Ranking molecules by SN1 reactivity

S_N1 reactions

The S_N1 reaction proceeds in two main steps:

1️⃣ The leaving group departs, forming a carbocation intermediate — this is the rate-determining step.

2️⃣ The nucleophile then attacks the carbocation to complete the substitution.

Because the rate-determining step is the formation of the carbocation, two key factors determine how reactive a molecule is in an S_N1 reaction:

• How stable the carbocation will be after the leaving group departs
• How stable the leaving group is once it leaves

Molecules that form more stable carbocations—and have better leaving groups—undergo S_N1 reactions more rapidly.

In this guide, we’ll walk through a systematic approach to:

• Classifying substrates
• Evaluating carbocation stability
• Assessing leaving group ability

By the end, you’ll be able to confidently rank molecules in order of their S_N1 reactivity.

Ready? Let’s dive in! 🚀

Step 1: Classify the substrate

In substitution (and elimination) reactions, the molecule that contains the leaving group is called the substrate.

✅ Start by locating the carbon that’s directly bonded to the leaving group. This carbon is called the alpha (α) carbon.

Now ask yourself:

How many alkyl (carbon) groups are attached to that α carbon?

Why is this important? Because it tells you what kind of carbocation will form when the leaving group departs, a key step in S_N1 reactions.

Substrates are classified as methyl, primary (1°), secondary (2°), or tertiary (3°) based on the number of alkyl groups connected to the α position.

But there’s more!

• If the α carbon is next to a double bond, it’s allylic.
• If it’s next to a benzene ring, it’s benzylic
• If the α-carbon is part of a double bond, it’s vinylic
• If the α-carbon is part of a benzene ring, it’s aryl.

🚫 Vinylic and aryl substrates do not undergo S_N1 reactions as vinylic and aryl carbocations are unlikely to form.

Step 2: Evaluate carbocation stability

In S_N1 reactions, the rate-determining step is when the leaving group leaves, forming a carbocation.

So, the more stable that carbocation is, the faster the S_N1 reaction goes.

Let’s look at what stabilizes carbocations:

1️⃣ Alkyl substituents stabilize carbocations

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.

As a result, we get the following trend for carbocation stability (and therefore S_N1 reactivity):

Methyl << Primary (1°) < Secondary (2°) < Tertiary (3°)

2️⃣ Adjacent C-C pi bonds stabilize carbocations

Key rule: Allylic and benzylic carbocations are more stable than regular carbocations with the same number of alkyl substituents.

Why? The positive charge in allylic and benzylic carbocations is delocalized (spread out) across multiple atoms, reducing the charge density and stabilizing the carbocation.

3️⃣ Adjacent lone pairs stabilize carbocations

If an atom next to the carbocation has lone pairs, it can donate electrons via resonance.

This gives the carbocation a full octet, making it much more stable than a structure with an empty p orbital.

So always check for nearby heteroatoms like oxygen or nitrogen—they can offer major stabilization through lone pair donation.

Key takeaway:

 

Carbocation stability is the major driver of S_N1 reactivity, and features like resonance and lone pairs significantly increase that stability.

Step 3: Assess the leaving group

In an S_N1 reaction, the leaving group is involved in the rate-determining step, so the better the leaving group, the faster the reaction.

Here’s how to assess reactivity:

• If molecules form different types of carbocations, prioritize carbocation stability.
• If they form the same type of carbocation, then compare leaving group ability.

What makes a good leaving group? It must be stable on its own after it leaves.

Leaving group reactivity (and thus S_N1 reactivity) trend:

For alkyl halides, the trend typically follows:

Step 4: Rank the molecules

Now that you have considered:

• Substrate type (Step 1)
• Carbocation stability (Step 2)
• Leaving group ability (Step 3)

You can confidently rank molecules in terms of S_N1 reactivity.

Quick recap

1️⃣ Classify the substrate

Find the α carbon → Count attached alkyl groups → Identify allylic/benzylic/vinylic character

2️⃣ Evaluate carbocation stability

The more stable the carbocation, the faster the reaction. Carbocations are stabilized by:

• • Alkyl substituents
  • Resonance (C=C or lone pairs nearby)

3️⃣ Assess leaving groups

• The better the leaving group, the faster the reaction.
• Rank molecules by leaving group reactivity if they form the same type of carbocation.

4️⃣ Rank the molecules

Combine carbocation stability and leaving group quality to predict S_N1 reactivity.

You did it! 🎉

With practice, these steps will become second nature—and you’ll be ranking molecules by S_N1 reactivity like a pro. Got questions or thoughts? Share them below!