2. TOPICS
Generation of Carbocation
Strucutre of Carbocation
Stability of Carbocation
Reactivity of Carbocation
Refrences of my seminar
3. INTRODUCTION
*INTERMEDIATE -- An organic species which is short
lived,highly reactive and formed during the course of
reaction.
In Organic chemistry there is many types of reaction
intermediate found
Some intermediates are
CARBOCATION,CARBANION,FREE RADICAL,
CARBENE AND NITRENE etc.
4. GENERATION OF CARBOCATION
Heterolytic cleavage is responsible for generation of
carbocations.
1- Unimolecular Aliphatic Nucleophilic Substitution Reactions
(SN1) :--
Leaving group attached to carbon leave with its pair of
electron and formation of carbocation occur.
5. 2- Aromatic electrophilic
substitution unimolecular:--
In step 1st there is formation of
arenium ion takes place which
is stabilize by resonance.
3- Protonation of unsaturated
systems:--
A protons and other positive
species adds to one atom of
unsaturated system, leaving
the other carbon atom as
positive charge
6. STRUCTURE
Carbon with positive
charge is sp2 hybridised.
Electronoic configuration of
carbon with positive charge
is [He]2s1 2px
1 2py
1 2pz
0
Bond angle is 1200.
Three sp2 hybrid orbitals
are in same plane.
Empty pz orbital at the
perpendicular to plane of
three hybrid orbitals.
7. Stability of Carbocation
The more stable Carbocation have been
prepared in solution.
In solution the carbocation may be free if
solution is polar in nature or it may exist
as ion pair means closely associated with
negative ion called counter ion( most
likely in non polar solvents).
Simple alkyl cation is not stable in
ordinary strong acid solution. Example
H2SO4.
Carbocations stable in Super Acid
solution (mixture of fluorosulphuric acid in
antimony pentafluoride in sulfer di oxide)
8. 1- On the basis of Inductive
Effect:-
The stability of the carbocation
will increase as the number of
donating ability of the attached
group increase.
Since the alkyl group has an
electron donating effect (+I)
Tertiary cation is more stable
than secondary and secondary
cation is more stable than
primary.
Many effects explain stability of Carbocation.
9. (2) On The Basis Of
Hyperconjugation:-
No Bond Resonance also
stabilize carbocation
Number of hyperconjugative
structure depends upon
number of Alpha Hydrogens
present.
Total number of
hyperconjugative structures
is equal to
(1+ Number of Alpha
Hydroger)
10. (3) Stability of Allylic or Benzylic
carbocation
The stability of Carbocation in
which the carbon bearing positive
charge is adjacent to double or
triple bond can be rationalize in
terms of resonance
Because carbon with positive
charge having a vacant pz orbital
in plane same as filled pz orbital
of carbon with double bond so
there is delocalization of pi bond
stabilizes carbocation.
11. REACTIVITY
1- Nucleophilic attack(SN1 reactions) :- In these types
of reactions, a carbocation may combine with a
species(Nucleophile) by accepting an electron pair.
12. (2) Rearrangement reaction:--
1-2 methyl shift or 1-2 hydride shifts are very common
in carbocation chemistry to attain a more stable
counterpart.
A primary carbocation will prefer to rearrange itself
into a more stable tertiary carbocation.
13. Example:-
1) Pinacol and pinacolone rearrangement:-
Pinacol-- An organic compund in which two hydroxy group is
present at adjacent positions is called pinacol.
