Ranking alkenes by stability

Challenge 1: Ranking alkenes by substitution pattern

Question:
Rank the following alkenes according to their stability.

Step 1: Classify the alkenes

The first step is to classify each alkene based on the number of carbon substituents (alkyl groups) attached to the double bond. Here’s how:

1. Highlight the carbons in the double bond.
2. Count the number of carbons directly bonded to these double-bond carbons. These are your substituents.

Let’s apply this to the alkenes in question:

• Alkene A: 3 carbon substituents → Trisubstituted
• Alkene B: 2 carbon substituents → Disubstituted
• Alkene C: 4 carbon substituents → Tetrasubstituted
• Alkene D: 1 carbon substituent → Monosubstituted

Great! Now that we’ve classified the alkenes, let’s move on to ranking them.

Step 2: Rank the alkenes by stability

To understand stability, we need to consider two important factors: substitution and steric effects.

Substitution

The golden rule is: The more alkyl groups attached to the double bond, the more stable the alkene.

Why? Because of hyperconjugation: The σ-bonds of the alkyl groups interact with the π-orbital of the double bond. This interaction spreads out the electron density across the molecule, lowering its overall energy and increasing stability.

General trend for alkene stability:

You’ll often see this trend summarized as:

Let’s apply this trend to our alkenes:

C (tetrasubstituted) is the most stable because it has the most alkyl groups. D (monosubstituted) is the least stable because it has only one alkyl group.

So far, our ranking looks like this:
C > A > B > D

Steric Effects

Now, let’s talk about steric effects, which come into play when we compare alkenes with the same level of substitution.

Steric effects arise from how atoms or groups are arranged in space. When bulky groups get too close to each other, their electron clouds start to repel one another. This repulsion creates steric strain, which raises the molecule’s energy and makes it less stable.

Now, how does this apply to alkenes?

• Trans alkenes are more stable because the bulky groups are on opposite sides of the double bond. This spacing reduces crowding and minimizes steric strain.
• Cis alkenes are less stable because the bulky groups are on the same side of the double bond. This brings them closer together, causing more repulsion and making the molecule more strained.

So, the bottom line is: trans alkenes are more stable than cis alkenes due to less steric strain.

Final ranking

From most stable to least stable:

C (tetrasubstituted) > A (trisubstituted) > B (disubstituted) > D (monosubstituted)

Challenge 2: Ranking alkenes by substitution pattern and steric effects

Question:
Rank the following alkenes from least stable to most stable.

Ready? Let’s dive in!

Step 1: Classify the alkenes

As before, we classify each alkene based on substitution:

• Alkene A: 3 carbon substituents → Trisubstituted
• Alkene B: 2 carbon substituents → Disubstituted
• Alkene C: 1 carbon substituent → Monosubstituted
• Alkene D: 3 carbon substituents → Trisubstituted

Step 2: Rank the alkenes by stability

We already know the general trend: More substituted alkenes are more stable. So, we can immediately rank:

C (monosubstituted) < B (disubstituted) < ? < ?

But now, we need to compare A and D. They are both trisubstituted, so what sets them apart? Let’s dive deeper into steric effects.

The E/Z System

For trisubstituted (and tetrasubstituted) alkenes, we use the E/Z system to describe their spatial arrangement:

• E (entgegen): High-priority groups are on opposite sides of the double bond, like in trans alkenes.
• Z (zusammen): High-priority groups are on the same side of the double bond, like in cis alkenes.

Now, let’s apply the E/Z system to Alkene A and Alkene D:

Alkene A has an E configuration (trans), meaning less steric strain and more stability. Alkene D has a Z configuration (cis), meaning more steric strain and less stability.

So, A is more stable than D.

Final ranking

From least stable to most stable:

C (monosubstituted) < B (disubstituted) < D (trisubstituted, cis) < A (trisubstituted, trans).

Linking stability and energy

Sometimes, you might be asked to rank alkenes based on their energy instead of their stability. Don’t let this throw you—it’s really just a different perspective on the same idea!

Here’s the key relationship to keep in mind:

Stability and energy are inversely related.
The more stable a molecule is, the lower its energy.

This means that ranking alkenes by energy is just like ranking them by stability, but in reverse:

• The most stable alkene has the lowest energy.
• The least stable alkene has the highest energy.

Let’s revisit the alkenes we analyzed earlier. When ranked by stability, the order (from least stable to most stable) was:

C (monosubstituted) < B (disubstituted) < D (trisubstituted, cis) < A (trisubstituted, trans).

Now, if we’re ranking them by energy instead, we simply reverse the logic:

A (trisubstituted, trans) < D (trisubstituted, cis) < B (disubstituted) < C (monosubstituted).

This new order reflects that the most stable alkene (A) corresponds to the lowest energy, while the least stable alkene (C) corresponds to the highest energy.

💡 Key takeaway: When you see a question framed in terms of energy, just remember that lower energy equals greater stability. Use the same steps to determine stability, then translate it into energy rankings. It’s that simple!

Quick recap

1. Classify each alkene by counting the number of alkyl substituents (groups) attached to the double bond.
2. Apply the substitution trend: The more substituted the alkene, the more stable it is.
3. For similarly substituted alkenes, use steric effects to further assess stability:
   • For disubstituted alkenes, compare their cis/trans configuration (trans is more stable).
   • For trisubstituted and tetrasubstituted alkenes, use the E/Z system (E is more stable than Z).