Hyperconjugation allows for the delocalization of electrons from C-H sigma bonds into adjacent pi systems. This increases the stability of compounds that can undergo hyperconjugation such as alkenes, carbocations, and alkyl radicals. The number of contributing hyperconjugative structures correlates with increased stability, explaining trends like the greater stability of tertiary carbocations and radicals compared to secondary or primary ones. Hyperconjugation also influences bond lengths and the orientation of substitution in aromatic compounds.

1. Hyperconjugation
Baker and Nathan suggested that an alkyl group
with at least one H-atom on the -carbon atom
when attached to an unsaturated C-atom, is able
to release electron by a mechanism similar to
that of electromeric effect.
C
H
C
C

2. • This type of electron release due to presence of
the system H–C–C=C is known as
hyperconjugation. For example, Propylene may
be regarded as the resonance hybrid of the
following structures due to hyperconjugation.
C
H
H
CH
H CH2 C
H
CH
H CH2
I II
H
C
H
H
CH CH2 C
H
CH
H CH2
III IV
H
H

3. • The various hyperconjugation forms of
propylene are called contributing structures.
In fact hyperconjugation effect is similar to
resonance effect.
• Since structure II, III & IV have no
definite bond between the -C – atom and
one of the H–atom, hence hyperconjugation
is also known as no bond resonance. It is
also known as second order resonance
or Baker Nathan effect.

4. • The orbital concept of hyperconjugation
may be explained with the help of
propylene. In this concept, the electron
pair of C–H bond ( bond) is involved in
conjugation with the  electron pair of the
double bond. Therefore, hyperconjugation
involves delocalization of  electrons of
C–H bond through overlapping of p-orbital
of double bond as shown below:
C
H
H
C C
H
 bond
CH  bond
 hyperconjugation
Orbital picture of hyperconjugation.

5. Structural requirements of
hyperconjugation:
• Any organic compound can show
hyperconjugation if it will fulfill the following
conditions:
1. Compound should have sp2 hybrid carbon of
alkene, arenes, carbocations and free radicals.
2. -Carbon atom with respect to sp2 hybrid C–
atom should have at least one hydrogen atom
and -carbon atom should be sp3 hybridized.
• Thus, hyperconjugation is of following types:

6. (i)  (C–H),  conjugation:
• This type of conjugation occurs in alkenes
and alkyl substituted aromatic compounds.
C
H
H
CH
H CH2

C CH3
H
H


7. (ii) (C–H), positive charge
(vacant p-orbital) conjugation:
• This type of conjugation occurs in alkyl
carbocations.
C CH2
H
H
H

C CH
H
H
H
C H
H
H

8. (iii) (C–H), odd electron (incomplete
p-orbital conjugation):
• This type of conjugation occurs in alkyl
free radicals.
C CH2
H
H
H

C CH
H
H
H
C H
H
H
 

9. Applications:
1. Stability of alkenes:
• Heat of hydrogenation shows that the
greater the number of alkyl groups
attached to the double bonded C – atom,
greater is the stability of alkene (lower is
the heat of hydrogenation). Thus on the
basis of heat of hydrogenation, the order
of stability of different alkenes have been
found to be: Tetrasubstituted >
Trisubstituted > Disubstituted >
Monosubstituted > Ethylene.

10. Alkenes
No. of
 - H atoms
12 9 6 3
No. of
Resonating
structures
13 10 7 4
•The above order of stability of substituted
alkenes can also be explained on the basis of
hyperconjugation.
C C
CH3
CH3
CH3
CH3
C C
CH3
H
CH3
CH3
CH CH CH3
C
H3 CH CH2
C
H3
The greater the number of resonating structures
of a molecule, greater will be its stability.

11. • Trans-2-Butene is more stable than cis-2-
butene, in which two methyl groups are close
together and hence their electronic cloud
(steric hindrace) repel each other.
C C
CH3
H
CH3
H
C C
H
CH3
CH3
H
Steric hendrace
in cis-2-butene
No Steric hendrace
in trans-2-butene

12. 2. Abnormal bond lengths:
• In hyperconjugation a single bond acquires a
double bond character and vice versa, hence
abnormality in bond, lengths is observed in the
compounds showing hyperconjugation. For
example, Ethane and ethylene, C–C and C=C
bonds show normal length 1.54Aº and 1.33Aº,
respectively due to no hyperconjugation in the
compounds but in propene, the bond lengths
are 1.47Aº and 1.35Aº for C–C and C = C
bonds, respectively. This change in bond
lengths may be explained by hyperconjugation.
1.54 Aº
Ethane
1.33 Aº
Ethylene
CH3 CH CH2
1.47 Aº
Propylene
1.35 Aº
CH2 CH2
C
H3 CH3

13. 3. Directive influence of alkyl groups:
• The –CH3 and other alkyl groups are o- and
p- directing which can be explained on the
basis of hyperconjugation as follows:
C H
H
H
C
H
H
H C
H
H
H C
H
H
H
Six more such structures
are possible due to other
two -H atoms.
As a result of hyperconjugation, electron density
at o- and p- position (w.r.t. methyl group)
increases and therefore electrophilic substitution
in toluene takes place at o- and p- positions.
Thus alkyl groups are o- and p- directing.

14. 4. Orienting power of methyl
group in p-substituted toluene:
• In p-t-butyl toluene, further substitution
occurs at o- position with respect to
methyl group even though the (+)
inductive effect of t-butyl group is far
greater than methyl group. This anomaly
is due to greater hyperconjugative effect
of methyl group (having 3 - H atoms)
which increases the electron density at o-
position w.r.t. methyl group than t-butyl
groups which have no  - H atoms and
hence no hyperconjugation occurs. Here
hyperconjugation over weight inductive
effect.

15. • Hyperconjugation in t-butyl toluene
C
H
H
C CH3
H3C
CH3
H H
C
H
C CH3
H3C
CH3
C H
H
C CH3
H3C
CH3
H
C CH3
H3C
CH3
C H
H
H H

16. 5. Stability of free radicals andcarbocations:
1. Stability of alkyl carbocations:
• The order of stability of different alkyl
carbocations is:
C
CH3
CH3
C
H3 C
H
CH3
C
H3 C
H
H
C
H3 C
H
H
H
t Butyl Isopropyl Ethyl Methyl
Carbocation (3º) Carbocation (2º) Carbocation (1º) Carbocation
This order of stability of alkyl carbocations can
be explained on the basis of inductive effect &
hyperconjugation as follows:

17. • hyperconjugation states that greater the number of α-
hydrogen atoms on a carbocation greater is the
number of hyperconjugative structures (greater the
dispersion of positive charge) and hence more is the
stability of carbocation.
• Thus t-butyl carbocation (3º) with nine α-hydrogen
atoms has one usual and nine hyperconjugative
structures is more stable than isopropyl carbocation
(2º) with six α-hydrogen atoms, having one usual and
six hyperconjugative structures which, in turn, is more
stable than ethyl carbocation (1º) with three α-
hydrogens, having one usual & three hyperconjugative
structures, while methyl carbocation with no α-
hydrogen, has one usual and no hyperconjugative
structure, is least stable.

18. • One usual and nine hyperconjugative
structures of t-butyl carbocation (3º)
H C C CH3
H
H
CH3



H C C CH3
H CH3
C C CH3
H
H
CH3
H C C CH3
H
CH3
etc.
t Butyl Carbocation
H
H
H

19. • One usual and six hyperconugative structures
of isopropyl carbocation (2º)
H C C C H
H
H
H H
H


I
H C C C H
H H H
H
II
C C C H
H
H
H H
H
III
H C C C H
H
H H
H
IV
H C C C H
H
H
H
V
H C C C
H
H
H H
H
VI
H C C C H
H
H H
VII
H
H
H
H
H
H

20. • One usual & three hyperconjugative
structures of ethyl carbocation (1º)
H C CH2
H
H
I
H C CH2
H
II
C CH2
H
H
III
H C CH2
H
IV
H
H
H

21. Stability of alkyl free radicals:
• The order of stability of different alkyl free
radicals is as follows:
C
CH3
CH3
C
H3 C
H
CH3
C
H3 C
H
H
C
H3 C
H
H
H
t-Butyl
free radical
( 3º )
Isoprppyl
free radical
( 2º )
Ethyl
free radical
( 1º )
Methyl
free radical

22. • The above order of stability can be explained
on the basis of hyperconjugation.
R'
C
R
C
H
H
H




p-orbital
 bonds of
having odd electron
alkyl group
R
C
R
C
H
H
H

overlaping of orbital due to hyperconjugation
R
C
R
C
H
H
H
Structure of a radical
having -H atoms

23. • In general, greater the number of α-hydrogens,
greater is the number of hyperconjugative
structures and hence more stable is the free
radical. Thus t-butyl free radicals (3º) with nine
α-hydrogens has one usual and nine
hyperconjugative structures, is more stable
than isopropyl free radical (2º) with six α-
hydrogens has one usual & six
hyperconjugative structures which in turn, is
more stable than ethyl free radical (1º) with
three α-hydrogens has one usual & three
hyperconjugative structures while methyl free
radical with no α-hydrogens has one usual & no
hyperconjugative structures is least stable.

24. • One usual & nine hyperconjugative
structures of t-butyl free radical (3º)
H C C CH3
H
H
CH3
H C C CH3
H CH3
C C CH3
H
H
CH3
H C C CH3
H
CH3
 

H
H
H
etc.

25. • One usual and six hyperconjugative
structures of isopropyl free radical (2º)
H C C C
H
H
H H
H
H
 
H C C C
H H H
H
H
H
C C C
H H H
H
H
H
H
H C C C
H H
H
H
H
H
H
C
C
C
H
H
H
H
H
C
C
C
H
H
H
H H
H
H
C
C
C
H
H
H H
H

26. • One usual and three hyperconjugative
structures of ethyl free radical (1º)
H
C
H
C
H
H
H
H
C
H
C
H
H
H
C
H
C
H
H
H
C
H
C
H
H
H
H
H