Write a note on stability of conjugated dienes
Introduction
Conjugated dienes are characterized by alternating carbon-carbon double bonds separated by
carbon-carbon single bonds.
Conjugated dienes are more stable than non conjugated dienes due to factors such as
delocalization of charge through resonance and hybridization energy.
The stability of conjugated dienes arises from several factors:
Hybridization in conjugated dienes:
The carbon atoms in conjugated dienes are sp² hybridized, meaning they have 33% s-character
and 67% p-character. This sp² hybridization results in the formation of a planar structure, which
is essential for effective p orbital overlap. The delocalization of π-electrons across the
conjugated system stabilizes the molecule, as the p orbitals are aligned for maximum
interaction.
In isolated dienes, where the double bonds are separated, the hybridization is the same (sp²),
but the lack of conjugation reduces stability.
Delocalization of electrons:
In conjugated dienes, the π-electrons are delocalized across the entire conjugated system,
which reduces the overall energy of the molecule and increases stability. The delocalization in
conjugated dienes, aided by sp² hybridization, allows for more effective electron distribution,
increasing stability.
Resonance stabilization:
The delocalization of electrons in conjugated dienes leads to resonance stabilization, making
these compounds more stable than their non-conjugated counterparts.
This electron delocalization allows the molecule to distribute energy more evenly over the
structure. This stabilization is often referred to as "resonance stabilization" or "conjugation
stabilization.
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Lower heat of hydrogenation:
Another useful resource to consider are the heats of hydrogenation of different arrangements
of double bonds. The stability of conjugated dienes can be quantitatively assessed through their
heats of hydrogenation.
The process of hydrogenation adds hydrogen to the double bonds, converting the diene into an
alkane. The higher the heat of hydrogenation the less stable the compound, it is shown below
that conjugated dienes (~54 kcal) have a lower heat of hydrogenation than their isolated (~60
kcal) and cumulated diene (~70 kcal) counterparts.
Compared to isolated double bonds, the heat of hydrogenation (the energy released when a
double bond is hydrogenated) for conjugated dienes is lower. This indicates that conjugated
dienes are more stable, as less energy is needed to hydrogenate them.
Here is an energy diagram comparing different types of bonds with their heats of
hydrogenation to show relative stability of each molecule:
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Molecular Orbital (MO) Theory and Stabilization of Conjugated dienes:
Molecular Orbital (MO) Theory explains the stability of conjugated dienes through the interaction and
delocalization of π-electrons over the conjugated system. In a conjugated diene, the individual p orbitals
of the carbon atoms involved in double bonds overlap, forming molecular orbitals that extend over the
entire conjugated system.
MO theory stabilizes conjugated dienes by allowing the delocalization of electrons across multiple
atoms, resulting in lower energy bonding molecular orbitals and a more stable overall structure.
In contrast to conjugated dienes, isolated dienes (where the double bonds are separated by more than
one single bond) do not benefit from the same degree of electron delocalization. The p orbitals in
isolated dienes cannot overlap as effectively, so the π-electrons remain localized in separate double
bonds. As a result, isolated dienes are less stable than conjugated dienes because they lack the energy-
lowering delocalization effect.
Understanding the Concept of MO Theory
The overlapping p orbitals form bonding and antibonding molecular orbitals. In a conjugated
diene, four p orbitals combine to form four π molecular orbitals – two bonding (lower energy)
and two antibonding (higher energy).
Two bonding molecular orbitals (π₁ and π₂) that have lower energy. Two antibonding molecular
orbitals (π₃* and π₄*) that have higher energy
For a simple conjugated diene like 1,3-butadiene, we have four p orbitals (one on each of the
sp²-hybridized carbon atoms involved in the double bonds). The formation of bonding and
antibonding molecular orbitals allows electrons to occupy lower-energy bonding states, resulting in
greater stabilization compared to isolated dienes. The reduced energy gap between bonding and
antibonding orbitals and the delocalization of electrons contribute to the overall stability of conjugated
dienes.
