Carbene%20ppts%20(1)@[1].pptx

2. CARBENE

3. Presenters
Usama Sajjad = 005
Zoha Mushtaq = 020
Sania Zahra = 035
Amina = 040
AneesNoor = 048
Zainab = 54
Sonaila = 56

4. Presented to = Mam . Ayesha
Amjad
From BS Chemistry 7th Group 9

5. Content List
• Intermediate Reaction
• Carbenes
• Types Of Carbenes
• Synthesis of Carbenes
• Reactions of Carbenes
• Applications Of
Carbenes

6. Intermediate Reaction
• Species Produced in between the reactant and products
during the reaction
• Produced due to cleavage of Covalent bond
• Short lived
• Highly Reactive Species

7. General Reactions
•
• A (Reactant ) B ( Product )
• A (Reactant) X ( Intermediate ) B ( Product )
Intermediate Reactions

8. Types Of Intermediate Reactions
• Free Radicals Carbocation Carbanions Carbenes
• C C C :CH2

9. Carbenes
• A carbenes is a molecule containing neutral Carbon
atoms with a valence of two and two unshared valence
electrons
• Carbenes are uncharged , electron deficient molecular
species that contain a divalent carbon atom surrounded
by a sextet of electrons , and two substituents .

10. • It is often called Methylene because it is derivatives from Methane
• It is strongly reactive ( Carbenes are named by indicating the
substituents attached to divalent carbon . E.g :CCl2 ,
Dichlorocarbene ( :CHPH ) .
• Short lived time .
• Highly unstable and strongly reactive .

11. α-elimination of HX or X2 from an organic
halide
• α- elimination means the elimination in which both the protons and the
leaving groups are located on the same atom.
• A strong base removes an acidic proton adjacent to an electron withdrawing
group to give a carbanion.
• Loss of a leaving group from the carbanion creates a carbene.

12. • One of the best known elimination reactions occur when chloroform is
treated with base forming a dichlorocarbene .
• The two non-bonding electrons of carbene are derived by hemolytic fission
of two bonds. The carbon atom in carbene has 6 electrons and so is electron
deficient species analogous to carbocation . Carbene normally reacts as a
strong electrophile .

13. Why the Carbenes has 2 Bonding and Non-
Bonding electrons

14. Types of Carbene
• Singlet Carbene Triplet Carbene
• Hybridization
• Sp2 Sp
• Violates Hund’s rule , Diamagnetic Follows Hund’s rule
Paramagnetic
• Electronic Configuration

15. In Case of Singlet
Carbene .
Ground State
In Case of Triplet
.
E.C of carbon
Excited state

17. Stability of Singlet and Triplet Carbene
• Singlet < Triplet ( in case of methylene ( :CH2 )
Singlet is less stable due to more repulsion and less Bond angle
(103°)
Triplet is more stable due to less repulsion between L.p and B.p and
increase bond angle (136º)
Triplet < Singlet in case of :CCl2

19. Generation Of Carbenes
 From Aliphatic Diazo Compounds
 From Ketenes
 From Epoxides
 From Alkyl Halides

20. From Aliphatic Di azo compound
• Aliphatic di azo compound can be decomposed either photo chemically or
thermally to generate carbene .
•
C
N
2 C
+
N
2
R
R
R
R
h
v

21. H2C N N H2C N N C
H
H
hv
-N2
C
H
2
N
2 C
H
2 + N
2
D
i
a
z
o
m
e
t
h
a
n
e C
a
r
b
e
n
e
R
C
O
C
H
N
2 R
C
O
C
H
+ N
2
A
c
y
lD
i
a
z
o
c
o
m
p
o
u
n
d A
c
y
lC
a
r
b
e
n
e
N
2
C
H
C
O
O
C
2
H
5 C
H
C
O
O
C
2
H
5 + N
2
D
i
a
z
o
a
c
e
t
i
c
e
s
t
e
r C
a
r
b
e
t
h
o
x
y
C
a
r
b
e
n
e
h
v
o
rh
e
a
t

22. From ketenes
• Ketenes can be decomposed thermally or photo chemically to generate
carbene .
C C O C +
C
O
C O C
H
2 C O
RC C
l R C C
H
N
2 R
C
H C O +N
2
R R
R R
C
H
3
C
H
3
O O
C
H
2
N
2
A
g
2
O
D
i
a
z
o
k
e
t
o
n
e
s K
e
t
e
n
e
s
h
v
o
r
h
e
a
t
k
e
t
e
n
e
s

23. From Epoxides and cyclopropane
Photolytic decomposition of epoxides generate carbene.
C C C + C
O
h
v
O
R
R
R
R
R
R R R

24. From Alkyl Halides
• This method is commonly used for the generation of chlorocarbenes , thus
loss of proton from chloroform by a base followed by expulsion of chloride
ion generates di chloro carbene .
• Reactions
•

25. CHCl3 CCl3 CCl2 + Cl
Ph2C CHBr Ph2C C Br Ph2C C
Base
Base
Dichloro carbene

27. Reactions of Carbenes
 Cyclo Addition reactions
 Insetion reaction
 Re- arrangements

28. Cycloadditions
• Carbenes add on to an olefinic double bond giving a cyclopropane
derivatives .
• When reaction is performed in liquid medium singlet carbene is
formed . Which adds on a sterospecific way . Cis-alkene gives Cis-
Cyclopropane and Trans- alkene gives Trans-Cyclopropane . In
gaseous medium , the attacking species is triplet carbene, which
does not add stereospecically .

29. C C
R
H
R
H
+ CH2
singlet carbene
Cis-Olefin
C C
R
R
H
H
CH2
Cis-Cylcopropane
C C
R
H
R
H
+ CH2
Triplet carbene
Cis-Olefin
C C
H
R
R
H
CH2
trana-Cyclopropane
C C
R
R
H
CH2
Cis-Cylcopropane
H

30. Insertion Reaction
• Carbene can be insert into a C H Bond
• Insertion reaction shows that carbene are very reactive .
C H + CH2 C H
CH2
C
H2
C H
One step process
C H + CH2 C + CH3 C CH3
Two step process

31. Re-arrangement Reactions
• Alkyl carbenes can undergo re-arrangement involving migration of an alkyl
group or hydrogen.
CH3 CH2 CH
H
CH2
1,2 Hydride shift
CH3 CH3 CH CH2

33. The reaction of pyrole with carbene give the product of 3-
chloro pyridine
N
H
Pyrole
:C
Cl
Cl
dichloro carbene
N
3 chloro pyridine
Cl
N
Cl
Cl
H

34. Carbene can couple with one another forming alkenes.
CHCl3 / KOH CR2 CR2
CH2:
carbene

35. H O
phenol
C H C l3
N aO H
O H
salicylaldehyde
C H O

36. Dichlorocyclopropanes, obtained from cycloalkene and
carbene on treatment with LiMe gives spiro compound.
Cycloalkene
CH3Li
C
spiro compound
Br2C:

37. F
F
Cl
chloro diflouro methane
H
H Cl
hydrochloric acid
+
+
NaOH
:
C
F
F
di flouro carbene
:
C
F
F
di flouro carbene
:
C
F
F
di flouro carbene
F
F
F
F
tetraflouro ethylene

38. F
F
Cl
chloro diflouro methane
H
H Cl
hydrochloric acid
+
+
NaOH
:
C
F
F
di flouro carbene
:
C
F
F
di flouro carbene
:
C
F
F
di flouro carbene
F
F
F
F
tetraflouro ethylene

39. CH2:
carbene ethane
H CH2 CH3 CH3 CH2 CH3
methyl
free
radicle
ethyl
free
radicle

40. “
”
Reference