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:−
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.
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 .