7.6 Stability of Alkenes

Alkene Stability and Structure

Alkene stability determines which products form in elimination reactions and which alkene isomers predominate at equilibrium. By understanding what makes one alkene more stable than another, you can predict reaction outcomes across many different contexts. The three main factors are degree of substitution, geometric isomerism (cis vs. trans), and conjugation.

Alkene Stability and Substitution

The more alkyl groups attached to the double bond carbons, the more stable the alkene. The stability ranking looks like this:

tetrasubstituted > trisubstituted > disubstituted > monosubstituted > unsubstituted (ethene)

Two effects explain why substitution increases stability:

• Hyperconjugation is the dominant factor. The filled $\sigma$ bonds on carbons adjacent to the double bond can overlap with the empty $\pi^*$ antibonding orbital. This interaction delocalizes electron density and lowers the overall energy of the molecule. More alkyl substituents means more $\sigma$ bonds available for this overlap, so tetrasubstituted alkenes benefit the most.
• Inductive effects play a smaller role. Alkyl groups are weakly electron-donating through the $\sigma$ framework, which helps stabilize the relatively electron-rich $\pi$ system.

For a concrete comparison: 2,3-dimethyl-2-butene (tetrasubstituted) is more stable than 2-methyl-2-butene (trisubstituted), which is more stable than 2-butene (disubstituted), which is more stable than propene (monosubstituted).

Conjugation can further stabilize an alkene. When a double bond is adjacent to another $\pi$ system (another double bond, a carbonyl, or an aromatic ring), the extended overlap of p orbitals delocalizes electron density and lowers energy. For example, 1,3-butadiene is more stable than 1,4-pentadiene (which has isolated double bonds) because of this conjugative stabilization.

Cis vs. Trans Alkene Stability

Trans alkenes are generally more stable than their cis isomers. The reason is steric strain: in the cis configuration, the two larger substituents are on the same side of the double bond, and their electron clouds repel each other. In the trans configuration, those groups point in opposite directions, minimizing this repulsion.

A few details to keep in mind:

• The stability difference grows with substituent size. For trans-2-butene vs. cis-2-butene (both with methyl groups), the energy difference is only about 4 kJ/mol. Replace those methyls with tert-butyl groups and the gap becomes much larger.
• In cycloalkenes, the cis/trans comparison gets more complicated because ring strain enters the picture. For small and medium rings, the trans isomer can actually be less stable or even impossible to form. Trans-cyclohexene, for instance, is extremely strained because the ring can't easily accommodate a trans double bond. Trans-cyclooctene is the smallest trans-cycloalkene that's reasonably stable, and even it is less stable than cis-cyclooctene.

Heats of Hydrogenation as a Stability Measure

Heat of hydrogenation ($\Delta H_{\text{hydrog}}$) is the enthalpy change when $\ce{H2}$ adds across a double bond to form an alkane. Since all alkenes in a comparison set hydrogenate to the same (or equivalent) alkane product, any difference in the energy released reflects a difference in the starting alkene's stability.

The key principle: a more stable alkene releases less energy upon hydrogenation (its $\Delta H_{\text{hydrog}}$ is less negative).

Here's how to use this in practice:

1. Hydrogenate each alkene to its corresponding alkane.
2. Measure the heat released for each reaction.
3. The alkene with the least exothermic (least negative) $\Delta H_{\text{hydrog}}$ is the most stable.

For example, 1-butene (monosubstituted) has $\Delta H_{\text{hydrog}} \approx -127$ kJ/mol, while trans-2-butene (disubstituted) has $\Delta H_{\text{hydrog}} \approx -115$ kJ/mol. Trans-2-butene releases less energy, confirming it's more stable.

Limitations to watch for:

• The energy differences between isomers can be small (a few kJ/mol), so experimental error matters.
• Heavily substituted alkenes may have additional steric strain that complicates the trend.
• Conjugated dienes and aromatic compounds don't follow the same pattern as isolated alkenes. Benzene, for instance, has a dramatically lower heat of hydrogenation than you'd predict from three isolated double bonds, because of aromatic stabilization.

Molecular Orbital Theory and Alkene Structure

The double bond in an alkene consists of one $\sigma$ bond and one $\pi$ bond. Both carbons of the double bond are sp² hybridized, which gives them trigonal planar geometry with bond angles of approximately 120°.

The $\pi$ bond forms from the sideways overlap of the unhybridized p orbitals on each carbon. This overlap is weaker than the head-on overlap of the $\sigma$ bond, which is why $\pi$ bonds are more reactive. The combination of $\sigma$ and $\pi$ bonding also makes $\ce{C=C}$ bonds shorter (~1.34 Å) and stronger (~614 kJ/mol) than $\ce{C-C}$ single bonds (~1.54 Å, ~348 kJ/mol).

From an MO perspective, the two p orbitals combine to form a bonding $\pi$ orbital (lower energy, where the electrons sit) and an antibonding $\pi^*$ orbital (higher energy, normally empty). Hyperconjugation, discussed above, involves donation of electron density from adjacent $\sigma$ bonds into this $\pi^*$ orbital, which is why it stabilizes the alkene without breaking the $\pi$ bond itself.