carban ion
TRANSCRIPT
Bond Cleavage
−+→ B:AB:A A : B A– : B+
+vely charged ion – carbocation-vely charged ion – carbaanion
Heterolytic Cleavage
• Reactant intermediate product
A very important way of generating carbon nucleophiles involves removal of a proton from a
carbon by a base. The anions produced are carbanions.
The negative charge gives good nucleophilic properties and it can be used in the formation of new
carbon carbon bond.
Carbanion• Pyramidal - sp3 hybridised
bond angle 109.28.geometery is thus tetrahedral
• Has eight electrons
• Stabilized by resonance or by inductive effect.
. . sp3 hybrid orbitalcontaining lone pair
Tetrahedral structure of carboanion
•The efficient generation of a significant equilibrium concentration of
a carbanion requires choice of a proper base.
•The equilibrium will favor carbanion formation only when the
acidity of the carbon acid is greater than that of the conjugate acid
corresponding to the base used for deprotonation.
O
CH3R+ + H2OOH-
O-
R CH2
Strong base, but it is sufficiently bulky so as to be relatively nonnucleophilic.
K>1 mean more product.K<1 mean more reactent.
Stability of Carbanion depends upon three factors:
(i) By resonance
H
-
H
-
H
Cyclopentadienyl carbanion
Stability of Carbanion(ii) By inductive
CH3C H
CH3
CH3
CH3 C
CH3
H
C
CH3
H
3° 2° 1°
Stability of Carbanion(iii) Electron-donating groups destabilize a carbanion while electron-withdrawing groups stabilize it.
N O 2 3O C H
>
−2C H −
2C H
• An ordering of some important substituents with respect to their ability to stabilize carbanion can be established.
NO2> COR>CN-CO2R>SOR>Ph-SR>H>R
• F stabilizes CH2X−
more effectively than Cl, Br, and I because of the fluorine electronegativity.
Consider the elimination of 2-fluoropentane …
A carbanion-like transition state
Same typical examples of proton abstraction equilibria
O
R'R
O
R'R+ NH2
- + NH3
O
OR'R
O
OR'R+ NR''2
-+ HNR''2
O
OR'+ R'O- + R'OH
R'O
O O
OR'R'O
O
O
OR'C + R'O- + R'OH
(-)
O
OR'C
(-)N N
NO2R + OH- + H2O
(-)
(-)
(-)NO2R
O
OR'+ R'O- + R'OH
O O
OR'R'O
O
(-)
Electron delocalization in the corresponding carbanions
O
R'R
O
OR'R
O
OR'R'O
O
(-)
O
OR'C
(-)N
(-) (-)
(-)N+R
O-
R'R
O-
OR'R
O-
OR'R'O
O O
OR'R'O
O-
O
OR'
O
(-)
O-
OR'
O O
OR'
O-
O-
OR'CN N C C
OR'
O
O
O- N+R
O-
O-
;
carbon nucleophiles.
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Organometallic Compounds
An organic compound containing a carbon–metal bond
Organolithium compounds and organomagnesium compounds are two of the most common organometallic compounds
19
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Coupling Reactions
Example of carbon–carbon formation using the Gilman reaction:
Alkylation of Alkynes:
Anions of Terminal Alkynes
CYANIDE ANIONS:
The witting reaction:
Wittig reported a method for the synthesis of alkenes from aldehydesand ketones using compounds called phosphonium
ylides.
m
EnolatesThe anions formed by deprotonation of the carbon
alpha to a carbonyl group bear most of their negative charge on oxygen and are referred to as
enolates
enolate anion: Enolate are important intermediate because they react at carbon to create new carbon-carbon
bonds in two types of reactions.
First, they can function as nucleophiles substitution reactions.
• Second, they function as nucleophiles addition. in carbonyl addition reactions
Why Carbon of enolate ion attack?
Kinetic controlExperimental conditions underwhich the composition of theproduct mixture is determined bythe relative rates of formation ofeach product.
Thermodynamic control
Experimental conditions that permit the establishment of equilibrium between two or more products of a reaction. The composition of the product mixture is determined by the relative stabilities of the products.
Kinetic vs. Thermodynamic Enolates
Kinetic vs. Thermodynamic Enolates
a) Aldol condensation. The reaction of an aldehyde or ketone with dilute base or acid to form a beta-hydroxycarbonyl product.
CH3CH=Odil. NaOH
CH3CHCH2CH O
OH
acetaldehyde 3-hydroxybutanal
CH3CCH3
Odil. NaOH
CH3CCH2CCH3
OOH
CH3acetone
4-hydroxy-4-methyl-2-pentanone
H3CC
CH3
O
OH
H3CC
CH2
O
H3CC
CH3
O
H3CC
O
CH2
C
O
CH3
CH3
+ H2O
+ H2O
H3CC
O
CH2
C
OH
CH3
CH3dil. NaOH