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SN1 vs SN2 Reactions: What's the Difference?

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University Course Reader · STEM

Because the substrate is a tertiary alkyl halide, the reaction proceeds through an SN1 mechanism via a carbocation intermediate.

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Overview

Both are substitution reactions, where one group attached to a carbon gets replaced by another. In SN2 the swap happens in one step, with the new group pushing in as the old one is pushed out. In SN1 it takes two steps, and how crowded the carbon is decides which route runs.
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Overview

Two ways to swap a group on a carbon. SN2: the newcomer kicks the door in and shoves the old group out — one step. SN1: the old group rage-quits first, the carbon sits there positively desperate, then the newcomer strolls in. 😎

A quick take — often all you need.

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Detail

SN2 first: the attacking group (the nucleophile) strikes the carbon from the side directly opposite the leaving group and bonds while pushing it out — one continuous step that flips the carbon's other three groups to the opposite side, inverting the molecule's 3D arrangement. Because that step is a collision between two molecules, the speed depends on the concentration of both — the "2," bimolecular — and the backside strike needs a clear path, so SN2 runs on uncrowded carbons. SN1 takes over on crowded carbons, where the path is blocked but neighboring groups stabilize positive charge: the leaving group departs first, leaving a carbocation, and the nucleophile then attaches to this flat intermediate from either side, producing a mix of 3D arrangements. The slow step involves one molecule only, so speed depends on the substrate alone — the "1," unimolecular. The exam shortcut: open carbon → SN2, crowded carbon → SN1.
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Detail

Read the crowd level — that's the whole game. Open carbon: SN2, a backside attack, one clean hit, and the molecule does the full umbrella-in-a-storm flip — inside-out. Crowded carbon: the backside is blocked, but the crowd babysits the positive charge, so the old group can dip early — SN1, and since the middle step is flat, the newcomer lands on either face for a scrambled 3D mix. The exam trap: the 1 and 2 are NOT step counts, they're about speed. SN2 speed depends on both molecules, because it's a collision. SN1 speed depends only on the one doing the leaving — the exit is the slow part, and it exits alone. 😎

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Analogy

SN2 is a stop shot in pool: the cue ball needs a clear straight line, comes in fast from the opposite side, and in one collision knocks the target ball out while stopping in its exact spot. SN1 is street parking, where the parked car pulls out on its own, the spot sits empty, and the nearest waiting car slides in from either direction. Access to the carbon decides which one runs.
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Analogy

SN2 is leaving someone for someone — the replacement is already at the door, one handoff, never single. SN1 is the breakup-first route: end it, survive your unstable era (only works if your life can support being single), then whoever's around gets the spot. Chemistry grades both routes entirely on your situation, not your morals.
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Formal definition — The same term, explained the usual way

SN1 and SN2 denote unimolecular and bimolecular nucleophilic substitution mechanisms respectively. SN2 proceeds through concerted backside displacement with second-order kinetics and inversion of configuration, favored by unhindered substrates and strong nucleophiles. SN1 proceeds through rate-determining ionization to a planar carbocation intermediate with first-order kinetics, favored by substrates forming stabilized carbocations, and yields substantial racemization.

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