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Home , Carbanion

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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 Module Tag CHE_P5_M7

CHEMISTRY PAPER No.5:Organic Chemistry-II (Reaction Mechanism-1) MODULE No.7: Generation, structure, stability and reactivity of carbanions

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TABLE OF CONTENTS

1. Learning Outcomes 2.Introduction 3. Generation of a carbanion 3.1 Proton abstraction 3.2 Decarboxylation 3.3 Addition of to 3.4 Formation of organometallic compounds 3.6 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 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

CHEMISTRY PAPER No.5:Organic Chemistry-II (Reaction Mechanism-1) MODULE No.7: Generation, structure, stability and reactivity of carbanions

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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 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 of electrons.

─ The basic form of carbanion is methide (CH3 ) also commonly known as methyl carbanion. It + is carbanion of (CH4) formed by loss of a proton (hydrogen ion, H ).

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Nuceophilic carbon species are required to form new carbon- carbon bonds. Carbanions are known to be good .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.

CHEMISTRY PAPER No.5:Organic Chemistry-II (Reaction Mechanism-1) MODULE No.7: Generation, structure, stability and reactivity of carbanions

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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 , NH2 , RO ).

CHEMISTRY PAPER No.5:Organic Chemistry-II (Reaction Mechanism-1) MODULE No.7: Generation, structure, stability and reactivity of carbanions

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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, , potassium, , zinc, mercury, lead, thallium) react with 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 .

For example, alkyl bromides react with magnesium in the presence of dry diethyl ether to form alkyl magnesium halides also known as .

Reaction of alkyl halides with lithium, form alkyl lithium having negative charge on carbon.

Reaction of acetylene with sodium in liquid results into the formation of sodium acetylide.

3.6 Ylides Ylidesgenerated by the reaction of triphenyphosphine with alkyl halides are also carbanion like species.

CHEMISTRY PAPER No.5:Organic Chemistry-II (Reaction Mechanism-1) MODULE No.7: Generation, structure, stability and reactivity of carbanions

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3.7 Dithiane Dithianeare generated by the reaction of propane thiol with carbonyl compound. The dithianes on further treatment with butyl lithium gives a dithiane anion which is a carbanion.

3.8 Carbanions of weak CH acids The fluoride ion has the ability to cleave a proton from those CH acids whose pKa value ranges from 20 to 28 and forms carbanions of weak CH acids. For example, phenylacetylene when treated with a I M solution of tetraethylammonium fluoride (TEAF.2H2O) in CH3CN forms the phenylacetylene anion.

3.9 Chiral Carbanion

Chiral carbanion can be generated from a chiral substrate.

4. Features of Carbanion

• Carbanion possess an unshared pair of electron thus it is electron rich

• Carbanion has octet of electron in outer shell • The negatively charged carbon is trivalent • If all the sustituents on the negatively charged carbon are different the carbanion will be chiral • Carbanion act as base or nucleophile • Carbanion has a pyramidal geometry CHEMISTRY PAPER No.5:Organic Chemistry-II (Reaction Mechanism-1) MODULE No.7: Generation, structure, stability and reactivity of carbanions

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• Carbanion undergoes rapid interconversion between two pyramidal forms. The energy barrier is different for different types of carbanions. For example for a methyl carbanion the energy barrier is 2 kcal/mol, while for trifluoromethyl carbanion value is around 120 kcal/mol. The higher energy barrier of trifluoromethyl carbanion is due to the more electronegativity of fluorine atom which is more stabilizing than a hydrogen atom.

• The negatively charged saturated carbon atom is sp3 hybridized in carbanion

• The lone pair of electrons occupies one of the sp3 hybrid orbitals. • Since the lone pair-bond pair repulsions are expected to be greater than bond pair-bond pair repulsions, the bond angles in carbanions are much less than regular tetrahedral angle of 109o28’. In other words, carbanions do not have regular tetrahedral geometry but have pyramidal structure in which bond angles are in between 97o – 100o. • Stability order of alkyl carbanions is methyl>1o>2o>3o • Stable bridge head carbanions are also known.

5. Stability of Carbanion

Factors which can stabilize or disperse the negative charge on carbon will stabilize a carbanion. The stability of carbanion depends on the following factors: • Inductive effect • Extent of conjugation of the anion • Hybridization of the charge-bearing atom • Aromaticity

5.1 Inductive effect If the groups attached to carbanion are electron releasing in nature they will increase the negative charge on carbon and thus destabilize it. However, electronegative atoms or electron withdrawing groups adjacent to the negatively charged carbon will stabilize the carbanion.

CHEMISTRY PAPER No.5:Organic Chemistry-II (Reaction Mechanism-1) MODULE No.7: Generation, structure, stability and reactivity of carbanions

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The alkyl groups are electron releasing in nature due to inductive effect (+I). More the number of alkyl groups attached lesser will be the stability. Carbanions prefer a lesser degree of alkyl substitution. Therefore the order of stability order of alkyl carbanion is methyl>1o>2o>3o.

Presence of electronegative atoms (F, Cl, Br) or electron withdrawing groups (NO2, CN, COOH, CO) close to the negatively charged carbon will stabilize the charge. Thus more the number of such groups in a carbanion greater will be the stability.

5.2 Extent of conjugation of the anion If negatively charged carbon is in conjugation with a double bond the effects will stabilize the anion by spreading out the charge by rearranging the electron pairs.

Which is more stable amongst benzyl and allyl carbanions? The negative charge is delocalized through resonance in both benzyl and allyl anions. But benzyl anion has more number of contributing structures. Thus, benzyl anion is more stable than allyl anion.

Allyl anion:

Benzyl anion:

CHEMISTRY PAPER No.5:Organic Chemistry-II (Reaction Mechanism-1) MODULE No.7: Generation, structure, stability and reactivity of carbanions

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Resonance in some carbanions:

5.3 Hybridization of the charge bearing atom

Stability of anion will depend upon the s character of carbanion i.e. more the s character, higher will be the stability of anion. The percentage s character in the hybrid orbitals is as follows: sp(50%)> sp2(33%)>sp3(25%).

CHEMISTRY PAPER No.5:Organic Chemistry-II (Reaction Mechanism-1) MODULE No.7: Generation, structure, stability and reactivity of carbanions

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Explanation of the order of stability:

Propargyl anion>propenyl anion>propyl anion

The reason for this order can be understood as follows. In propargyl anion, the triply bonded carbon is sp hybridized, in propenyl anion the doubly bonded carbon is sp2 hybridized while in propyl anion the carbon is sp3 hybridized. Orbital with greater s character is more close to the nucleus and feels more nuclear charge. The sp hybridized atoms (50% s character) are more electronegative than sp2(33% s character) and sp3(25% s character). The distance of lone pair and nucleus is less if the lone pair is sp hybridized than in a sp2 hybrid orbital. Since, it is more favourable for the negative charge of an anion to be in an orbital close to the positively charged nucleus. Therefore sp hybrid anion is more stable than sp2.

5.4Aromaticity In some carbanions, the lone pair of electrons of the negative charge is involved in delocalization to add on to the aromatic character of the molecule which gives them extra stability. For example, in cyclopentadienyl anion there are 6 π electron and thus it obeys Huckel rule, (4n+2) π electron. It has pKa value 16. This anion is stabilized by aromatization.

Cyclooctatetraene on reaction with potassium gets converted to cyclooctatetraenyldianion potassium . This is 10 π electron system which is stable due to aromaticity.

CHEMISTRY PAPER No.5:Organic Chemistry-II (Reaction Mechanism-1) MODULE No.7: Generation, structure, stability and reactivity of carbanions

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6. Reactivity of Carbanion

Carbocations are strong Lewis acids while carbanions are strong bases (Lewis and Bronsted bases). Carbanions are part of most of the common reaction types such as displacement, elimination, condensation, addition, rearrangement, polymerisation etc.

6.1 Displacement Reaction

Carbanion can combine with positive species in a displacement reaction.

6.2 Elimination In a Conjugate Base Elimination reaction (E1cb) the C-H bond breaks with formation of carbanion as intermediate. The developed negative charge on carbon assist in the loss of , leading to the formation of alkene.

6.3 Condensation reactions

6.3.1 : Reaction of aldehydes or ketones to give β-hydroxy carbonyl compounds is known as . The reaction can be base or acid catalyzed. The base catalyzed reaction is more common.

CHEMISTRY PAPER No.5:Organic Chemistry-II (Reaction Mechanism-1) MODULE No.7: Generation, structure, stability and reactivity of carbanions

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In a base catalyzed reaction the carbonyl compound is deprotonated at the α-carbon by base (alkali ) to give carbanion which is resonance stabilized with the anion.

The next step is the nucleophilic attack of the carbanion to the of another, non- enolized, aldehyde molecule. The product which is obtained after workup is a β– hydroxyaldehyde or ketone.

6.3.2 Claisen condensation: Formation of β-keto esters from carboxylic esters is known as Clasien ester condensation.

The mechanism involves the formation of carbanionfrom the ester(ethyl acetoacetate) by reaction with base.

The carbanion generated can add to another ester molecule. The resulting anionic species results into β-keto ester by loss of an alkoxide anion.

6.3.3 Dieckmann condensation: It is intramolecular condensation of diesters to give cyclic β- keto ester. The base abstracts α-proton to one of the ester group to form a carbanion. The carbanion then undergoesnucleophilic attack at the carbonyl carbon of other ester group present within the same molecule. The product obtained is a cyclic ketone.

CHEMISTRY PAPER No.5:Organic Chemistry-II (Reaction Mechanism-1) MODULE No.7: Generation, structure, stability and reactivity of carbanions

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6.4 Addition Reaction

6.4.1 : Addition of a carbanion to α,β-unsaturated carbonyl compounds is called the Michael reaction or Michael addition.

6.5 Rearrangement reactions

6.5.1 Favoroskii rearrangement: The rearrangement of cyclopropanones and α-halo ketones in the presence of base to derivatives is known as Favorskii rearrangement.

For example, in cyclic α-halo ketones, reaction involve abstraction of alpha proton by base to give a carbanion intermediate, which is resonance stabilized with its enolate form. This carbanion or enolate cyclize to a cyclopropanone intermediate, which is then attacked by the hydroxide nucleophile to for carboxylate derivative.

CHEMISTRY PAPER No.5:Organic Chemistry-II (Reaction Mechanism-1) MODULE No.7: Generation, structure, stability and reactivity of carbanions

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6.6Anionic Polymerization

Polymersisation involving anionic intermediates are known. For example, anionic polymerization of styrene in presence of sodamide proceeds with the formation of a carbanion intermediate in the chain initiation and chain propagation steps.

6.7 Wittig reaction: In a Wittig reaction, or olefins are formed by the reaction of phosphoniumylides with aldehydes or ketones. The ylides have carbanion character.

CHEMISTRY PAPER No.5:Organic Chemistry-II (Reaction Mechanism-1) MODULE No.7: Generation, structure, stability and reactivity of carbanions

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6.8 Dithiane (Umploung reaction)

Dithiane on reaction with butyl lithium gives an anion which reacts with alkyl halides, this on subsequent hydrolysis gives carbonyl compounds.

Some other important reactions which involve carbanion intermediate are:

• Claisen-Schmidt reaction • Benzoin condensation • Perkin reaction • Mannich reaction • Malonic acid synthesis • Acetoacetic acid synthesis

7. Summary

• Heterolytic cleavage of a bond with shared pair of electrons on carbon gives a carbanion. • Carbanion has pyramidal geometry • Factors affecting stability of carbanion are inductive effect, extent of conjugation of the anion, hybridization of the charge bearing atom, aromaticity. • Carbanions are part of most of the common reaction types such as displacement, elimination, condensation, addition, rearrangement, polymerisation etc. • Some example of reactions involving carbanion intermediate are aldol condensation, Claisen condensation, Favoroskii rearrangement, Wittig reaction, Benzoin condensation, Perkin reaction, Mannich reaction.

CHEMISTRY PAPER No.5:Organic Chemistry-II (Reaction Mechanism-1) MODULE No.7: Generation, structure, stability and reactivity of carbanions