Step 1: Count the number of alkyl substituents on the double bond
Key rule:
The more alkyl groups attached to the double bond, the more stable the alkene.
Why?
Because of hyperconjugation. The sigma (σ) bonds of alkyl groups interact with the pi (π) orbital of the double bond.
This overlap allows the electron density to spread across a larger area, lowering the overall energy of the molecule and increasing stability.
General trend for alkene stability:
Your task:
Locate the double bond and count how many alkyl groups are directly attached.
Step 2: Check for cis vs trans (or E/Z) isomers
Key rule:
Trans alkenes are more stable than their cis counterparts.
Why?
This stability difference is due to steric strain.
Trans alkenes have bulky groups on opposite sides of the double bond, minimizing repulsion and making the molecule more stable.
Cis alkenes have bulky groups on the same side of the double bond, creating repulsion and increasing the molecule’s energy, thus reducing stability.
When to use E/Z:
Use the E/Z system when simple cis/trans labels cannot clearly describe the arrangement of substituents around the double bond. The logic remains consistent:
- E-isomers: High-priority groups on opposite sides (like trans)
- Z-isomers: High-priority groups on the same side (like cis)
Consider cis/trans (or E/Z) when comparing alkenes with the same number of alkyl substituents.
Step 3: Look for conjugation
Key rule:
Alkenes conjugated with another double bond or an aromatic ring are more stable.
What is conjugation?
Conjugation occurs when pi bonds are separated by exactly one sigma bond, creating a continuous chain of overlapping p orbitals.
This arrangement allows electrons to delocalize (spread out) across multiple atoms.
Why does this matter?
Just like hyperconjugation, delocalizing electron density lowers the molecule’s energy, making conjugated alkenes more stable.
Step 4: Use heats of hydrogenation (ΔH° hydrogenation) if provided
Key rule:
More stable alkenes have lower heats of hydrogenation.
Why?
The heat of hydrogenation indicates how much energy is released when hydrogen is added to an alkene, converting it to an alkane. If less energy is released, it means the starting alkene was already relatively stable (low in energy).
Thus, if you’re provided with ΔH° values, the alkene with the lowest number is the most stable.
Step 5: Rank the alkenes
Now that you’ve walked through each step, let’s quickly recap the factors you’ve considered:
1️⃣ Alkyl substitution:
More alkyl substituents → more stable
2️⃣ Cis/trans (or E/Z) isomerism:
Trans (or E) isomers → more stable than Cis (or Z) isomers
3️⃣ Conjugation:
Conjugated alkenes → more stable than isolated alkenes
4️⃣ Heats of hydrogenation (ΔH°):
Lower heat of hydrogenation → more stable
Using these key principles, you now have all the tools to accurately and confidently rank any set of alkenes by their stability.
