Substitution

Learning Goals

• Describe the general pattern of a nucleophilic substitution reaction.
• Explain what the 1 and 2 in SN1 and SN2 actually mean.
• Classify alkyl halides as methyl, primary, secondary, or tertiary.
• Rank carbocation stability and explain why tertiary is most stable.
• Draw the SN2 mechanism and predict inversion of configuration.
• Explain what makes a strong nucleophile and rank nucleophilicity.
• Draw the SN1 mechanism through its carbocation intermediate.
• Identify good and bad leaving groups.
• Choose SN1 or SN2 from the substrate, nucleophile, solvent, and leaving group.

Key Terms

Tap any term to see its definition.

Section 1

What Is Substitution?

A substitution reaction is exactly what it sounds like: one group leaves and another takes its place. The general pattern is R-LG + Nu⁻ → R-Nu + LG⁻.

Nucleophile in, leaving group out

Here is the textbook example. CH₃Br + OH⁻ → CH₃OH + Br⁻. Bromide is the leaving group, hydroxide is the nucleophile, and OH ends up where Br used to be.

CH₃Br + OH⁻ → CH₃OH + Br⁻

Section 2

What Does SN Mean?

Let me break the name down properly, because the label tells you almost everything. SN stands for nucleophilic substitution: the S is substitution, the N is nucleophilic. Then the number: SN1 is unimolecular nucleophilic substitution, and SN2 is bimolecular nucleophilic substitution.

Setup

Alkyl Halides and Carbocation Stability

Before we split into SN1 and SN2, you need two quick tools, because the rest of the lesson leans on them constantly. The first is classifying the substrate. An alkyl halide is a carbon bonded to a halogen, and we label that carbon by how many other carbons are attached to it: methyl (zero), primary (one), secondary (two), or tertiary (three).

Methyl, primary, secondary, tertiary

The second tool is carbocation stability, and this is the one I really want you to understand rather than memorize. A carbocation is an electron-poor carbon with a positive charge. Anything that donates electron density toward that carbon stabilizes it. Alkyl groups do exactly that, through hyperconjugation (donation from neighboring C-H and C-C bonds) and a small inductive push. So the more alkyl groups attached, the more stable the cation.

Tertiary > secondary > primary > methyl

Hyperconjugation stabilizes the positive charge

Section 3

What SN2 Really Means

SN2 is one-step substitution. The nucleophile attacks at the exact same time the leaving group leaves, so two things are involved in that single slow step: the substrate and the nucleophile. That is why the rate law is rate = k[substrate][nucleophile].

That rate law tells you what speeds SN2 up. A stronger or more concentrated nucleophile makes it faster. A better leaving group makes it easier. And less steric hindrance makes it easier, because the nucleophile needs room to get in.

Section 4

The SN2 Mechanism

The key to the whole mechanism is backside attack. The nucleophile comes in from the side directly opposite the leaving group, because that is where it can reach the antibonding orbital of the C-LG bond.

Backside attack, opposite the leaving group

Because the nucleophile comes in from the back, the carbon flips. This gives inversion of configuration, like an umbrella turning inside out in the wind. If you tracked stereochemistry in Lesson 6, this is where it starts paying off.

Inversion: the umbrella flips

Section 5

What Favors SN2

Three conditions decide whether SN2 is happy. Here is how I check each one.

SN2 substrate: methyl > primary > secondary, tertiary blocked

Going deeper

What Actually Makes a Good Nucleophile

I keep saying SN2 wants a strong nucleophile, so let me tell you what strong actually means. Nucleophilicity is just how readily a species donates its electrons to form a new bond. The rule underneath everything: the more available the electrons, the more nucleophilic the species. So the first thing I do is find the nucleophilic center, usually a lone pair or an anion, and then ask how freely those electrons are willing to move.

Three trends tell you almost everything, and two of them line up with basicity while the third does not.

Across a row

Nucleophilicity falls left to right, so C⁻ > N⁻ > O⁻ > F⁻. Rising electronegativity holds the lone pair tighter and makes it less available. This matches the basicity order.

Same atom, more charge

For the same central atom, more electron density means a better nucleophile, so HO⁻ > H₂O. An anion always beats its neutral version. This also matches basicity.

Down a group

Nucleophilicity rises down a group, so I⁻ > Br⁻ > Cl⁻ > F⁻. Bigger atoms are more polarizable, and that loosely held electron density reaches out to form the new bond more easily. This is the opposite of the basicity order.

The three nucleophilicity trends

The trend I want you to remember most

Down a group, nucleophilicity and basicity go opposite ways. I⁻ is a weaker base than F⁻ but a far better nucleophile, because a big, polarizable iodide reaches out to form the new C-X bond much more easily. Polarizability barely helps basicity, since a proton is tiny, but it matters a lot for attacking carbon.

And here is a working ranking of common nucleophiles, roughly strongest to weakest, so you have something concrete to reach for.

Strength	Nucleophiles
Very good	I⁻, HS⁻, RS⁻
Good	Br⁻, HO⁻, RO⁻, CN⁻, N₃⁻
Moderate	NH₃, Cl⁻, F⁻, RCO₂⁻
Weak	H₂O, ROH
Very weak	RCO₂H

Section 6

What SN1 Really Means

SN1 is two-step substitution. First the leaving group leaves, then the nucleophile attacks. The slow step is only the substrate losing its leaving group, so the rate law is rate = k[substrate]. The nucleophile is nowhere in that rate-determining step.

That is the whole reason SN1 can run with a weak nucleophile. If the nucleophile is not in the slow step, it does not need to be strong to get the job done.

Section 7

The SN1 Mechanism

The SN1 mechanism runs in two or three moves. Step one: the leaving group leaves and a carbocation forms. Step two: the nucleophile attacks the carbocation. Step three, only if needed: a quick deprotonation cleans up the charge to give a neutral product.

Leaving group leaves, carbocation forms, nucleophile attacks

That carbocation is the whole catch. SN1 only works if the carbocation is reasonably stable, which is exactly why the stability ranking from the setup section matters so much here.

A flat carbocation gets attacked from either face

Section 8

What Favors SN1

The same three conditions, read the opposite way from SN2.

SN1 substrate: tertiary > secondary, primary and methyl out

Section 9

Leaving Groups

Neither pathway works without a good leaving group, so this section matters for both. A good leaving group is stable after it leaves, which usually means it is a weak base.

Stable, weak bases leave best

Protonate OH to make it a good leaving group

Section 10

SN1 vs SN2 Side by Side

This is the table I want you able to reproduce from memory by the end of the lesson.

Section 11

The Decision System

When you are handed a reaction and asked which pathway it takes, run these four questions in this order. This is the exact sequence I use every time.

SN1 vs SN2 decision flowchart

Common Mistakes

Watch Out for These

Lesson Summary

Substitution swaps a nucleophile in for a leaving group. The number in SN1 and SN2 counts species in the rate-determining step, not the number of steps. SN2 is one concerted step with a backside attack that gives inversion, and it wants a strong nucleophile, a methyl or primary substrate, and a polar aprotic solvent. SN1 forms a carbocation first (so it needs a stable one, tertiary > secondary > primary > methyl), tolerates a weak nucleophile, prefers a polar protic solvent, and gives racemization. Both need a good leaving group. To call a reaction, walk the substrate, then the nucleophile, then the solvent, then the leaving group.