2. CONJUGATION
Conjugation is the overlap of one p-orbital with another across an
intervening sigma bond
Conjugated molecules have alternating single , double or triple bonds
For example:
3. The molecule is non conjugated when it do not have alternating single double or triple bond
For example :
Conjugated and non-conjugated dienes :
Conjugated dienes are two double bond separated by a single bond
For example:
4. Non Conjugated Dienes
Non-conjugated dienes having two double bonds are separated by more than one single
bond
For example:
Cyclic conjugated system
Linear conjugated system
5. Stability of conjugated dienes :
Conjugated dienes are more stable than non-conjugated dienes due to factors such as De
localization of charge through resonance and heat of hydrogenation , It is shown that as
alkene become more stable , They contain less energy so release less heat during
hydrogenation
conjugated dienes have a lower heat of hydrogenation that is -226 kJ/mol
Than non conjugated dienes have a heat of hydrogenation that is -252 kJ/mol
6. The stability of conjugated dienes is due to the positioning of the p-orbitals and ability to overlap
which give strength to single bond between two double bonds
Types of conjugation
Hyper conjugation
CROSS conjugation
Hyper conjugation
It is the stabilizing interaction that result from interaction of the electrons (usually c-h c-c)
with an adjacent empty or partially filled orbital or pi orbital to give an extended molecular
orbital that increases the stability of system.
example
7. Cross conjugation:
Cross-conjugation is a special type of conjugation in a molecule, when in a set of
three pi bonds only two pi bonds interact with each other by conjugation, while the
third one is excluded from interaction.
Example:
8. Hyperconjugation
Hyper-conjugation is a special case of resonance in which sigma electrons of a
carbon-hydrogen
or
carbon-carbon bond are in conjugation with pi-electrons of the delocalized
system or with p-orbitals.
9. Explanation
Hyper-conjugation may involve the interaction of the electrons in a
sigma (σ) orbital (e.g. C–H or C–C) with an adjacent unpopulated
non-bonding p or antibonding σ* or π* orbitals to give extended
molecular orbitals.
Hyper-conjugation is directly proportional to stability.
According to classical resonance theory, electron delocalization could
occur only via parallel overlap of p orbitals. According to hyper-
conjugation, a variant of resonance theory, electron delocalization could
also occur via parallel overlap of p orbitals with hybridized orbitals involved
in forming the structures
11. 1. Relative stability of alkyl carbocations
consider the ethyl carbocation (CH3CH2 + ), a primary carbocation.
The empty p orbital on C1 and the sp3 -hybridized orbital on C2
participating in C2— H1 σ-bond are more or less parallel, allowing parallel
overlap, which lowers the electron deficiency at C1 but makes the H1
electron deficient. In same way, C2—H2 σ-bond and C2—H3 σ-bond could
also share electrons with the empty p orbital on C1.
14. Cont.…
Based on the above resonance contributing structures, the resonance hybrid
of the ethyl carbocation can be shown roughly as follows.
Due to hyper-conjugation, in the ethyl carbocation, the net charge of +1 is
shared by a total of four atoms, one carbon atom and three hydrogen atoms.
No. of hyper-conjugated structures is directly proportional to stability.
15. Stability of Carbocations
More the number of CH3 groups bonded to positively charged C of
carbocation, the greater is the interaction and so is stability.
tertiary> secondary> primary
16. 2.Relative stability of free radicals
Hyper-conjugation in alkyl free radicals disperses unpaired electron over large
volume of space thus providing stability as shown for ethyl free radical
(CH3CH2˙).
The order of stability depends on the extent of delocalization. The greater the
delocalization, the more is the stability. The order of alkyl free radical stability
is,
3° free radical > 2° free radical > 1° free radical > free radical
17. Cont.…
No. of sigma hydrogens is directly proportion to resonating structures
18. 3. Relative stability of alkenes
Overlap of σ-orbital of C–H bond with π-orbital of adjacent C–C double bond
gives rise to canonical structures. Delocalization of electrons occurs over
three nuclei and thus stabilizes the alkene.
19. More the substituents, more is the opportunity for hyper-conjugation and
more stable is the alkene. Stability of alkenes will increase with increase in
number of Hydrogens α to unsaturated system.
21. Negative hyper-conjugation
It is also called “reverse hyper-conjugation”
It operates in opposite direction to that of hyper-conjugation.
The interaction between filled π or p orbitals and adjacent antibonding σ*
orbitals is referred to as "negative hyper conjugation", as for example in the
fluoro-ethyl anion:
22. it is exerted by sigma bonded electrons alpha-halo alkyl group.
alpha-c-x bonds when interact with nearby sigma or pi-bonding orbital.
Negative hyper conjugation stabilizes the molecule by dispersion of negative
charge.
23. Effect on bond length
Hyper-conjugation leads to shortening of C–C sigma (σ) bond adjacent to
multiple bond. For example, C–C σ-bond in methyl acetylene is 1.46 A° in
length, much less as compared to 1.54 A° found in saturated
hydrocarbons.
For methyl acetylene, this can be explained by hyper conjugation between
alkyl and alkynyl parts.
