Ranking molecules by SN2 reactivity

S_N2 reactions

In S_N2 reactions, the nucleophile attacks the substrate expelling the leaving group in a single step.

Since both events occur at the same time, the reaction follows a concerted mechanism and its rate depends on both the nucleophile and the substrate.

In this guide, we’ll walk you through how to analyze and rank substrates based on their S_N2 reactivity, so you can confidently predict which reactions will proceed quickly — and which ones won’t.

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. This is where the nucleophile will attack during the S_N2 process.

Now ask yourself:

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

Why is this important? This is because S_N2 reactions are highly sensitive to steric hindrance — or how crowded the α carbon is.

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_N2 reactions. The geometry of these systems doesn’t allow for the required backside attack.

Step 2: Assess steric hindrance

Key rule: The more substituted (crowded) the α and β carbons are, the slower the S_N2 reaction.

Why?

In S_N2 reactions, the nucleophile performs a backside attack at the α carbon.

The more crowded the carbon is, the harder it is for the nucleophile to attack— making the substrate less reactive.

At the α carbon:

The rate of an S_N2 reaction is most sensitive to the number of substituents at the α carbon.

Here’s the general trend:

At the β carbon:

Steric effects from groups one carbon away (β position) also slow the S_N2 reaction — just not as dramatically as α-substitution does.

Step 3: Look for adjacent π systems

Key rule: Substrates with adjacent C–C or C–O π bonds undergo S_N2 faster than simple alkyl substrates.

Why? These adjacent π systems help stabilize the transition state and increase the rate of the reaction.

For example, allylic and benzylic systems enhance S_N2 reactivity.

α-Halo carbonyl compounds — where the α carbon is adjacent to a carbonyl (C=O) — are among the fastest S_N2 substrates.

Step 4: Evaluate the leaving group 

In an S_N2 reaction, the leaving group is expelled with a negative charge. So, the best leaving groups are those that stabilize that negative charge well.

If two molecules have similar steric hindrance, then compare them by leaving group ability.

Leaving group reactivity (and thus S_N2 reactivity) trend:

For alkyl halides, the trend typically follows:

Step 5: Rank the molecules

Now that you have considered:

• Substrate type (Step 1)
• Steric hindrance (Step 2)
• Presence of adjacent C-C or C-O π systems (Step 3)
• Leaving group ability (Step 4)

Put it all together to confidently rank molecules by their S_N2 reactivity!

Quick recap

1️⃣ Classify the substrate

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

2️⃣ Assess steric hindrance

The more substituted (crowded) the α and β carbons are, the slower the S_N2 reaction.

3️⃣ Look for adjacent pi systems

• Substrates with adjacent C–C or C–O π bonds undergo S_N2 faster than simple alkyl substrates.
• Especially fast: α carbon next to a carbonyl (C=O).

4️⃣ Check leaving group ability

• The better the leaving group, the faster the reaction.
• Rank molecules by leaving group reactivity if they have the same type of steric hindrance.

5️⃣ Rank the molecules

Combine steric hindrance + π systems + leaving group ability.

You did it! 🎉

With practice, these steps will become second nature—and you’ll be ranking molecules by S_N2 reactivity like a pro. Got questions or thoughts? Share them below!