SN1 and SN2: what actually decides which one happens
Both replace a leaving group with a nucleophile, and they differ in whether the bond breaks before the new one forms. SN2 does it in one motion and turns the carbon inside out; SN1 lets the leaving group go first, makes a carbocation, and whatever attacks it can attack from either face.
The one-step version
In SN2 the nucleophile arrives on the side opposite the leaving group and pushes it out in the same motion. There is no intermediate — bond-making and bond-breaking happen together, through a single transition state where the carbon is briefly bonded to five things.
Because the nucleophile has to reach the back face, anything crowding that carbon slows it down badly. A methyl or primary carbon is easy, secondary is sluggish, tertiary is effectively closed. And because the attack comes from behind, the carbon ends up inverted, like an umbrella in the wind. If the carbon was a stereocentre, the product has the opposite configuration.
The two-step version
In SN1 the leaving group leaves on its own, which is the slow step, and leaves behind a carbocation. The nucleophile then attacks that flat, positively charged carbon.
That ordering flips every preference. Now the question is not whether the nucleophile can reach the back of the carbon but whether the carbocation is stable enough to form at all — so tertiary is easiest and primary is effectively closed, exactly the reverse of SN2. And because the carbocation is flat, attack happens from both faces, so a single enantiomer going in gives a mixture coming out.
Reading a substrate
The substrate usually settles it. Primary carbon with a decent nucleophile: SN2. Tertiary carbon in a polar solvent that can stabilise ions: SN1. Secondary is the genuinely ambiguous case, and there the conditions decide — a strong nucleophile pushes toward SN2, a weak one in an ionising solvent pushes toward SN1.
A strong base rather than a good nucleophile changes the question entirely, because elimination starts competing for the same substrate. That is a different fork, and it is worth knowing you are at it.
Where this stops being simple
- SN1 and SN2 are the two ends of a range, not a pair of boxes. Plenty of real reactions, particularly at secondary carbons, sit somewhere between and have kinetics that match neither cleanly.
- The platform can draw either pathway on your own structures, but drawing a mechanism is not evidence that it is the one that runs. It is a rule-derived pathway that arrives at the product, and where two pathways both arrive, both can be drawn.
See it happen
The SN2 mechanism, drawn
Every step on real structures, with the arrows and what each one assumes.
The SN1 mechanism, drawn
Ionisation, the carbocation, and why the stereochemistry is lost.
What makes a leaving group good
Both routes need one to go, and the same property governs both.
Try it on your own substrate
Draw the halide and the nucleophile and see what the engine proposes, with its precedent.
Other concepts
What is a leaving group, and why are some better than others?
A leaving group is whatever leaves taking the bonding electrons with it. How good it is comes down to one question: how comfortable is it holding that negative charge once it has gone?
What is the difference between a nucleophile and an electrophile?
A nucleophile has electrons to give and an electrophile has somewhere to put them. Every curly arrow in every mechanism runs from the first to the second, which is why getting this right makes mechanisms readable rather than memorised.
What is Markovnikov's rule?
Adding HX across an alkene puts the hydrogen on the carbon that already has more of them, and the X on the other. The memorable phrasing hides the actual reason, which is that the reaction goes through whichever carbocation is more stable.
What is a functional group?
A functional group is an arrangement of atoms that behaves roughly the same way wherever it appears. It is why a chemist can look at an unfamiliar molecule with an ester in it and already know several things it will do.
What do the curly arrows in a mechanism actually mean?
Each curly arrow means one pair of electrons moved: the tail sits where the pair is now, the head where it goes. Nothing else about the drawing is arbitrary, which is why a mechanism can be checked rather than memorised.
Everything here is open without an account. If you want to try the idea rather than read it, the mechanisms run on structures you supply and prediction will tell you what it thinks a flask would give, with the evidence it is reasoning from attached.