Conjugated dienes contain alternating double and single carbon bonds. The single bond between conjugated double bonds is shorter than a typical alkane bond due to delocalization of pi electrons across the carbon chain. This delocalization through resonance structures makes conjugated dienes more stable than non-conjugated dienes. The extra stability of conjugated dienes is also due to the lower hybridization energy from sp2 hybridized carbons compared to the sp3 carbons in non-conjugated dienes.

1. B-Pharm Students
Presented By- Sandhya Punetha

2.  Conjugated dienes are carbon structures
which maintain 2 double bond separated by a
single bond.
 The C-C single bond between conjugated
double bonds is shorter than a typical alkane
C-C.
 This is due to the difference in the
hydridisation of the C atom involved and the
conjugation of the 2 pi bonds.

3.  Two conformations of conjugated dines are
important – s-cis and s-trans.
Eg- 1,3- butadiene
 The s refers rotation about a sigma bond.

4.  Conjugated dienes- Double bonds that
alternate with single bonds.
Eg- CH3-CH=CH-CH3
 Isolated dienes- Double bond that are
separated by more than one single bond
Eg- CH2=CH-CH2-CH2-CH3
 Allenes- Contains cumulated double bond.
Eg- -C=C=C-

5.  Conjugated dienes are more stable than non
conjugated dienes due to factors such as
delocalisation of charge through resonance
and hybridisation energy.
 This can also explain why allylic radicals are
much more stable than secondary or even
tertiary carbocations.
 This is all due to the positioning the pi orbitals
and ability for overlap to occur to strengthen
the single bond between the 2 double bond.

6.  The resonance structure shown below gives a
good understanding of how the charge is
delocalized across the 4 carbons in this
conjugated diene.
 This delocalization of charges stabilizes the
conjugated diene-

7.  Along with resonance, hybridisation energy effect
the stability of the compound.
 Eg- 1,3- Butadiene the carbon with the single bond
are sp2 hybridised unlike in non-conjugated dienes
where the carbons with single bonds are sp3
hybridised.
 This difference in hybridisation shows that the
conjugated dienes have more s character and draw
in more of the pi electrons, thus making the single
bond stronger and shorter than an ordinary alkane
C-C bond.

9.  Another useful resource to consider are the
heat of hydrogenation of different
arrangements of double bonds. Since, the
higher the heat of hydrogenation the less
stable the compound, it is shown below that
conjugated dienes ( 54kcal) have a lower heat
of halogenation than their isolated ( 60kcal)
and cumulated diene (70kcal) counterparts.

10.  Here is an energy diagram comparing different
types of bonds with their heats of hydrogenation
to show relative stability of each molecule.