Alkene Stability: Videos & Practice Problems

Alkenes exhibit stability through a phenomenon known as hyperconjugation, which is also a stabilizing force for carbocations. This concept is crucial when analyzing the stability of alkenes, as it relates to the presence of adjacent sigma bonds that can interact with the pi bond formed by overlapping p orbitals. The more substituents (R groups) present around the double bond, the greater the stabilization due to hyperconjugation, as these groups can share electron density with the pi bond.

The stability of alkenes can be ranked based on the number of substituents around the double bond. The trend in stability is as follows: tetra-substituted alkenes are the most stable, followed by tri-substituted, di-substituted, mono-substituted, and finally unsubstituted alkenes, which lack any stabilizing groups and are therefore the least stable.

When considering di-substituted alkenes, there are different configurations that can affect stability. These configurations include:

• cis: Both substituents are on the same side of the double bond, leading to steric hindrance and reduced stability.
• trans: Substituents are on opposite sides, allowing for more spatial separation and increased stability.
• geminal: Both substituents are attached to the same carbon atom, which is referred to as geminal (from the Latin word for twins). This configuration is surprisingly the most stable due to unique interactions, despite being less intuitive than the trans configuration.

In summary, the stability of alkenes is significantly influenced by the number and arrangement of substituents around the double bond. Understanding these concepts is essential for predicting the behavior and reactivity of alkenes in organic chemistry.

Understanding the heat of combustion is crucial in evaluating the stability of different compounds. The heat of combustion is a measure of the energy released when a substance undergoes complete combustion. A higher heat of combustion indicates a higher enthalpy and, consequently, lower stability of the compound. Therefore, when ranking compounds from lowest to highest heat of combustion, the most stable compound will have the lowest heat of combustion, while the least stable will have the highest.

To determine the stability of various compounds based on their substitution patterns around double bonds, we categorize them as follows:

1. **Trisubstituted**: This compound has three branches attached to the double bond, making it the most stable due to increased steric hindrance and hyperconjugation. It will have the lowest heat of combustion.

2. **Monosubstituted**: This compound has only one branch attached to the double bond, resulting in lower stability compared to trisubstituted compounds. It will have the highest heat of combustion among the group.

3. **Disubstituted**: Compounds with two branches can be further analyzed. Among disubstituted compounds, the stability can be influenced by the arrangement of the substituents:

- **Geminal Disubstituted**: Both substituents are on the same carbon atom, which is generally more stable than the cis arrangement.

- **Cis Disubstituted**: Both substituents are on the same side of the double bond, leading to increased steric strain and lower stability.

In summary, the order of stability from lowest to highest heat of combustion is as follows: trisubstituted (most stable) > geminal disubstituted > cis disubstituted > monosubstituted (least stable). This understanding is essential for predicting the behavior of compounds in chemical reactions and is likely to appear in examinations.