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, so no deprotonation needed.

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!