organic chemistry

1. CARBOCATIONS
BOBBY SOLANKI
M.Sc Chemistry
Sharda University
Greater Noida

2. CARBOCATIONS
• A carbocation is an ion with a positively-charged carbon
atom.
• It has Planar structure and sp2 hybridized
• bond angle 120o
• Has six electrons
C
Empty unhybridised
p-orbital
sp2
Hybridisation of
carbon
Planar Strucutre of carbnion
+

3. Generation of carbocation:
2.From alkenes:
1.

4. R
+
N 2
+
R N 2
+
•By the heterolysis of alkyl diazonium salt
R R R R R
R
+
-e
-e
+
.
R
+
.
.
R
+
•By the removal of an electron from a neutral molecule or a free radical

6. The most stable of all alkyl cations is the tert-butyl cation.
: The electron-donating effect of alkyl groups
increases the electron density at the charge-bearing carbon,
reducing the net charge on the carbon, and in effect spreading
the charge over the α carbons.
+
CH3
H C C
3
CH3
+ + +
CH3 H3C CH2 H3C CH CH3
+I effect

7. : Tertiary carbocations are more stable
(and form more readily) than secondary carbocations;
primary carbocations are highly unstable because, while
ionized higher order carbons are stabilized by
hyperconjugation, unsubstituted (primary) carbons are not.
CH3
+
CH2
H
H
+ +
H
H H
H
H CH C
+
H
H
H
H
NONE Three for 3C-H
bonds
H H
Six for 6C-H bonds
H H HH
H
Nine for 9C-H bonds

8. 3. +R groups stabilize the carbocations:
2
+
H3 C O CH H C
3
O
+
CH 2
CH2
+
+
CH
CH2
CH
+
CH2
+
HC
CH2 CH2
+
R
CH2
+
R C
+
H
CH2
CH2
+
H2C
+
CH2
4. Some carbocations are stabilized due to aromatization. For eg;
+
+
Cyclopropenyl cation is stable due to aromatization

9. 1. R+
cation act as an electrophile to react with nucleophiles.For
Eg;
R+ + Nu- R Nu
2. Some carbocations act as Bronsted acid to lose a proton . For
Eg;
H
H
R C+
R
R
R R
R R
H
+
+
→
Reactions of carbocations:

10. O
R
H
+ +
C OH
R
R
+
R
H2C CH2 + H
+
3
H C CH2
+
O
R
R + H3C F BF3
O
+
CH
3
R
R
+ BF4
_
R R O
CH3
+
+
By the addition of a cation to a neutral molecule.
Addition reaction:

11. Wagner-Meerwein rearrangement:
• When alcohol containing more than two alkyl or aryl group
on β carbon are treated with acid, the product formed is
generally a rearranged product, rather than simple
substitution or elimination product. This reaction is called
Wagner - Meerwein rearrangement.
• Newly generated carbocation is stabilized generally by loss
of a proton to give olefin.

12. The earliest examples of Wagner-Meerwein rearrangement was noticed in
bicyclic terpenes.
R H
R1
OH H
R2
R3
R H
R1 OH2
R2 R3
R
R1
R2
H
R3
R2
R1 H
R3
R
R2
R1 R
R3
Note: that in the initial step a proton is consumed and in the last step a proton
is released.
Mechanism involves rearrangement of the carbocation
intermediate.

13. The migrating group can be alkyl, aryl, hydrogen or ethoxycarbonyl.
In the case of unsymmetrical diols, which one of the hydroxy group gets
protonated it is important. As seen earlier in this module, in general, the
hydroxyl group that can generate a more stabilized carbocation is the one
which gets protonated.
CH3
H
CH3 CH3
H3C C C
OH OH
Pinacol
CH3 O
H3C C C CH3
CH3
Pinacolone
Pinacol Rearrangement:
When vicinal diol (also known as pinacol) is treated with acids, it
rearranges to give aldehyde or ketone. This reaction is called as Pinacol
rearrangement.

14. Mechanism :
R1 C R3
C C
A plausible mechanism can be represented as follows;
R2
R4 R2 R4
H R1
C R3
OH OH2
-H2O
R1
C
OH
C
OH OH
R2 R4
R3 R1 C
OH
C
R2
R4
R3 -H
O
R1
C C
R2
R3
R4

15. In this reaction, the hydroxyl group on carbon bearing two phenyl
groups will be protonated faster to form a more stable benzylic
carbocation.
OH OH
Ph
Ph
H2SO4
O
Ph
Ph

16. The following reaction represents a special case of Wagner-Meerweinrearrangement.
Generally, a mixture of rearranged and non-rearranged products isformed.
NH2
HNO2
OH
2
CH OH
+
HNO2
CH2NH2
NH2 OH
HNO2
OH +
Demjanov rearrangement:

17. Mechanism is as follows:
N
O
HO N
O
H2O
N
O
O
N
O O
N
O
H
-H2O
N
O
N
O
+ O
NH2
N N OH
H
NO2
N N OH2
N N
H
-HNO2 H
HN N O
-H2O
-N2 H2O
OH