Rearrangement of Carbocation for B.pharm Students

1. Rearrangement of
Carbocation
For B-Pharm Students
Presented By- Sandhya Punetha

2. Carbocation
 A carbocation is molecule in which a carbon atom bears
three bond with a positive charge.
 Carbocations are usually unstable because they do not
have eight electrons to satisfy the octet rule.
 It generate through heterolysis fusion
- C:𝑍−
- 𝐶+
+ 𝑍:−

3. Important
Facts
 All carbocations are known as carbonium ions.
 Some carbocations may have 2 or more positive
charge on carbon atom.

4. Classification
of
carbocations
 Primary Carbocations- It is one in which there is
one carbon group attached to the carbon
bearing the positive charge.
Eg- H
𝐶𝐻3 𝐶+
H
 Secondary Carbocations- It is one in which there
is two carbon group attached to the carbon
bearing the positive charge.
Eg- H
𝐶𝐻3 𝐶+
𝐶𝐻3
 Tertiary Carbocations- It is one in which there is
three carbon group attached to the carbon
bearing the positive charge.

5. Eg- 𝐶𝐻3
𝐶𝐻3 𝐶+
𝐶𝐻3
 Allylic Carbocation- The carbon bearing the
positive charge is immediately adjacent to a
carbon-carbon double bond, this type of
carbocation is known as allylic carbocation.
 Eg- 𝑪𝑯 𝟐 𝐶+ 𝐻2

6.  Benzylic Carbocation- If the carbon bearing the positive
charge is immediately adjacent to the benzene ring the
carbocation is termed as benzylic carbocation.
 Eg-
H
𝑪+
H
 Vinyl Carbocation- If the carbon bearing the positive
charge is part of alkene is called vinyl carbocation.
 Eg- C𝑯 𝟐= 𝑪+- H

7.  Phenyl Carbocations- If the carbon bearing the
positive charge is a part of a benzene ring, the
carbocation is termed as an aryl or phenyl
carbocation.
 Eg-
+

8. Structure of
Carbocation
 The structure of carbocations is trigonal planar.
 A stylized orbital structure of the methyl cation.The
bonds are sigma bonds formed by overlap of the carbon
atoms 3 𝑠𝑝2 orbitals with 1s orbitals of the hydrogen
atoms.The p orbital is vacant.
 A dashed line-wedge representation of the tert-butyl
cation.The bonds between carbon atoms are formed by
overlap of 𝑠𝑝2orbitals of the central carbon atom.

9. Rearrangement
of
Carbocation
 The bonding electrons of a carbocation may shift
between adjacent atoms to form a more stable
carbocation.
 Eg- Rearrangement will occur if a secondary carbocation
can be formed from a primary carbocation because a
secondary carbocation is more stable than the primary
carbocation.
 Types of shift-

10.  Of these two rearrangement examples hydride
shift leads to a tertiary carbocation whereas alkyl
shift leads to a secondary carbocation, the
hydride shift is favored in preference to the alkyl
shift.
 Any C-H or C-C bond adjacent to a carbocation
may shift , but onlyC-C and C-H bonds can
migrate during carbocation rearrangement.
 The most common carbocation rearrangement
involve a carbocation rearranging into a more
stable carbocation such as, secondary to tertiary
with a resonance.
 Rearrangements that transform a carbocation
into another of apparently equal stability are less
common , but they do occur.
 Rearrangement to a less stable carbocation is
very unusual , but also does occur.

11. Factors
affecting
stability in
carbocation
 Factors that destabilize the carbocations are-
1) Effect of substitution
2) Neighboring electron withdrawing groups
3) Hybridisation

12. 1) Effect
of
substitution
 The stability of carbocation decreases as the
number of carbons attached to the carbocation
decreases.
 Stability of carbocation-
Tertiary > Secondary> Primary
 Explanation-The methyl group acts as an
electron donor and therefore stabilizes the
positively charged cation and as well as due to
hyperconjugation.

13. 2)
Neighbouring
electron
withdrawing
group
 If there will be the presence of any electron
withdrawing group with the carbocation than
inductive effect may occur.
 The greater the number of electron withdrawing
group less will be the stability.
 The donation or withdrawal of electrons
through a sigma bond is called as inductive
effect.

14. 3) Hybridisation  The stability of carbocation decreases as it
moves from 𝑠𝑝3 > 𝑠𝑝2 > sp .

15. THANK
YOU
“You never fail until you stop trying”….