Explain why each of the following alkenes is stable or unstable.
(h)
(i)

Verified step by step guidance

1. Step 1: Analyze the first alkene structure (image h). The double bond is part of a large ring system. Consider the ring strain and steric hindrance in the molecule. Larger rings generally have less ring strain compared to smaller rings, making the alkene more stable.
2. Step 2: Evaluate the second alkene structure (image i). The double bond is part of a bicyclic system. Bicyclic systems often have significant ring strain due to the rigid structure and limited flexibility, which can destabilize the alkene.
3. Step 3: Consider the hybridization of the carbon atoms involved in the double bond for both structures. Sp2 hybridized carbons in alkenes are planar, and any deviation from planarity due to ring strain can affect stability.
4. Step 4: Assess the substituents around the double bond in both structures. Substituents can stabilize alkenes through hyperconjugation or inductive effects. For example, alkyl groups attached to the double bond can donate electron density, stabilizing the alkene.
5. Step 5: Compare the overall molecular geometry and steric interactions in both structures. The first structure (h) likely has less steric hindrance and strain compared to the second structure (i), making it more stable. The second structure (i) has a rigid bicyclic framework, which increases strain and reduces stability.

Key Concepts

Alkene Stability

The stability of alkenes is influenced by the degree of substitution around the double bond. More substituted alkenes, such as tetrasubstituted ones, are generally more stable due to hyperconjugation and the electron-donating effects of alkyl groups, which help to stabilize the double bond.

Geometric Isomerism

Alkenes can exhibit geometric isomerism (cis/trans or E/Z isomerism) due to the restricted rotation around the double bond. The spatial arrangement of substituents affects the steric interactions and overall stability of the molecule, with trans isomers typically being more stable than their cis counterparts due to reduced steric strain.

Resonance and Conjugation

Resonance and conjugation can significantly enhance the stability of alkenes. When alkenes are part of a conjugated system, the delocalization of electrons across multiple bonds can lower the overall energy of the molecule, making it more stable compared to isolated double bonds.