Challenge 1
Question:
Rank the following carbocations according to stability.
Step 1: Classify the carbocation
Alright, let’s get started by identifying the carbocation. Your mission: find the carbon atom with the positive charge—the one marked with a big “+.” That’s your carbocation center.
Once you’ve located it, the next question is: How many carbon (alkyl) groups are directly bonded to this positively charged carbon?
Here’s how we classify carbocations based on the number of attached carbon groups:
- Primary (1°): Attached to 1 carbon group.
- Secondary (2°): Attached to 2 carbon groups.
- Tertiary (3°): Attached to 3 carbon groups.
But there’s more! If the carbocation is next to:
- A double bond, it’s allylic.
- A benzene ring, it’s benzylic.
If the carbocation is part of a double bond, it’s vinylic.
We can classify the carbocations as follows:
- A: Three carbon groups → Tertiary (3°)
- B: Three carbon groups and next to a double bond → Tertiary allylic (3° allylic)
- C: One carbon group → Primary (1°)
- D: Three carbon groups and next to a double bond → Tertiary allylic (3° allylic)
- E: Two carbon groups → Secondary (2°)
Step 2: Consider what stabilizes or destabilizes a carbocation
Now that you’ve classified the carbocations, it’s time to think about what makes some carbocations more stable than others. There are two big stabilizing factors: substitution and resonance. Let’s break them down.
1. Substitution: The more, the merrier
Key rule: The more carbon substituents directly attached to the carbocation, the more stable it is.
Why? Two big reasons:
- Hyperconjugation: Neighboring C–H bonds can overlap slightly with the empty p-orbital of the carbocation, donating electron density to help stabilize the positive charge.
- Inductive Effect: Alkyl groups push electron density toward the carbocation, reducing the charge density.
Here’s the stability ranking based on substitution alone:
Methyl < Primary (1°) < Secondary (2°) < Tertiary (3°)
2. Resonance: Sharing is caring
Carbocations LOVE resonance because it spreads the positive charge over multiple atoms, dramatically increasing stability.
a. Resonance with pi bonds
Key rule: Allylic and benzylic carbocations are more stable than regular carbocations with the same level of substitution.
Why? The positive charge in allylic and benzylic carbocations is spread out through resonance, reducing the charge density and stabilizing the carbocation.
When you combine substitution and allylic/benzylic resonance, you get this overall trend for carbocation stability:
For this question:
- B and D are both tertiary allylic carbocations, making them the most stable because they benefit from both substitution and resonance.
- Among A, C, and E, stability is determined by substitution: A (3°) > E (2°) > C (1°).
Ranking so far (most stable to least stable):
? > ? > A > E > C
b. Resonance with atoms with lone pairs
Key rule: Atoms with lone pairs (like oxygen or nitrogen) next to a carbocation are powerful stabilizers.
Why? They can donate their lone pair of electrons through resonance, spreading out the positive charge and creating a more stable structure.
Let’s compare B and D, our tertiary allylic carbocations.
Carbocation B
The oxygen atom donates a pair of electrons to the carbocation, forming a resonance structure that gives the carbocation a full octet—much more stable than having an empty orbital.
Carbocation D
While D does get some resonance stabilization from the nearby pi bond (because it’s allylic), it doesn’t have a nearby atom with a lone pair to help out. In both of its resonance structures, the carbon doesn’t have a full octet, which leaves it less stable overall.
Wait, isn’t oxygen electronegative? Won’t that destabilize the carbocation?
Great question! While it’s true that oxygen is highly electronegative, its ability to donate electrons (via resonance) more than makes up for any destabilization caused by electronegativity.
So, the final takeaway: B is more stable than D because of the stabilizing effect of the nearby oxygen atom.
Final ranking:
From most stable to least stable: B > D > A > E > C
Challenge 2:
Question:
Rank the following carbocations according to stability.
Step 1: Classify the carbocation
First things first—just like in the previous challenge, we need to identify and classify each carbocation. Look for the positively charged carbon (the one with the “+”) and count the number of carbon groups directly attached to it. Ask yourself:
- Is the carbocation next to a double bond? If so, it’s allylic.
- Is it part of a benzene ring? Then it’s benzylic.
Now, let’s classify the carbocations:
- A: Three carbon groups and next to two double bonds → Tertiary allylic (3° allylic)
- B: Two carbon groups and next to one double bond → Secondary allylic (2° allylic)
- C: Three carbon groups → Tertiary (3°)
- D: Two carbon groups → Secondary (2°)
Step 2: Consider what stabilizes or destabilizes a carbocation
Now that we’ve classified the carbocations, let’s remember what makes some carbocations more stable than others. As we discussed earlier, there are two main factors: substitution and resonance.
- Substitution: More alkyl (carbon) groups → more stability.
- Resonance: The ability to spread out the positive charge across multiple atoms → better stability.
Here’s the general stability trend when combining substitution and resonance:
Using the trend above, we can already make some initial observations:
- D (2°): The least stable—it has no resonance and only moderate substitution.
- C (3°): More stable than D because it has higher substitution, but it still doesn’t benefit from resonance.
- A (3° allylic) and B (2° allylic): Both are stabilized by resonance, but we might initially think A is more stable because it’s tertiary.
However, 2° allylic carbocations can be roughly as stable as 3° carbocations, thanks to resonance. To confirm the exact ranking of A, B, and C, we need to dig deeper into the resonance structures.
Analyzing resonance structures
Let’s take a closer look at A and B by drawing out their resonance structures.
Carbocation A:
The positive charge is delocalized across two 2° carbons and one 3° carbon.
Carbocation B:
The positive charge is delocalized across one 2° carbon and two 3° carbons.
Although we initially classified B as a 2° allylic, it turns out that B is actually more stable than A because its positive charge is spread over more 3° carbons (which are more stable than 2° carbons).
Carbocation C (3°):
Unlike A and B, C doesn’t have resonance stabilization. Its positive charge is stuck on a single tertiary carbon. Although C is stable because it’s tertiary, it’s still less stable than A and B, both of which have resonance delocalizing the positive charge across three atoms.
Final ranking
- B: Most stable because the positive charge is delocalized across two 3° carbons and one 2° carbon.
- A: Second most stable because the positive charge is delocalized across one 3° carbon and two 2° carbons.
- C: The positive charge is stuck on one 3° carbon, which is less stable than the resonance-stabilized carbocations A and B.
- D: Least stable, with the positive charge stuck on one 2° carbon with no resonance stabilization.
From most stable to least stable: B > A > C > D
Recap
- Substitution matters: More alkyl groups = more stability.
- Resonance is key: Whether it’s with pi bonds or lone pairs, resonance spreads the positive charge and stabilizes the carbocation.
- Combine these effects: Look for both substitution and resonance to determine overall stability.
And there you have it—everything you need to confidently classify and rank carbocations like a pro! 🚀
