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As a reactive intermediate

CARBANION


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Carbanion

Carbanions are units that contain a negative chargeon a carbon atom

In many reactions bond breaks heterolytically andnegative charge comes on Carbon that trivalentcarbon having negative charge is called Carbanion


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Geometry of Carbanion

Structurally, carbanions have a tetrahedral molecular

geometry if the lone pair is counted. This means thatcarbon, the central atom, is symmetricallysurrounded by the electron orbitals in the shape of atetrahedron.


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Geometry of Carbanion TRIGONAL PYRAMIDAL

If only the three bonds are included in thegeometry, the molecule is trigonal pyramidal, withthe carbon atom at the apex of the pyramid, thethree bonded atoms forming the base, and the

lone pair floating on top.
http://en.wikipedia.org/wiki/File:Carbanion_Structural_Formulae_V.1.svg


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Sources of carbanion

Organolithium reagents

n-BuLi, PhLi, MeLi commercially available

t-BuLi > sec-BuLi> n-BuLi in base strength

Bu-Br + 2 Li c o l d Bu-Li + LiBr

ether or hexane

H3CH2C Li H3C CH2+ Li


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2. Carbon Acids


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3. Enolization


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Stability

1. Hybridization

More s character, more acidic is the molecule

S character pulls the charge towards nucleus. Sorbitals are closer to the nucleus than corresponding porbitals , and they are at a lower energy level. Theelectron pair in sp orbital is held closer to, and moretightly by the carbon atom than an electron pair in sp2and sp3 orbital. This serves not only to make H atomlose more easily without its electron pair I,e., moreacidic, but also to stabilize the resultant Carbanion.


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2. Inductive Effects

Electron-withdrawing substituents will inductivelystabilize negative charge on nearby carbons.


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3. Conjugation - Delocalization Delocalization of negative charge, especially onto

electronegative atoms, provides potent stabilizationsof carbanionic centers. Since almost all conjugatingsubstituents are also more electronegative than H orCH3, there is usually a significant inductive

contribution to the stabilization.


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4. Second and Third Row ElementEffects ("d-orbital" effects)

All measures of acidity show that there is an unusuallevel of carbanion stabilization for all second rowelements (Cl, S, P, Si, as well as higher elements)when these are bonded to a carbanion center.


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5. Lone Pair Effects

For the first row elements N, O, F, andperhaps also for higher elements, the

presence of lone pairs has a strongdestabilizing effect on a directly bondedcarbanion center.


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Reactions of Carbanion


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1.Aldol condensation

CH3CH=Odil. NaOH

CH3CHCH2CH O

OH

acetaldehyde 3-hydroxybutanal

OH

CH2CH=O CH3CH+ OCH3CHCH2CH O

O

+ H2O

+ H2O


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Ketones

H3CC

CH3

O

OH

H3C C CH2

O

H3C C CH3

O

H3C C

O

CH2

C

O

CH3

CH3

+ H2O

+ H2O

H3CC

O

CH2

C

OH

CH3

CH3dil. NaOH


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Cyclic compounds

O

dil. OH-

O

OH

OH

O

O

O

O + HOH


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Acidic Medium

dil. H+

O

+ H2O

O

With dilute acid the final product is the ,-unsaturatedcarbonyl compound!

CH2CH O

phenylacetaldehyde

dil NaOH CH2 CH

CH

OH

CH=O

dilute H+

CH2 CH

C CH=O

note: double bond is conjugated

with the carbonyl group!

+ H2O


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In general, in these reactions the concentration ofbase has to be controlled. With higher concentration,

dehydration of aldol products occurs rather readilydue to the formation of stable conjugatedunsaturated carbonyl compounds.


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2. Claisian condensation


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3. Substitution reactions

Malonate synthesis of carboxylic acids.

1. Diethyl malonate has acidic alpha-hydrogens

2. When reacted with sodium metal, the ester isconverted into its conjugate base (an enolate

anion)

CO2CH2CH3

CH2

CO2CH

2CH

3

CO2CH

2CH

3CH2

CO2CH2CH3

NaCO

2CH

2CH

3CH

CO2CH2CH3

+ Na+ + H2


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3. The enolate can be used as the nucleophile in an

SN2 reaction with a 1o

or CH3 alkyl halide.

4. Upon hydrolysis, the substituted malonic acid willdecarboxylate when heated.

5. Product is a carboxylic acid derived from aceticacid.

CO2Et

CH

CO2Et

+ R-XSN2 CO2Et

CH

CO2Et

R

CO2Et

CHCO2Et

R

H2O, H+

heat

CO2H

CHCO2H

R

- CO2

heat CH2CO2HR


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CCH2

O

C

O

OEt

OEt

diethyl malonate

Na CCH

O

C

O

OEt

OEtNa

RX CCH

O

C

O

OEt

OEtR

H+,H2O

heat

CCH

O

C

O

OH

OHR

heat

-CO2

CH2COOH

Na

C

C

O

C

O

OEt

OEt

R

R'XC

C

O

C

O

OEt

OEt

R

H+,H2O

heat

C

C

O

C

O

OH

OH

R

-CO2heat

R

CHCOOHRR'

R'R'


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4. Michael addition reaction


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5. Organomettalic compounds

1. With Formaldehyde: product is 1o alcohol

2. With Aldehydes R'CHO: product is a 2o alcohol with R and

R' on the a-C.


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3. With Ketones R'COR": product is a 3o alcohol

4. With Esters: product is a 3o alcohol with 2 identical a-substituents.


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6. Wurtz reaction One electron from the metal is transferred to the halogen

to produce a metal halide and an alkyl radical.

R-X + M R + M+X

The alkyl radical then accepts an electron from anothermetal atom to form an alkyl anion and the metal becomescationic. This intermediate has been isolated in a severalcases.

R + M RM+

The nucleophilic carbon of the alkyl anion then displacesthe halide in an SN2 reaction, forming a new carbon-carbon covalent bond.

R

M+

+ R-X R

-R + M+

X

Limitations

The Wurtz reaction is limited to the synthesis ofsymmetric alkanes. If two dissimilar alkyl halides are

taken as reactants, then the product is a mixture ofalkanes that is often difficult to se arate


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Corey-House Synthesis

R X + 2 Li

diethylether(Et2O) R Li + LiX

alkyl lithium

May be a primary, secondary or tertiary alkyl halide

H3C

H2C

CH2

H2C

Cl H3C

H2C

CH

CH3

Cl

H3C

C

CH3

CH3

Cl

e.g.

R Li2 + CuI R2 CuLi + LiI

Lithium dialkyl cuprate

R2 CuLi + R' X R R' + LiX + R Cu

May be a methyl halide, primary halide or a secondary cyclic halide

H3C X

H3C

H2C

CH2

H2C

Cl

e.g.H2C

H2C

CH2

CH

H2C

Br


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7. Beta elimination of vicinal dihalides

Dehalogenation of vicinal dihalides to form

alkenes: Beta Elimination. a) Overall reaction:

b) Mechanism:

Example:


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8. Dickmann condensation


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9. Reformatsky reaction


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10. Favorskii reaction
http://upload.wikimedia.org/wikipedia/commons/a/a7/Favorskii_Rearrangement_Scheme.pnghttp://en.wikipedia.org/wiki/File:Favorskii_Rearrangement_Mechanism.png


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11. Reimer Tiemenn Reaction
http://upload.wikimedia.org/wikipedia/commons/a/a7/Favorskii_Rearrangement_Scheme.pnghttp://upload.wikimedia.org/wikipedia/commons/4/43/Reimer_Tiemann_Reaktion_mechanismus.svg


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Thank You
http://upload.wikimedia.org/wikipedia/commons/a/a7/Favorskii_Rearrangement_Scheme.png

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