On paper, SN1 and SN2 reactions look almost identical. A nucleophile comes in, a leaving group goes out, and one carbon swaps partners! What separates them is how and when that swap happens, and the MCAT likes to test it.
In this note, we’ll put SN1 vs SN2 side by side, then walk through the rate laws, inversion versus racemization, and the four factors that decide which path a reaction takes. Let’s get started 🙂
SN1 vs SN2: The Difference at a Glance
Both are nucleophilic substitution reactions. A nucleophile, an electron-rich species with a lone pair to share, replaces a leaving group on a saturated (sp3) carbon.
The S stands for substitution, the N for nucleophilic, and the number for how many molecules take part in the slow step. That’s the heart of SN1 vs SN2!
Organic chemistry makes up about 15% of the Chem/Phys section, by the AAMC’s estimate, and these two mechanisms come up often in its reaction questions. Here’s the whole SN1 vs SN2 comparison in one chart!
The 2 in SN2 says two molecules, the substrate and the nucleophile, meet in its one step, so both appear in the rate law. The 1 in SN1 says only the substrate takes part in the slow step.
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The SN1 vs SN2 Cheat Sheet
Which mechanism a substrate takes, what the solvent does to it, and how stereochemistry comes out. Three pages.
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What Are SN2 Reactions?
SN2 stands for substitution, nucleophilic, bimolecular. The nucleophile attacks the carbon at the same moment the leaving group leaves, so the whole reaction happens in one step with no intermediate!
The nucleophile comes in from the back side, 180° from the leaving group. At the transition state, the carbon is partly bonded to both, with its other three groups flattened into one plane.
As the new bond forms, the old bond breaks, and the leaving group takes that electron pair with it.
SN2 in action
Bromomethane and hydroxide give methanol and bromide in one step.
CH3Br + OH− → CH3OH + Br−
Hydroxide is a strong nucleophile, and a methyl carbon has almost nothing blocking its back side. That’s an ideal SN2 setup! Our nucleophilic substitution notes cover the general reaction if you’d like a refresher first.
What Are SN1 Reactions?
SN1 stands for substitution, nucleophilic, unimolecular. It takes two steps, and the order they happen in decides everything else about it. Here’s how it goes!
- The leaving group leaves. It takes the bonding electrons with it and leaves a carbocation, a carbon with three bonds and a positive charge. This step is slow and rate-determining.
- The nucleophile attacks the carbocation. This step is fast, since a positive carbon pulls on any electron pair nearby.
SN1 in action
tert-Butyl chloride in water reacts by SN1. First the chloride leaves.
(CH3)3CCl → (CH3)3C+ + Cl−
Then water, the solvent, adds to the carbocation and loses a proton to give tert-butyl alcohol. When the solvent acts as the nucleophile like this, it’s called a solvolysis. Nice and tidy!
(CH3)3C+ + H2O → (CH3)3COH + H+
Carbocations can rearrange
Because SN1 runs through a carbocation, a hydrogen or methyl group on the next carbon can shift over (a hydride or methyl shift) when that makes a more stable carbocation.
So the nucleophile can end up on a different carbon from the one the leaving group left! SN2 never does this, and it’s one more SN1 vs SN2 clue to look for in a product.
SN1 and SN2 Rate Laws
Each rate law comes from the slow step. In SN2, the substrate and nucleophile meet in that one step, so the rate depends on both. The reaction is first order in each and second order overall.
SN2 rate = k[R-LG][Nu−]
In SN1, the slow step is the leaving group falling off on its own. The nucleophile arrives afterward, so the SN1 rate of reaction depends only on the substrate. It’s first order, and that’s it!
SN1 rate = k[R-LG]
![SN1 vs SN2 energy diagrams: SN2 has one transition state and rate = k[R-LG][Nu-], SN1 has a slow first step, a carbocation intermediate and rate = k[R-LG]](https://medlifemastery.com/wp-content/uploads/2025/02/sn1-sn2-energy-diagram.png "SN2 v.s. SN1 Reactions - MCAT Content 2")
Double the nucleophile in SN2, and the rate doubles. Do the same in SN1, and the rate stays put.
Double the substrate and the nucleophile together, and SN2 runs 4 times faster (2 × 2) while SN1 runs 2 times faster. Nice work if you got that one!
Our reaction rates and rate laws notes cover reaction orders in more depth, if you’d like more practice!
Stereochemistry: Inversion vs Racemization
The stereochemistry of the product often tells you which mechanism ran, so it’s a favorite SN1 vs SN2 question!
In SN2, the backside attack pushes the other three groups through a flat transition state and out the other side, like an umbrella flipping in the wind. That’s inversion of configuration, also called Walden inversion.
In SN1, the carbocation is flat (sp2, with an empty p orbital), so the nucleophile can add to either face. Starting from one enantiomer, you get both, a racemic mixture.
In the lab, the mix often leans a little toward inversion. On the MCAT, though, SN1 means racemization.
Need a refresher on chirality? Our notes on enantiomers and naming R and S configurations walk through it step by step!
Factors Affecting SN1 and SN2 Reactions
Four factors decide which mechanism wins. Start with the substrate, since it often settles the question on its own!
1. Substrate structure
SN2 needs room at the back of the carbon. Each carbon group attached there blocks the nucleophile (steric hindrance), so SN2 rates fall from methyl to primary to secondary. Tertiary carbons don’t react by SN2 at all!
SN1 needs a stable carbocation. Carbon groups donate electron density and steady the positive charge, so tertiary carbocations form fastest. Methyl and primary carbocations are too unstable to form.
Secondary substrates can go either way, so the nucleophile and solvent decide. Allylic and benzylic carbons are a special case, since resonance spreads out their positive charge, and they can react by either path.
2. Nucleophile strength
SN2 needs a strong nucleophile, because the nucleophile is part of the slow step. Strong ones usually carry a negative charge, like hydroxide (OH−), alkoxides (RO−), cyanide (CN−) and azide (N3−).
SN1 works with weak, neutral nucleophiles like water and alcohols. The nucleophile only arrives after the carbocation forms, and a carbocation takes electrons from almost anything.
The solvent changes nucleophile strength too. Our effects of solvents on nucleophilicity notes explain why iodide is the strongest halide nucleophile in protic solvents and fluoride is in aprotic ones.
3. Solvent
Polar aprotic solvents, like acetone, DMSO, DMF and acetonitrile, favor SN2. They dissolve the nucleophile but can’t hydrogen bond to it, so it stays free and reactive!
Polar protic solvents, like water, methanol, ethanol and acetic acid, favor SN1. Their O-H groups hydrogen bond to the departing leaving group, and their lone pairs steady the carbocation. With both ions stabilized, the slow step speeds up. Pretty neat!
4. Leaving group
A good leaving group is a weak base that’s stable holding an extra pair of electrons.
The conjugate bases of strong acids leave well, so iodide, bromide and chloride work, and so do sulfonates like tosylate (TsO−). Fluoride and hydroxide are poor leaving groups.
Both mechanisms break the bond to the leaving group in their slow step, so a better leaving group speeds up SN1 and SN2. An alcohol’s OH becomes a good leaving group once it’s protonated, since it then leaves as water. Handy, right?
- Methyl or primary carbon + strong nucleophile + polar aprotic solvent → SN2
- Tertiary carbon + weak nucleophile + polar protic solvent → SN1
- Secondary carbon → let the nucleophile and solvent break the tie
Why Bonds Break in SN1 and SN2, in Simple Terms
In both reactions, the bond between carbon and the leaving group is polar. The leaving group is more electronegative, so it pulls the shared electrons toward itself and leaves the carbon slightly positive (δ+). That partial charge draws in the nucleophile!
In SN2, the bond breaks because it’s pushed. Carbon can’t keep five bonds, so as the nucleophile’s lone pair forms a new bond on the back side, the old bond’s electron pair shifts onto the leaving group.
In SN1, the bond breaks on its own, before any nucleophile shows up, and both electrons go to the leaving group. That happens at a useful rate only when the leaving group is stable as an anion.
The carbocation left behind has to be stable too, held up by its carbon groups and by a polar protic solvent.
How to Tell SN1 vs SN2 Apart on the MCAT
When a question asks you to call SN1 vs SN2, check these in order. You’ll get faster with practice!
- The substrate. Find the carbon holding the leaving group and count its carbon groups. Methyl and primary point to SN2, and tertiary points to SN1.
- The nucleophile. A strong, charged one pushes toward SN2. Water or an alcohol pushes toward SN1.
- The solvent. Acetone or DMSO suggests SN2, and water or ethanol suggests SN1.
- The stereochemistry. A single inverted product fits SN2, and a racemic mix fits SN1.
A passage might hand you rate data instead. If changing the nucleophile’s concentration changes the rate, it’s SN2!
Two SN1 vs SN2 worked examples
1-Bromobutane with sodium hydroxide in DMSO has a primary carbon, a strong nucleophile and a polar aprotic solvent. That’s SN2, and the product is 1-butanol!
2-Bromo-2-methylpropane in ethanol has a tertiary carbon, a weak nucleophile and a polar protic solvent. That’s SN1. Ethanol adds to the carbocation, and the product is an ether, 2-ethoxy-2-methylpropane, plus HBr. You’ve got it!
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SN1 vs SN2 Practice Questions
Try these SN1 vs SN2 questions before you open each answer! Don’t peek :)
Question 1. What type of substrate is most reactive in SN2 reactions?
A. Tertiary
B. Secondary
C. Primary
D. All three react at the same rate
Show the answer
C. Primary substrates have the least steric hindrance of these choices, so the nucleophile reaches the back side most easily. A methyl substrate would be faster still, but it isn’t an option here.
Question 2. Which solvent is ideal for SN1 reactions?
A. Acetone
B. DMSO
C. Water
D. Hexane
Show the answer
C. Water is polar protic, so it stabilizes the carbocation and the leaving group as they separate. Acetone and DMSO are polar aprotic (SN2 solvents), and nonpolar hexane won’t dissolve the ions.
Question 3. During SN2, you double the concentration of the nucleophile and halve the concentration of the alkyl halide. What happens to the rate?
A. It doubles
B. It halves
C. It stays the same
D. It quadruples
Show the answer
C. The rate law is rate = k[R-LG][Nu−], so the rate changes by a factor of ½ × 2 = 1.
Question 4. (R)-2-Bromobutane reacts with sodium iodide in acetone. What is the main product?
A. (R)-2-iodobutane
B. (S)-2-iodobutane
C. A racemic mixture of 2-iodobutane
D. 2-Butanol
Show the answer
B. A secondary carbon, a strong nucleophile and a polar aprotic solvent point to SN2, so the configuration inverts. Iodine takes bromine’s spot as the top priority group, so R becomes S.
If you’d like more passage practice, our Chem/Phys & Bio/Biochem Mastery Course has 100+ strategy lessons from 130+ scorers, including 70+ AAMC passage walkthroughs.
SN1 vs SN2 Terms to Know
SN1 vs SN2 FAQs
What is the difference between SN1 and SN2 reactions?
SN2 happens in one step, with the nucleophile pushing out the leaving group from the back side. SN1 happens in two steps, with the leaving group leaving first to form a carbocation. That one difference explains their rate laws, stereochemistry and favorite substrates!
What is the SN1 rate law?
The SN1 rate law is rate = k[substrate]. It’s first order, because the slow step involves only the substrate losing its leaving group. Changing the nucleophile’s concentration doesn’t change the rate!
Does SN1 or SN2 give inversion of configuration?
SN2 does, because the nucleophile attacks from the side opposite the leaving group. SN1 gives racemization, since the nucleophile can add to either face of the flat carbocation.
Why do tertiary substrates favor SN1 over SN2?
Three carbon groups crowd the back side, which blocks SN2. Those same groups donate electron density and stabilize the carbocation, which speeds up SN1.
Why do polar aprotic solvents favor SN2?
They dissolve charged nucleophiles without hydrogen bonding to them. The nucleophile stays free and reactive, and SN2 depends on a strong nucleophile. That’s why acetone and DMSO show up in SN2 questions so often!
Can a secondary substrate react by SN1 or SN2?
Yes, it can go either way! A strong nucleophile in a polar aprotic solvent pushes it toward SN2, and a weak nucleophile in a polar protic solvent pushes it toward SN1.
Final Thoughts on SN1 vs SN2
Once you know the one-step versus two-step story, the rest of SN1 vs SN2 follows from it, from the rate laws to the stereochemistry. Check the substrate, nucleophile and solvent, and these questions start to feel predictable!
If you’d like help building that habit, our 6-Week Live MCAT Accelerator pairs live strategy classes with seven 1:1 sessions with a 515+ scorer.
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VII. Supplemental Resources
Want to go deeper on this topic? Here's where to read, watch and practice it next.
Read
- Kaplan MCAT Organic Chemistry Review 2026-2027, §4.2
Watch
- “Sn1 vs Sn2: Summary” (Khan Academy)
- “Steric hindrance” (Khan Academy)
Official AAMC practice that tests this topic
- Chem/Phys Section Bank, Q13
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