Write out the starting material and the product of a nucleophilic substitution reaction, and two completely different mechanisms can produce exactly the same overall transformation on paper — one atom or group swapped for another. Look more closely at how the molecule's three-dimensional shape changes during the reaction, though, and the two mechanisms, SN1 and SN2, reveal themselves as genuinely different processes, distinguishable by a detail the flat chemical equation never shows.
SN2: one step, one direction, one inversion
In an SN2 reaction, the incoming nucleophile attacks the carbon atom from the side directly opposite the leaving group, in a single concerted step — bond-breaking and bond-forming happen simultaneously, with no intermediate stage in between. This backside attack forces the molecule's other three groups attached to that carbon to flip through, like an umbrella caught by a gust of wind turning inside out, a phenomenon chemists call Walden inversion. Because there's only one mechanistic pathway and it only ever attacks from the back, the outcome is completely predictable: if you start with a specific three-dimensional arrangement, you get its mirror-image arrangement out, every time.
SN1: two steps, and the molecule forgets which way it started
SN1 proceeds completely differently: the leaving group departs first, on its own, forming a flat, planar intermediate called a carbocation, before the nucleophile ever gets involved. Because that intermediate is flat, the incoming nucleophile can attack from either face with roughly equal likelihood, since there's no longer any "back side" being blocked by anything — the original three-dimensional information about which way the molecule was arranged has effectively been erased in the intermediate stage. The result is a mixture of both possible three-dimensional outcomes, in contrast to SN2's single, predictable inversion, which is exactly the kind of stereochemical fingerprint that lets chemists tell the two mechanisms apart even when the starting material and final product look identical in a simple equation.
What we're still unsure about
The mechanistic distinction between SN1 and SN2, and the stereochemical evidence that reveals it, are well-established, thoroughly confirmed organic chemistry, not something in dispute. What remains genuinely harder to predict from first principles is exactly which mechanism a given real-world reaction will actually favour — while chemists have strong general rules of thumb based on factors like the structure of the substrate, the strength of the nucleophile, and the solvent used, borderline cases exist where a reaction proceeds through a genuine mixture of both pathways, and precisely quantifying that balance for a novel substrate can still require experimental testing rather than confident prediction alone.
This sits inside Nucleophilic Substitution (SN1 & SN2), one of eight topics in Organic Chemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.