SN2 vs SNA vs E1 vs E2- Reaction Mechanisms Compared

SN1 vs SN2 vs E1 vs E2: What You Actually Need to Know

If you're taking organic chemistry, you've probably stared at a reaction and had no idea whether it'll go SN1, SN2, E1, or E2. That's normal. Most students do. The good news is these four mechanisms follow predictable rules. Learn the patterns, and you'll stop guessing.

These are the four major reaction mechanisms you'll encounter for alkyl halides and similar substrates. Each one has specific conditions that favor it. Get those conditions down, and you can predict products without memorizing everything.

SN2: Bimolecular Nucleophilic Substitution

SN2 reactions involve a single step where the nucleophile attacks the electrophile at the same time the leaving group leaves. It's a backside attack, which means stereochemistry inverts—like an umbrella flipping inside out in the wind.

Key Characteristics of SN2

The nucleophile must physically reach the carbon bearing the leaving group. If there are three bulky groups in the way, it's not happening. That's why methyl halides react fastest, followed by primary, then secondary. Tertiary? Forget it.

Good Nucleophiles for SN2

You need strong nucleophiles for SN2. Charge helps—negatively charged species are better than neutral ones. Some reliable ones:

Weak nucleophiles like water or alcohols won't drive SN2. They'll sit around waiting for something else to happen.

SN1: Unimolecular Nucleophilic Substitution

SN1 reactions happen in two steps. First, the leaving group leaves on its own, forming a flat carbocation intermediate. Then the nucleophile attacks from either side of that carbocation.

Key Characteristics of SN1

Because the carbocation is planar, the nucleophile can attack from either face. If you start with a single enantiomer, you get both R and S products. That's racemization.

When SN1 Dominates

SN1 wins when:

Polar protic solvents like water or alcohols actually help SN1 by solvating and stabilizing the charged intermediate. That's the opposite of SN2, where these solvents slow things down by hydrogen-bonding to the nucleophile.

E2: Bimolecular Elimination

E2 is elimination, meaning you're removing a hydrogen and a leaving group to form a double bond. Like SN2, it's a single concerted step. The base pulls off the hydrogen at the same time the leaving group departs.

Key Characteristics of E2

The anti-periplanar requirement matters. If the hydrogen and leaving group can't get opposite each other due to ring strain or sterics, E2 won't happen easily. Cyclohexane rings in particular lock in anti-periplanar geometry for axial substituents.

Strong Bases for E2

These bases are strong enough to rip a proton off while the leaving group exits. Weaker bases can't keep up with the timing.

E1: Unimolecular Elimination

E1 follows the same first two steps as SN1—leaving group leaves, carbocation forms. Then instead of a nucleophile attacking, a base removes a proton from a carbon adjacent to the carbocation, forming the double bond.

Key Characteristics of E1

Because the carbocation can lose a proton from different carbons, you often get mixtures of alkene products. The more substituted double bond usually predominates, but you're not guaranteed a single product.

E1 vs SN1: Same Starting Point, Different Outcome

After the carbocation forms, it either:

The nucleophile versus base competition determines which pathway dominates. Strong nucleophiles favor SN1. Weak nucleophiles with strong bases favor E1.

Side-by-Side Comparison

Feature SN2 SN1 E2 E1
Steps One Two One Two
Kinetics Second-order First-order Second-order First-order
Rate depends on Substrate + nucleophile Substrate only Substrate + base Substrate only
Stereochemistry Inversion Racemization Anti-periplanar required No stereochemical preference
Best substrate Methyl, 1°, 2° 3°, allylic Unhindered 2°, 3° 3°, allylic
Leaving group Must be good Must be good Must be good Must be good
Base/nucleophile strength Strong nucleophile needed Weak nucleophile OK Strong base needed Weak base OK
Solvent effect Polar aprotic speeds it up Polar protic stabilizes carbocation No strong preference Polar protic helps

How to Predict Which Mechanism Occurs

Here's the decision tree that actually works on exams:

Step 1: Check the Substrate

Tertiary substrate? → Elimination wins (E1 or E2). SN2 is impossible. SN1 is possible but elimination usually dominates with strong bases.

Primary substrate? → SN2 is your main option. E2 is possible with a very strong base, but SN2 usually wins unless the base is huge (like t-butoxide).

Secondary substrate? → This is where it gets complicated. You need to look at other factors.

Step 2: Check the Reagent

Strong nucleophile AND strong base? → SN2 or E2 (depending on substrate). If it's also bulky, E2 wins even at secondary carbons.

Weak nucleophile, weak base? → SN1 or E1 (carbocation pathway). Which one actually happens depends on what else is present.

Strong nucleophile, weak base? → SN2. The nucleophile attacks before elimination can happen.

Step 3: Check the Solvent

Polar aprotic solvent (acetone, DMSO, DMF) → SN2 or E2. These solvents don't stabilize the nucleophile, so it's more reactive.

Polar protic solvent (water, alcohols) → SN1 or E1. These stabilize carbocations and slow down anions.

The Quick Rules

Getting Started: Working Through a Problem

Say you see 2-bromopropane reacting with hydroxide. How do you figure out what happens?

1. Identify the substrate. 2-bromopropane is secondary. That rules out SN1 (tertiary only) and makes SN2 unlikely but possible.

2. Identify the reagent. Hydroxide is both a strong nucleophile and a strong base. That's your clue.

3. Check conditions. If this is in water (polar protic), SN1/E1 becomes more likely. In an alcohol solvent, similar. In DMSO (polar aprotic), SN2/E2 dominates.

4. Make your call. With hydroxide and a secondary substrate in polar aprotic solvent, you'll get mostly SN2 with some E2. In polar protic, you'll get a mixture of SN1, E1, and E2—probably elimination products dominate.

This is why exam questions always specify conditions. Change the solvent, change the product distribution.

Carbocation Rearrangements

Both SN1 and E1 involve carbocations. Those carbocations can rearrange—a hydrogen or methyl group shifts to form a more stable carbocation. When that happens, your product reflects the rearranged structure, not the original.

You'll know rearrangement happened if the carbon skeleton of your product doesn't match the starting material. Watch for this on exams. If you draw a product and the carbon chain looks different, you probably had a rearrangement.

What About Competing Pathways?

In the real world, reactions don't give you one clean product. You get mixtures. A secondary substrate with a moderate base might give you SN2 product, E2 product, and maybe some SN1 if conditions allow. That's normal.

Your job isn't to predict a single product—it's to predict the major product and explain why. Focus on the factors that favor one pathway over others. The substrate is usually the biggest factor, followed by the reagent strength.

The Bottom Line

SN1 and SN1 differ in steps, stereochemistry, and what controls the rate. E1 and E2 are elimination counterparts with similar distinctions. The patterns are consistent:

Once you internalize these patterns, you stop having to guess. You just read the conditions and the mechanism follows.