Challenge 1

Question:

Rank the following carbocations according to stability (1 = most stable, 5 = least stable.)

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°)
• C: One carbon group → Primary (1°)
• D: Three carbon groups and next to a double bond → Tertiary allylic (3°)
• 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

Recap

1. Substitution matters: More alkyl groups = more stability.
2. Resonance is key: Whether it’s with pi bonds or lone pairs, resonance spreads the positive charge and stabilizes the carbocation.
3. 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! 🚀