Thermodynamic Considerations
The hydrogenation of alkenes to alkanes is exothermic. The amount of energy released during a hydrogenation reaction, known as the heat of hydrogenation, is inversely related to the stability of the starting alkene: the more stable the alkene, the lower its heat of hydrogenation. Examining the heats of hydrogenation for various alkenes reveals that stability increases with the amount of substitution.
| Compound Name | Structure | Molar Heat of Hydrogenation | Degree of Substitution | |
|---|---|---|---|---|
| in kj/mol | in kcal/mol | |||
| Ethylene | 137 | 32.8 | Unsubstituted | |
| 1-Butene | 127 | 30.3 | Monosubstituted | |
| trans-2-Butene | 116 | 27.6 | Disubstituted | |
| 2-Methyl-2-butene | 113 | 26.9 | Trisubstituted | |
| 2,3-Dimethyl-2-butene | 111 | 26.6 | Tetrasubstituted | |
The increase in stability associated with additional substitutions is the result of several factors. Alkyl groups are electron donating, and increase the electron density on the π bond of the alkene. Also, alkyl groups are sterically large, and are most stable when they are far away from each other. In an alkane, the maxiumum separation is that of the tetrahedral bond angle, 109.5°. In an alkene, the bond angle increases to near 120°. As a result, the separation between alkyl groups is greatest in the most substituted alkene.
Hyperconjugation, which describes the stabilizing interaction between the HOMO of the alkyl group and the LUMO of the double bond, also helps explain the influence of alkyl substitutions on the stability of alkenes. In regards to orbital hybridization, a bond between an sp2 carbon and an sp3 carbon is stronger than a bond between two sp3-hybridized carbons. Computations reveal a dominant stabilizing hyperconjugation effect of 6 kcal/mol per alkyl group.
Read more about this topic: Zaitsev's Rule