Predicting the products of SN2 reactions

S_N2 – Substitution Nucleophilic Bimolecular

The S_N2 reaction happens in a single step—the nucleophile attacks causing the leaving group to exit.

Because the nucleophile approaches from the opposite side of the leaving group (a backside attack), the reaction causes an inversion of configuration at the chiral center.

This means that if the starting material was R, the product will be S, and vice versa.

Since the configuration of the product directly depends on the structure of the starting material, S_N2 reactions are stereospecific.

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_N2 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^–

📌 Pro tip: The leaving group must be attached to an sp³-hybridized carbon. No S_N2 on sp² (alkenes) or sp-hybridized centers (alkynes)!

If the leaving group isn’t good, the S_N2 reaction likely won’t proceed—unless you convert it into a better one.

Step 2: Predict the product 

Now that we’ve confirmed the leaving group is good, let’s predict the product.

Net result: The nucleophile replaces the leaving group.

If the electrophilic carbon is chiral, draw the product with the opposite configuration (R → S or S → R).

R → S

S → R

You did it!  🎉

With practice, these steps will become second nature—and you’ll be predicting the products of S_N2 reactions like a pro. Got questions or thoughts? Share them below!