Stability of alkenes

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

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

8. Summary: Stability of Alkenes 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 ). 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 . 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.