Predicting the products of SN1 reactions

Step 1: Identify a good leaving group

Let’s start by checking if the molecule has a good leaving group—this is essential for any S_N1 reaction to occur.

A good leaving group can stabilize a negative charge it carries after departure.

Common good leaving groups:

• Halides: Cl, Br, I
• Sulfonate esters: OTs (tosylate), OMs (mesylate), OTf (triflate)

Poor leaving groups:

• F^–
• OH^– (unless it’s protonated to become H₂O)

📌 Pro tip: The leaving group must be attached to an sp³-hybridized carbon. No S_N1 on sp² (alkenes) or sp-hybridized centers (alkynes)!

If the leaving group isn’t good, the S_N1 reaction likely won’t proceed—unless it’s first transformed into a better one under acidic conditions.

Step 2: Draw the carbocation intermediate

Once the leaving group departs, a carbocation intermediate is formed. This is a key feature of the S_N1 mechanism.

At this stage, it’s important to pause and draw the carbocation.

You’ll need this structure to evaluate rearrangements, resonance, and potential reaction pathways in the next steps.

Step 3: Check for carbocation rearrangements 

Now that you have your carbocation drawn, ask yourself: Can it rearrange to form a more stable carbocation?

Because carbocations are inherently unstable, they will rearrange if a more stable form is possible.

Three common types of rearrangements:

1️⃣ 1,2-Hydride shifts (movement of a hydrogen with its bonding pair)

2️⃣ 1,2-Methyl shifts (movement of a methyl with its bonding pair)

3️⃣ Ring expansions (common in small rings to relieve strain)

🧠 Recognizing when and how rearrangements occur is a critical skill—and one we explore in more detail in a separate guide.

Step 4: Predict the final product

Now it’s time for the nucleophile to enter the scene and replace the leaving group.

Here’s what to keep in mind:

1️⃣ Since the carbocation intermediate is planar (flat), the nucleophile can attack from either side.

If the carbocation is chiral (attached to three different groups), this leads to a racemic mixture—equal amounts of both stereoisomers.

If the carbocation is achiral, only one stereoisomer is formed.

2️⃣ If a neutral nucleophile is used (like H₂O, MeOH, or NH₃), the initial product will be protonated (e.g., NH₃ → NH₃⁺). It will then lose a proton to form the neutral final product.

3️⃣ If a negatively charged nucleophile (like Br^– or CN^–) is used, the product is neutral right away—no deprotonation needed.

Special cases to consider

Allylic substrates

In allylic systems, the carbocation formed is resonance-stabilized.

As a result, the positive charge is delocalized across two carbons. This means the nucleophile can attach at either site, depending on the conditions.

This leads to a mixture of regioisomers.

Benzylic substrates

In benzylic systems, the benzyl carbocation is stabilized by resonance with the aromatic ring.

However, to preserve aromaticity, nucleophilic attack always happens on the side chain, not the ring.

With practice, these steps will become second nature—and you’ll be predicting the products of S_N1 reactions like a pro. Got questions or thoughts? Share them below!