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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.svg8/2/2019 Car Bani On
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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.png8/2/2019 Car Bani On
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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.svg8/2/2019 Car Bani On
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Thank You
http://upload.wikimedia.org/wikipedia/commons/a/a7/Favorskii_Rearrangement_Scheme.png