Overview
Section 7.6 covers alkene stability concepts used to predict major products in elimination reactions. Stability depends on geometric configuration, substitution degree, and ring strain in cyclic systems.
Steric Strain and Isomer Stability
- Trans-2-butene positions methyl groups far apart, minimizing steric strain and maximizing stability
- Cis-2-butene places methyl groups closer together, creating steric strain and reducing stability
- Higher steric strain raises molecular energy, making the isomer less stable
- Trans isomers generally produce major products due to lower energy
Heat of Combustion Evidence
- Cis-2-butene releases more energy upon combustion (2686 kJ/mol) than trans-2-butene (2682 kJ/mol)
- Greater energy release indicates cis isomer starts at higher energy state
- Cis isomer is approximately 4 kJ/mol higher in energy than trans isomer
- Both isomers combust to identical products, confirming relative stability difference
Alkene Substitution and Stability
| Substitution Level | Alkyl Groups | Relative Stability |
|---|
| Monosubstituted | 1 | Least stable |
| Disubstituted | 2 | More stable |
| Trisubstituted | 3 | Very stable |
| Tetrasubstituted | 4 | Most stable |
- More alkyl groups attached to double bond carbon atoms increase overall stability
- Hyperconjugation explains stability increase: alkyl groups stabilize C=C ฯ bond
- Tetrasubstituted alkenes represent the most stable configuration with four alkyl substituents
- Stability ranking helps predict major products in elimination reactions
Cyclic Alkene Stability
- Cyclic alkenes with fewer than seven carbons only exist stably in cis configuration
- Trans configuration requires excessive ring strain energy for small rings
- Cyclopropene cannot exist with trans double bond due to geometric impossibility
- Rings with seven or more carbons can accommodate trans configurations
Bredt's Rule
- Applies specifically to bicyclic alkene compounds with bridgehead double bonds
- Bridgehead double bonds are unstable in bicyclic systems with fewer than eight carbons
- Trans ฯ bond at bridgehead position prevented by poor p orbital overlap
- One ring must contain at least eight carbons for stable bridgehead double bond
Key Terms & Definitions
- Steric strain: Repulsive interactions between atoms/groups forced into close proximity
- Heat of combustion: Energy released when compound burns completely; reveals relative energy content
- Hyperconjugation: Stabilizing interaction where alkyl groups donate electron density to ฯ bond
- Bredt's rule: Bridgehead double bonds in bicyclic systems require minimum eight-carbon ring
Action Items / Next Steps
- Continue with elimination reactions discussion in next session
- Apply stability concepts to predict major products in upcoming elimination reaction problems