18 2 Relative stability of alkenes

Thermodynamic Control: Reactions favoring the formation of more stable products.
Importance of stability in products formed during reactions, particularly alkenes.

Steric Hindrance:
- Bulky groups on the same side (cis) lead to higher steric interactions.
- Trans alkenes, with groups on opposite sides, experience less interaction, resulting in increased stability.

Hydrogenation Reaction:
- Involves reacting alkenes with hydrogen in the presence of a metal catalyst.
- Energy released during this process can be measured to assess alkene stability (delta H).
Energy Diagram Analysis:
- Less stable alkenes release more heat upon hydrogenation, indicating higher energy states.

Alkenes Tested:
- 1-butene, cis-2-butene, and trans-2-butene.
Results of Hydrogenation:
- All yield butane as the final product.
- Heat released comparison:
  - Trans-2-butene: 115 kJ/mol
  - Cis-2-butene: 120 kJ/mol
Stability Conclusion:
- Trans isomer is more stable by 5 kJ/mol over cis.
Disubstituted vs. Monosubstituted Alkenes:
- Disubstituted alkenes are more stable than monosubstituted by 7 kJ/mol.

General Rule:
- More alkyl substituents increase alkene stability.
Stability Trends:
- Unsubstituted alkene (all hydrogens) is least stable.
- Ethylene as the highest energy alkene.
- Substituting a hydrogen with an alkyl group increases stability.
- Terminal alkenes (less substituents) are less stable compared to internal (more substituents) alkenes.
Disubstituted Alkene Stability Order:
- Alkyl groups on the same side (cis) < Alkyl groups on opposite sides (trans).
Tri- and Tetra-substituted Alkenes:
- Tri-substituted alkenes are more stable than disubstituted.
- Tetra-substituted alkenes are the most stable overall.

Stability of Alkenes

Thermodynamic Control: This principle of chemical thermodynamics suggests that in a reaction, products that are thermodynamically more stable will form preferentially. These reactions typically reach equilibrium, where the concentration of the stable products is higher due to their lower energy state compared to unstable configurations.
Importance of Stability: In the formation of alkenes, understanding stability offers crucial insights into reaction pathways and outcomes, affecting everything from industrial synthesis to biological processes.

Steric Interactions: Cis alkenes possess bulky substituents on the same side of the double bond, leading to increased steric repulsion and therefore heightened instability. This significant hindrance can hinder the rotation about the double bond.
Trans Alkenes Stability: For trans alkenes, groups are located on opposite sides, which minimizes steric hindrance and allows for greater spatial separation, resulting in a more favorable energy state and increased stability.

This process involves the addition of hydrogen (H₂) to alkenes in the presence of a suitable metal catalyst (such as palladium or platinum). Measuring the energy released during this reaction (delta H) helps assess the relative stability of the alkenes.

Energy Profiles: Thermodynamic studies utilizing energy diagrams reveal that less stable alkenes release greater heat energy upon hydrogenation due to their high-energy states declining to form stable products, indicating the unfavorable nature of those reactants.

Alkenes Tested: A comparative evaluation includes 1-butene, cis-2-butene, and trans-2-butene.
Results of Hydrogenation:
- Each alkene undergoes hydrogenation to yield butane as the final product.
- Heat Released Comparison:
  - Trans-2-butene: 115 kJ/mol (more stable)
  - Cis-2-butene: 120 kJ/mol (less stable)
Stability Conclusion: Evidence indicates that trans-2-butene is more stable, showing a ΔH difference with cis-2-butene of 5 kJ/mol.

Comparative Stability: Disubstituted alkenes exhibit enhanced stability over monosubstituted alkenes, evidenced by a stability difference of 7 kJ/mol. This stability arises due to increased availability of alkyl groups that can donate electron density, thus stabilizing the double bond.

Influence of Alkyl Substituents: Alkene stability is directly proportional to the number of alkyl substituents attached to the double bond; the more substituents, the more stable the alkene is.

Least Stable: An unsubstituted alkene (having all hydrogen substituents) is considered the least stable due to no stabilizing alkyl groups.
Highest Energy Alkene: Ethylene (C₂H₄) represents the highest energy configuration among alkenes as it has no substituents and exists in a planar form.
Substituent Impact: Each hydrogen replaced with an alkyl group spurs increased stability.
Terminal vs. Internal Alkenes: Terminal alkenes (with fewer substituents) are comparatively less stable than internal alkenes (which have greater substitution).

Stability Comparison: Alkenes with bulky groups on the same side (cis) are less stable than their counterparts where the bulky groups are on opposite sides (trans).

Ranking of Stability: Among substituted alkenes, trialkyl-substituted alkenes are more stable than di-substituted due to additional alkyl group stabilization. Tetra-substituted alkenes rank as the most stable category due to maximal alkyl group influence on the double bond.