2. Carbocation
ο A carbocation is molecule in which a carbon atom bears
three bond with a positive charge.
ο Carbocations are usually unstable because they do not
have eight electrons to satisfy the octet rule.
ο It generate through heterolysis fusion
- C:πβ
- πΆ+
+ π:β
4. Classification
of
carbocations
ο Primary Carbocations- It is one in which there is
one carbon group attached to the carbon
bearing the positive charge.
Eg- H
πΆπ»3 πΆ+
H
ο Secondary Carbocations- It is one in which there
is two carbon group attached to the carbon
bearing the positive charge.
Eg- H
πΆπ»3 πΆ+
πΆπ»3
ο Tertiary Carbocations- It is one in which there is
three carbon group attached to the carbon
bearing the positive charge.
5. Eg- πΆπ»3
πΆπ»3 πΆ+
πΆπ»3
ο 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.
ο Eg- πͺπ― π πΆ+ π»2
6. ο Benzylic Carbocation- If the carbon bearing the positive
charge is immediately adjacent to the benzene ring the
carbocation is termed as benzylic carbocation.
ο Eg-
H
πͺ+
H
ο Vinyl Carbocation- If the carbon bearing the positive
charge is part of alkene is called vinyl carbocation.
ο Eg- Cπ― π= πͺ+- H
7. ο 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.
ο Eg-
+
8. Structure of
Carbocation
ο The structure of carbocations is trigonal planar.
ο A stylized orbital structure of the methyl cation.The
bonds are sigma bonds formed by overlap of the carbon
atoms 3 π π2 orbitals with 1s orbitals of the hydrogen
atoms.The p orbital is vacant.
ο A dashed line-wedge representation of the tert-butyl
cation.The bonds between carbon atoms are formed by
overlap of π π2orbitals of the central carbon atom.
9. Rearrangement
of
Carbocation
ο The bonding electrons of a carbocation may shift
between adjacent atoms to form a more stable
carbocation.
ο 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.
ο Types of shift-
10. ο 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.
ο 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.
ο The most common carbocation rearrangement
involve a carbocation rearranging into a more
stable carbocation such as, secondary to tertiary
with a resonance.
ο Rearrangements that transform a carbocation
into another of apparently equal stability are less
common , but they do occur.
ο Rearrangement to a less stable carbocation is
very unusual , but also does occur.
12. 1) Effect
of
substitution
ο The stability of carbocation decreases as the
number of carbons attached to the carbocation
decreases.
ο Stability of carbocation-
Tertiary > Secondary> Primary
ο 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
ο If there will be the presence of any electron
withdrawing group with the carbocation than
inductive effect may occur.
ο The greater the number of electron withdrawing
group less will be the stability.
ο The donation or withdrawal of electrons
through a sigma bond is called as inductive
effect.
14. 3) Hybridisation ο The stability of carbocation decreases as it
moves from π π3 > π π2 > sp .