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

Classify an organic reaction by asking what changed: a group replaced on an $sp^3$ carbon means substitution, a new pi bond means elimination, and atoms added across an existing $C=C$ or $C\equiv C$ bond means addition. That first split already narrows most early problems to SN1, SN2, E1, E2, or addition.

The Five Mechanisms Side By Side

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

What Each Mechanism Actually Means

The five labels are short, but each names a specific structural story, and reading them out loud makes the table easier to use.

SN1 stands for substitution nucleophilic unimolecular. The slow step involves only the substrate, because the leaving group departs first and forms a carbocation. That makes SN1 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 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 at all.

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, and heat tends to push toward elimination, though that is a tendency, not a guarantee.

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 a beta hydrogen is available; on secondary and tertiary substrates, a strong base often pushes the reaction toward E2 instead of substitution.

Addition usually starts 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.

A Decision Rule For Picking The Path

When you want to choose quickly, walk this sequence:

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

That one structural question separates most beginner problems before you even think about curved arrows. Then ask whether the reagent acts more like a nucleophile or a base: strong nucleophiles often help substitution, strong bases often help elimination, and 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. The carbon bearing bromine is secondary, so both substitution and elimination are possible in principle. Ethoxide is a strong base and a good nucleophile; on a secondary substrate that already makes E2 a strong candidate. E2 needs a leaving group and a beta hydrogen, and 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, giving propene as the organic product as the base removes a beta hydrogen while bromide leaves in the same step. The example works for three reasons worth separating:

• a secondary substrate means real competition between substitution and elimination 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, because direct backside attack becomes easy and a primary carbocation is too unstable to favor the SN1 or E1 routes.

Common Confusion Points

The mix-ups that derail mechanism problems:

• Treating strong base and strong nucleophile as the same. Some reagents are both; the substrate matters as much. A strong nucleophile with a primary substrate points to SN2; a strong base with a hindered substrate points to E2.
• Assuming tertiary automatically means SN1. Tertiary blocks normal SN2 but does not force SN1; with a strong base, E2 is often the better call.
• Forgetting SN1 and E1 share a carbocation. Once a carbocation forms, rearrangements become possible and substitution and elimination compete.
• Calling every alkene reaction addition. Addition adds atoms across a pi bond; if a reaction creates the double bond instead, that is elimination.

Where These Reactions Are Used

These five types are the backbone of introductory organic synthesis and mechanism problems. They predict whether a molecule becomes more substituted or more unsaturated, whether a reagent replaces a leaving group or removes a hydrogen, how conditions change the major product, and why one substrate behaves differently with different reagents. The same ideas help chemists plan routes between carbon skeletons. To practice, take $1$-bromobutane and ask how the prediction shifts with sodium cyanide, sodium ethoxide, or water, changing one condition at a time. For a close follow-up, compare this page with nucleophilic substitution.