Carban ion

carbon carries negative charge

1. Forman Christian College
A chartered University
Furqan Alee
14-10239
[email protected]

3. Bond Cleavage
-+
¾®¾ B:AB:A A : B A
–
: B
+
+
vely
charged ion – carbocation
-
vely
charged ion – carbaanion

Heterolytic Cleavage
•Reactant intermediate product

4. 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.

5. Carbanion
•Pyramidal - sp
3
hybridised
bond angle 109.28.geometery is thus tetrahedral

•Has eight electrons
•Stabilized by resonance or by inductive effect.
. .
sp
3
hybrid orbital
containing lone pair
Tetrahedral structure of carboanion

6. •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
CH
3R
+ + H
2OOH
-
O
-
RCH
2

7. Strong base, but it is sufficiently bulky so as to be relatively nonnucleophilic.
K>1 mean more product.
K<1 mean more reactent.

8. Stability of Carbanion
depends upon three factors:
(i) By resonance
H
-
H
-
H
Cyclopentadienyl carbanion

9. Stability of Carbanion
(ii) By inductive
CH
3
C H
CH
3
CH
3
CH
3
C
CH
3
H
C
CH
3
H
3° 2° 1°

10. Stability of Carbanion
(iii) Electron-donating groups destabilize
a carbanion while electron-withdrawing
groups stabilize it.
NO
2 3O C H
>
-
2C H
-
2C H

11. •An ordering of some important substituents
with respect to their ability to stabilize
carbanion can be established.
NO
2
> COR>CN-CO
2
R>SOR>Ph-SR>H>R

12. • F stabilizes CH
2
X
−
more effectively than
Cl, Br, and I because
of the fluorine
electronegativity.

13. Consider the elimination of 2-fluoropentane …

14. A carbanion-like transition state

15. Same typical examples of proton abstraction
equilibria
O
R'
R
O
R'
R+ NH
2
-
+NH
3
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
NO
2R + OH
-
+H
2O
(-)
(-)
(-)
NO
2R
O
OR'
+ R'O
-
+R'OH
O
O
OR'R'O
O
(-)

16. 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
NCC
OR'
O
O
O
- N
+
R
O
-
O
-
;

17. carbon nucleophiles.

18. 18
Organometallic Compounds
An organic compound containing a carbon–metal bond
Organolithium compounds and organomagnesium
compounds are two of the most common organometallic
compounds

19. 19

20. 20
Coupling Reactions
Example of carbon–carbon formation using the
Gilman reaction:

21. Alkylation of Alkynes:

22. Anions of Terminal Alkynes

23. CYANIDE ANIONS:

24. The witting reaction:
Wittig reported a method for the synthesis of alkenes from
aldehydesand ketones using compounds called phosphonium
ylides.

25. m

26. Enolates
The 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

28. 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

29. Why Carbon of enolate ion
attack?

30. Kinetic control
Experimental conditions under
which the composition of the
product mixture is determined by
the relative rates of formation of
each 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

31. Kinetic vs. Thermodynamic
Enolates

32. a) Aldol condensation. The reaction of an aldehyde or ketone
with dilute base or acid to form a beta-hydroxycarbonyl product.
CH
3CH=O
dil. NaOH
CH
3CHCH
2CHO
OH
acetaldehyde 3-hydroxybutanal
CH
3CCH
3
O
dil. NaOH
CH
3CCH
2CCH
3
OOH
CH
3
acetone
4-hydroxy-4-methyl-2-pentanone

33. H
3C
C
CH
3
O
OH
H
3C
C
CH
2
O
H
3C
C
CH
3
O
H
3C
C
O
C
H
2
C
O
CH
3
CH
3
+ H
2O
+ H
2O
H
3C
C
O
C
H
2
C
OH
CH
3
CH
3
dil. NaOH