
3.1 Proton abstraction 
 3.2 Decarboxylation 
3.3 Addition of nucleophile to alkene 
3.4 Formation of organometallic compounds 
3.6 Ylides 
3.7 Dithiane 
3.8 Carbanions of weak CH acids 
3.9 Chiral carbanion 
 
4. Features of carbanion 
5. Stability of carbanion 
5.1 Inductive effect 
5.2 Extent of conjugation of the anion 
5.3 Hybridization of the charge-bearing atom 
5.4 Aromaticity 
6. Reactivity of carbanion 
6.1 Displacement reaction  
6.2 Elimination reaction  
6.3 Condensation reaction 
6.4 Addition reaction  
6.5 Rearrangement reaction 
6.6Anionic polymerisation reaction  
6.7 Wittig reaction 
7. Summary 
  
 
____________________________________________________________________________________________________ 
 
 
  
 
 
1. Learning Outcomes 
After studying this module, you shall be able to 
• Know the various types of carbanions 
• Learn the features of carbanions 
• Learn the stability of carbanion 
• Identify different types of reaction involved in the formation of carbanions 
• Learn the reactivity of carbanions 
2. Introduction 
Heterolytic cleavage of a bond, where carbon retains both the shared pair of electrons results into 
the formation of a carbanion (i.e, carbon atom having negative charge).In these species, carbon 
atom carrying negative charge has eight electrons in the valence shell- six from three covalent 
bonds and two from lone pair of electrons. 
 
The basic form of carbanion is methide ion (CH
3 
-
) also commonly known as methyl carbanion. It 
is carbanion of methane (CH
4
) formed by loss of a proton (hydrogen ion, H
+
). 
. 
Nuceophilic carbon species are required to form new carbon- carbon bonds. Carbanions are 
known to be good nucleophiles.Thus, carbanions are important in chemical synthesis as 
intermediates and thus they are used in the preparation of other substances.Carbanions are used 
for making important industrial products such as plastics. 
Stable carbanions do exist. In 1984 Olmstead synthesised lithiumcrown ethersaltof the 
triphenylmethyl carbanion. It was obtained by adding n-butyllithium to triphenylmethane in THF 
at low temperatures followed by addition of 12-crown-4. 
 
____________________________________________________________________________________________________ 
 
 
  
 
 
 
3. Generation of Carbanion 
Carbanion are generated as intermediate in various organic reactions. Some of the methods for 
the generation of carbanion are: 
• Proton abstraction 
• Decarboxylation 
• Addition of nucleophile to alkene 
• Formation of organometallic compounds 
 
3.1 Proton abstraction 
When proton is abstracted from a carbon centre then the resulting anion is called a carbanion. 
 
The acidic hydrogen of an organic substrate can be abstracted by an appropriate base. For 
example carbanion generated from carbonyl compounds. 
Here, are some examples showing generation of carbanion by abstraction of the acidic proton 
using a base (OH
-
, NH
2
-
, RO
-
). 
 
 
____________________________________________________________________________________________________ 
 
 
  
 
 
3.2 Decarboxylation 
Decarboxylation of carboxylates leads to formation of carbanion intermediate.  
 
 
3.3 Addition of nucleophile to alkene 
Carbanion are generated by the attack of nucleophiles on one of the carbon of an alkene. It results 
into the development of negative charge on the other carbon atom. 
 
3.4 Formation of organometallic compounds 
Metals which are less electronegative than carbon (such as magnesium, lithium, potassium, 
sodium, zinc, mercury, lead, thallium) react with alkyl halides under appropriate conditions to 
form a carbon-metal bondwhere the carbon carries negative charge and metal positive charge. 
Although the carbon does not carry full negative charge but it acts like a carbanion in its 
reactions. Thus, metallation reverse the polarity of the carbon from positive in reactant to 
negative in the organometallic compound this is known as umpolung. 
 
For example, alkyl bromides react with magnesium in the presence of dry diethyl ether to form 
alkyl magnesium halides also known as Grignard reagent. 
 
 
 
Reaction of alkyl halides with lithium, form alkyl lithium having negative charge on carbon. 
 
Reaction of acetylene with sodium in liquid ammonia results into the formation of sodium 
acetylide.  
 
 
3.6 Ylides 
Ylidesgenerated by the reaction of triphenyphosphine with alkyl halides are also carbanion like 
species.