SN2 stands for Substitution, Nucleophilic, Bimolecular (2), and every part of that name is meaningful. The defining feature is that it happens in one single concerted step: the nucleophile attacks the electrophilic carbon at the exact same moment the leaving group departs. There is no intermediate — bond-forming and bond-breaking happen together in one smooth motion, passing through a single high-energy transition state.

"Bimolecular" (the "2") refers to the kinetics: because both the substrate and the nucleophile are present in that single rate-determining step, the reaction rate depends on the concentration of both. Rate = k[substrate][nucleophile]. Double either one and the rate doubles. This experimental fact is actually the evidence that told chemists the mechanism is concerted.

Two consequences flow from the geometry of that single step, and both are heavily tested:

1. Backside attack and inversion of configuration. The nucleophile must approach from the side directly opposite the leaving group (the leaving group's electron cloud blocks any other approach). As the reaction proceeds, the three other bonds on the carbon flip through — exactly like an umbrella turning inside-out in a strong wind. If the carbon is a stereocenter, this inverts its configuration — a process called Walden inversion. An R starting material becomes an S product (though the label can occasionally stay the same if priorities reshuffle — track the actual 3D geometry, not just the letter).

2. Steric hindrance controls the rate. Because the nucleophile must squeeze in for backside attack, anything crowding the carbon slows the reaction dramatically. This gives the master rule for SN2: it works best on unhindered carbons. The reactivity order is methyl > 1° > 2° >> 3°, and tertiary substrates essentially do not undergo SN2 at all — they are too crowded for the nucleophile to reach the carbon.

SN2 is favored by a strong, unhindered nucleophile (it needs a good electron donor to drive the concerted attack) and by polar aprotic solvents — solvents like acetone, DMSO, or DMF that lack an O–H or N–H. Why aprotic? A protic solvent would surround and hydrogen-bond to the nucleophile, "caging" it and blunting its reactivity; an aprotic solvent leaves the nucleophile "naked" and aggressive. Put the whole profile together and the ideal SN2 reaction is: methyl or primary substrate + strong nucleophile + polar aprotic solvent → clean inversion.

SN1 stands for Substitution, Nucleophilic, Unimolecular (1), and it is the mechanistic opposite of SN2 in nearly every respect. Instead of one concerted step, SN1 occurs in two distinct steps:

1. The leaving group leaves first, on its own, before the nucleophile arrives. This generates a carbocation — a carbon bearing a full positive charge and only three bonds. This step is slow, difficult, and rate-determining, because forming a charged, electron-deficient intermediate costs energy.

2. The nucleophile then attacks the carbocation in a fast second step, forming the product.

"Unimolecular" (the "1") again refers to kinetics: only the substrate is involved in that slow first step, so the rate depends only on the substrate concentration. Rate = k[substrate]. Notice the profound implication — adding more nucleophile does not speed up an SN1 reaction, because the nucleophile isn't in the rate-determining step. That contrast with SN2 is a classic exam discriminator.

Everything about SN1 is governed by that carbocation intermediate, so you must understand carbocation stability, one of the most important recurring themes in all of organic chemistry. Carbocations are stabilized by electron donation toward the positive carbon, and the two big sources are:

Alkyl groups (hyperconjugation and induction): more attached carbons donate more electron density and spread the charge. So stability runs 3° > 2° > 1° > methyl — the exact opposite of the SN2 substrate preference. This is why SN1 is the mechanism of tertiary substrates: they can't do SN2 (too crowded) but they form the most stable carbocations.
Resonance: if the positive charge can be delocalized over multiple atoms (as in allylic or benzylic cations), it is dramatically stabilized.

Two more consequences distinguish SN1:

Racemization. The carbocation intermediate is flat (sp², trigonal planar), so the incoming nucleophile can attack from either face with roughly equal probability. Starting from a single enantiomer, you get a mixture of both product enantiomers — a racemic (or nearly racemic) mixture. Contrast this sharply with SN2's clean inversion.
Carbocation rearrangements. Because a more stable carbocation is always preferred, if a nearby carbon would give a more stable cation, a hydrogen or an alkyl group will shift over (a 1,2-hydride or 1,2-alkyl shift) to relocate the positive charge to the better position. This can give a "rearranged" product with a scrambled skeleton — a hallmark of carbocation mechanisms (SN1 and E1) and something SN2/E2 never do. Seeing an unexpected rearranged product on an exam is a strong signal that a carbocation was involved.

SN1 is favored by stable carbocations (3°, allylic, benzylic), a weak nucleophile (a strong one would just do SN2 instead — the weak nucleophile bides its time for the cation to form), a good leaving group, and polar protic solvents (water, alcohols), whose O–H groups actively stabilize the developing ions through solvation, helping the difficult first step along.

"Front and Back"