Why is SN1 often not perfectly racemic?

The leaving group can linger near one face of the carbocation as an ion pair, partly blocking it, so attack from the opposite face is slightly favored and a small excess of inversion results.

Why do polar aprotic solvents speed up SN2?

They dissolve ionic nucleophiles but cannot hydrogen bond to the anions, so the nucleophile is not wrapped in solvent and attacks more readily.

SN1 vs SN2: how do you tell them apart?

SN2 is one step: the nucleophile attacks from the back as the leaving group leaves, inverting the stereocenter, with a rate that depends on both reactants. SN1 is two steps: the leaving group leaves first to form a carbocation, which the nucleophile then attacks from either face, with a rate that depends only on the substrate.

Why does SN2 invert the stereocenter but SN1 doesn't?

Watch it explained

2D video · 34 seconds

When does a reaction go SN1 instead of SN2?

Shows SN1, where bromide leaves first and a carbocation forms, beside SN2's one-step backside attack, then crowds the carbon with three methyl groups so SN2 stalls while the cation is stabilized by the methyls and by polar protic water: tertiary carbons, weak nucleophiles and protic solvents favor SN1.

Scenes, in order:

The leaving group goes first
SN2: one step, backside attack
Crowded carbon blocks attack
Stable cation, polar protic solvent
SN1: tertiary, weak nucleophile, protic

Side by side

Mechanism: SN2 is concerted (one step). SN1 goes through a carbocation intermediate (two steps).
Rate law: SN2, rate = k[substrate][nucleophile]. SN1, rate = k[substrate], because forming the carbocation is the slow step.
Substrate: SN2 is fastest for methyl, then primary, slower for secondary, and does not happen at tertiary carbons (too crowded for backside attack). SN1 favors tertiary, can occur at secondary, and needs a stabilized cation (allylic or benzylic) to happen at primary.
Nucleophile: SN2 needs a strong one (CN−, I−, RS−, N3−). SN1 works with weak ones, often the solvent itself (water, alcohols).
Solvent: SN2 prefers polar aprotic solvents (acetone, DMSO, DMF), which leave the nucleophile unsolvated. SN1 prefers polar protic solvents, which stabilize the carbocation and the leaving group.
Stereochemistry: SN2 inverts the stereocenter. SN1 gives a mix of both configurations, often close to racemic.
Rearrangements: possible in SN1 (hydride or methyl shifts to a more stable carbocation), not in SN2.

Both need a good leaving group: iodide, bromide, chloride or a sulfonate such as tosylate. Hydroxide is a poor leaving group unless it is protonated first.

Two worked examples

(R)-2-bromobutane with sodium cyanide in DMSO: a secondary substrate, a strong nucleophile and a polar aprotic solvent point to SN2. Cyanide attacks opposite bromide and the product, 2-methylbutanenitrile, is formed with inverted geometry. Here the label also flips to (S), but always reassign R or S from scratch: inversion of geometry does not guarantee a new letter if the priorities change.

tert-Butyl bromide in water: a tertiary substrate and a weak, protic nucleophile point to SN1. Bromide leaves, the tertiary carbocation forms, and water attacks; loss of a proton gives tert-butyl alcohol. Doubling the water concentration would not change the rate.

Mistakes that cost points

Forgetting the competing eliminations (E1 and E2), especially with strong, bulky bases or heat.
Calling a secondary substrate automatically SN1. With a strong nucleophile in an aprotic solvent, it is usually SN2.
Drawing a carbocation in an SN2 mechanism. There is none; the transition state has partial bonds to both groups.

Watching backside attack

Inversion is the part that clicks once you see the three other groups flip like an umbrella in the wind. Ask Isotoon "Why does SN2 invert the stereocenter but SN1 doesn't?" for a short animated video of both pathways. See also what makes a molecule chiral, the organic chemistry page, and how to study for organic chemistry exams.

Questions

Why is SN1 often not perfectly racemic?: The leaving group can linger near one face of the carbocation as an ion pair, partly blocking it, so attack from the opposite face is slightly favored and a small excess of inversion results.
Why do polar aprotic solvents speed up SN2?: They dissolve ionic nucleophiles but cannot hydrogen bond to the anions, so the nucleophile is not wrapped in solvent and attacks more readily.