alkene stability conjugation

1. Alkene Stability

2. It was found that hydrogenation of ethylene released 32.5
kcal/mol (136 kJ/mol) of heat.

3. Stability of Alkenes Increases With Increasing
Substitution
The most notable trend that was found is that the heat of
hydrogenation decreases as C-H bonds are replaced with
C-C bonds.
This means that the lower the heat of hydrogenation,
the greater the stability of the alkene.

4. Alkene stability increases with increasing
substitution of hydrogen for carbon

5. Heats Of Hydrogenation For Some Monosubstituted
Alkenes

6. The Relative Stability of cis- and trans- Alkenes

8. Alkenes Stabilized By Conjugation: Resonance
Energy
1. Take but-1-ene. As we saw above the heat of hydrogenation is
about 30.1 kcal/mol.
2. Add a double bond, and you might expect the heat of
hydrogenation to double as well.
3. But it doesn’t! It’s actually a little bit less. [56.6 kcal/mol] .
4. The difference (that extra 3.6 kcal/mol of additional
stabilization) is called “resonance energy“.

10. Summary: Stability of Alkenes
1. One important factor is the substitution pattern. As C-H bonds are
replaced by C-C bonds, the stability of the alkene gradually increases in the
order mono (least stable) < di < tri < tetrasubstituted (most stable).
2. When hydrogenation liberates more energy than expected given the
substitution pattern, that’s likely a sign of strain. This is exemplified in the
difference in enthalpy of hydrogenation between cis- and trans- alkenes,
where the trans- alkene is more stable by about 1 kcal/mol.
3. When hydrogenation liberates less energy than expected given the
substitution pattern, that’s a sign that some extra factor is stabilizing the
molecule. Among commonly encountered factors, conjugation ranks high.
The difference in energy between the “expected” heat of hydrogenation and
the measured heat of hydrogenation is called the resonance energy. The
conjugation of one pi bond with an additional pi bond is “worth” about 2-3
kcal/mol.