hyperconjugation ,orbital structure, stability of alkene

1. PT. MOHAN LAL SD COLLEGE FOR WOMEN,
GURDASPUR
Class- B.Sc SemI
Subject – Organic Cemistry
Chapter Name – Structure and Bonding
Topic – Hyperconjugation Effect
Submitted By – Mrs. Gurdeep kaur
Asst. Prof. in Chemistry

2. WHAT IS HYPERCONJUGATION EFFECT
When alkyl groups are attached to an
unsaturated system like double bond
or a benzene ring, the +I –effect is
reversed. This effect is called
hyperconjugation.

3. ORBITAL CONCEPT
Hyper conjugation involves delocalization of
σ-electrons through overlapping of p-orbitals
of the double bond with σ-orbital of the
adjacent single bond . It is also known as σ-
π-conjugation or Baker-Nathan effect or no
bond resonance.

4. Hyperconjugation in Propene

6. SIGNIFICANCE OF HYPERCONJUGATION
 Directive influence of alkyl groups:- Alkyl groups
are o,p-directing in nature because as a result of
hyperconjugation, the electron density at o- and p-
positions w.r.t the CH3 group increases and hence
electrophilic substitution reactions in toluene will
occur at o- and p- positions w.r.t the CH3

7. 1) Stability of alkenes:
The stability of alkenes increases with increase in the
number of alkyl groups (containing hydrogens) on the
double bond. It is due to increase in the number of
contributing no bond resonance structures.
For example, 2-butene is more stable than 1-butene. This
is because in 2-butene, there are six hydrogens involved
in hyperconjugation whereas there are only two
hydrogens involved in case of 1-butene. Hence the
contributing structures in 2-butene are more and is more
stable than 1-butene.

8. The increasing order of stability of alkenes with
increases in the number of methyl groups on the
double bond is depicted below.
This order is supported by the heat of
hydrogenation data of these alkenes. The values
of heats of hydrogenation decrease with increase
in the stability of alkenes.
Also the heats of formation of more substituted
alkenes are higher than expected.
.

9. It is important to note that the alkyl groups
attached to the double bond must contain at
least one hydrogen atom for hyperconjugation.
For example, in case of the following alkene
containing a tert-butyl group on doubly bonded
carbon, the hyperconjugation is not possible

10. However propene is more stable than 1-butene.
This is because there are three hydrogens on α-
methyl group involved in hyperconjugation.
Whereas, in 1-butene there are only two
hydrogen atoms on -CH2 group that can take
part in hyperconjugation.

11. 2) Stability of carbocations :-
The ethyl carbocation, CH3-CH2
+ is more stable than
the methyl carbocation, CH3
+. This is because, the σ-
electrons of the α-C-H bond in ethyl group are
delocalized into the empty p-orbital of the positive
carbon center and thus by giving rise to 'no bond
resonance structures' as shown below. Whereas
hyperconjugation is not possible in methyl carbocation
and hence is less stable.

12. The stability of carbonium ions increases with
increase in the number of alkyl groups
attached to the positively charged carbon due to
increase in the number of contributing structures
to hyperconjugation.

13. Thus the increasing order of stability of
carbocations can be given as:
methyl < primary < secondary < tertiary

14. THANK YOU