Organic Reactions — SN1, SN2, E1, E2 & Addition

Organic reactions are easiest to classify by asking what changed. If one group on an $sp^3$ carbon was replaced, think substitution. If a new pi bond formed, think elimination. If atoms were added across an existing $C=C$ or $C\equiv C$ bond, think addition.

For most early organic chemistry questions, that first split already narrows the answer to SN1, SN2, E1, E2, or addition. Then check the conditions: whether there is a leaving group, whether the reactive carbon is primary, secondary, or tertiary, whether a strong base or nucleophile is present, and whether a carbocation is realistic.

How To Tell SN1, SN2, E1, E2, And Addition Apart Fast

Type	What changes	Often favored when	Core idea
SN1	leaving group is replaced	the substrate can form a relatively stable carbocation	two-step substitution
SN2	leaving group is replaced	the substrate is not too crowded and a good nucleophile can attack directly	one-step substitution
E1	a pi bond forms	a carbocation can form and elimination competes after that intermediate appears	two-step elimination
E2	a pi bond forms	a strong base removes a beta hydrogen while the leaving group leaves	one-step elimination
Addition	atoms add across a pi bond	the starting molecule already has a $C=C$ or $C\equiv C$ bond	the pi bond becomes new sigma bonds

This table is a guide, not a law. Real outcomes depend on the substrate, reagent, solvent, and sometimes temperature.

A Quick Decision Rule That Usually Works

• If the starting molecule has a leaving group on an $sp^3$ carbon, the main competition is usually substitution vs. elimination.
• If the starting molecule already has a pi bond and no leaving group is central to the problem, addition is the better first guess.
• If the reactive carbon is primary, SN2 is often more plausible than SN1.
• If the reactive carbon is tertiary, normal SN2 is blocked, so the common competition is SN1, E1, or E2 depending on conditions.
• If a strong base is present and a beta hydrogen is available, E2 becomes much more likely.

What Each Reaction Type Actually Means

SN1

SN1 stands for substitution nucleophilic unimolecular. The slow step involves only the substrate because the leaving group leaves first and forms a carbocation.

That makes SN1 more plausible when the carbocation would be relatively stable, especially for many tertiary substrates. If a rearrangement would make the carbocation more stable, the product can change.

SN2

SN2 stands for substitution nucleophilic bimolecular. The nucleophile attacks as the leaving group leaves, so the reaction happens in one step.

Because the attack must reach the reactive carbon directly, crowding matters a lot. Primary substrates often favor SN2 much more than tertiary ones, and tertiary substrates do not undergo normal SN2 at that crowded carbon.

E1

E1 stands for elimination unimolecular. Like SN1, it starts with loss of the leaving group to form a carbocation. Then a base removes a beta hydrogen, and a pi bond forms.

Because SN1 and E1 both pass through a carbocation, they often compete under similar conditions. Heat often makes elimination more important, but that is a tendency, not a guarantee.

E2

E2 stands for elimination bimolecular. In one concerted step, a base removes a beta hydrogen while the leaving group leaves and the pi bond forms.

E2 is common when a strong base is present and there is a beta hydrogen available. On secondary and tertiary substrates, a strong base often pushes the reaction toward E2 instead of substitution.

Addition

Addition reactions usually start from an alkene or alkyne. Instead of losing a group, the molecule gains new atoms across the pi bond.

A common intro example is adding $HBr$ across an alkene. The double bond breaks, and the atoms from the reagent end up on the two carbons that were previously double-bonded.

The Intuition That Makes These Reactions Click

The fastest way to make these reactions click is to ask what the molecule is structurally able to do.

If there is a leaving group on a saturated carbon, the molecule can either replace that group or remove a neighboring hydrogen and form a pi bond. If there is already a pi bond, the molecule may instead react by adding across it. That one structural question separates most beginner problems before you even think about curved arrows.

After that, ask whether the reagent is acting more like a nucleophile or more like a base. Strong nucleophiles often help substitution. Strong bases often help elimination. Some reagents can do both, so the substrate and conditions decide which path wins.

Worked Example: Why 2-Bromopropane Often Gives E2

Consider $2$-bromopropane reacting with sodium ethoxide, $\mathrm{NaOEt}$, in ethanol.

Start with the substrate. The carbon bearing bromine is secondary, so both substitution and elimination are possible in principle.

Now check the reagent. Ethoxide is a strong base and also a good nucleophile. On a secondary substrate, that already makes E2 a strong candidate.

Next ask what E2 needs. It needs a leaving group and a beta hydrogen. This molecule has both, so elimination can happen in one step.

Under common classroom conditions, E2 is often predicted as the major pathway, especially if heat is present. The organic product is propene because the base removes a beta hydrogen while bromide leaves in the same step.

Why this example matters:

• a secondary substrate means real competition is possible
• a strong base makes elimination more likely
• formation of an alkene identifies the reaction as elimination, not substitution

If you change the substrate or the reagent, the prediction can change. A less hindered primary substrate would make SN2 much more competitive.

Common Mistakes When Classifying Organic Reactions

Treating Strong Base And Strong Nucleophile As The Same Thing

Some reagents can act as both. The substrate matters just as much. A strong nucleophile with a primary substrate often points toward SN2, while a strong base with a more hindered substrate often points toward E2.

Assuming Tertiary Automatically Means SN1

Tertiary substrates cannot do normal SN2, but they do not automatically go SN1. With a strong base, E2 is often the better prediction.

Forgetting That SN1 And E1 Share A Carbocation

If a carbocation forms, rearrangements can become possible, and substitution and elimination can compete. That is why these two mechanisms are often discussed together.

Calling Every Alkene Reaction Addition

Addition requires atoms to add across the pi bond. If a reaction creates the double bond instead, that is elimination.

Where These Organic Reaction Types Are Used

These reaction types are the backbone of introductory organic synthesis and mechanism problems. They help you predict:

• whether a molecule becomes more substituted or more unsaturated
• whether a reagent is likely to replace a leaving group or remove a hydrogen
• how reaction conditions change the major product
• why the same substrate can behave differently with different reagents

They also matter beyond exams. The same ideas help chemists plan routes from one carbon skeleton to another.

Try A Similar Classification Problem

Take one substrate with a leaving group, such as $1$-bromobutane, and ask how the prediction changes if you pair it with sodium cyanide, sodium ethoxide, or water. Changing only one condition at a time is one of the fastest ways to make SN1, SN2, E1, and E2 feel logical instead of memorized. If you want a close follow-up, compare this page with nucleophilic substitution.