SN2 — Substitution & Elimination

SN2 is the first real mechanism of the course, and it is worth learning thoroughly, because everything about it is characteristic: a rate law you can measure, a stereochemical outcome you can predict, and a steric dependence that explains itself. Almost every later mechanism is described by comparison with this one.

The setup

A nucleophile attacks an electrophilic carbon and displaces a leaving group, all in a single concerted step. Nothing is formed before anything is broken; the two happen together, in one motion, through one transition state.

Finding the electrophilic carbon is the first move: the leaving group pulls electron density toward itself, leaving the carbon it is attached to short of electrons. That carbon is the target.

The two arrows

Arrow 1: tail on the nucleophile's lone pair, head on the electrophilic carbon — the new bond forming. Arrow 2: tail on the C–leaving-group bond, head onto the leaving group — the old bond breaking, with the leaving group taking both electrons (a heterolytic cleavage).

Both arrows fire together. That simultaneity is not a stylistic choice; it is the mechanism's defining claim, and it is what the "2" refers to — bimolecular, because two species are involved in the single rate-determining step.

In the transition state the carbon is momentarily bonded to five things: three fully, and two partially — a partial bond forming to the nucleophile and a partial bond breaking to the leaving group. It is sp²-hybridized at that instant, with the three spectator groups arranged in a plane and the nucleophile and leaving group on the axis perpendicular to it. This is a transition state, not an intermediate: it sits at an energy maximum and has no lifetime.

Backside attack: the nucleophile attacks from the side of the carbon exactly opposite the leaving group, 180° away, because that is the only approach that does not run into the leaving group's electron cloud. Orbital theory says the same thing more precisely — the nucleophile must donate into the C–LG antibonding (σ*) orbital, and that orbital's large lobe points directly away from the leaving group. As the new bond forms on one side and the old one breaks on the other, the three spectator groups sweep through the plane like an umbrella turning inside out in the wind.

The orbital reason behind the geometric one. The electrons the nucleophile is donating have to go somewhere, and where they go is the empty antibonding orbital of the C–Br bond — filling it is exactly what breaks that bond. That orbital is lopsided, with its large lobe on the opposite side from the bromine, so a backside approach is not merely the least obstructed one; it is the only direction with anything to aim at.

One step, start to finish. Nothing here is an intermediate — the middle picture is a transition state, an energy maximum with no lifetime, where the carbon is briefly bonded to five things: three fully and two by half. Watch the three hydrogens: they start leaning toward the nucleophile's side, pass through flat, and end up leaning the other way. That sweep is the inversion, and it is why an SN2 on a stereocentre gives one product rather than a mixture.The nucleophile can only come in at 180° from the leaving group — every other approach runs into it. So the three spectator groups turn inside out, like an umbrella in the wind. That is Walden inversion, and because there is no intermediate to lose track in, EVERY molecule inverts. No partial racemisation.

Inversion of configuration

The umbrella flip has a consequence you can test. If the carbon under attack is a stereocenter, SN2 always inverts its configuration — a clean, complete, one-for-one inversion known as Walden inversion. Every molecule inverts; there is no partial racemization, because there is no intermediate to lose stereochemical information.

The umbrella, in three frames. The nucleophile can only arrive from directly opposite the leaving group, so as the new bond forms the three spectator hydrogens are pushed past the carbon and out the other side. The middle frame is the moment they are coplanar with it — that is the transition state, not an intermediate, and nothing sits there. Watch any one hydrogen across the three and you have watched the inversion: it starts on the nucleophile’s side and finishes on the bromine’s. Because there is no intermediate to lose track in, every molecule that reacts does this, which is why an SN2 on a stereocentre gives one product and not a mixture.

One caution: inversion of the three-dimensional arrangement does not automatically mean the R/S letter changes. The descriptor depends on CIP priorities, and if the incoming nucleophile has a different priority rank from the departed leaving group, the label can stay the same while the geometry has genuinely inverted. Reason about the geometry first and assign the descriptor afterwards.

Worked examples

HO⁻ + CH₃Br → CH₃OH + Br⁻. Hydroxide attacks the backside, bromide leaves with the bonding pair, methanol results. Methyl has no stereocenter, so there is nothing to invert — but the rate is the fastest of any substrate class.

⁻CN + CH₃Cl → CH₃CN + Cl⁻. Same mechanism, and synthetically valuable: it forms a carbon–carbon bond and installs a nitrile, which can later be hydrolyzed to a carboxylic acid or reduced to an amine.

(S)-2-bromobutane + ⁻SH. Backside attack inverts the center, giving the (R) thiol as a single enantiomer. One reaction, one product, no mixture — this is what makes SN2 so useful for stereochemical control.

Steric effects: the dominant variable

Backside attack requires the nucleophile to physically reach the carbon from behind, so substituents on that carbon slow the reaction down — steeply.

Sterics, which is the variable that decides most SN2 questions. Since the nucleophile must arrive from directly behind the leaving group, anything sitting on that carbon is in the way. One methyl costs a factor of thirty; three close the approach entirely. Note that branching one carbon further out counts too — neopentyl bromide is formally primary and still reacts about 10⁵ times slower than ethyl bromide, because the quaternary carbon next door blocks the path.Six orders of magnitude from methyl to tertiary — and the last step is not "very slow", it is ZERO. A tertiary carbon has no accessible backside, so proposing an SN2 there is a mechanism error, not a slow reaction.

Substrate	Relative rate
CH₃Br	~30
CH₃CH₂Br (1°)	1
(CH₃)₂CHBr (2°)	0.03
(CH₃)₃CBr (3°)	~0 (no reaction)

Six orders of magnitude from methyl to tertiary. A tertiary carbon simply has no accessible backside, and SN2 does not occur there at all — not slowly, but not at all. Branching one carbon further out matters too: neopentyl bromide, (CH₃)₃C–CH₂Br, is formally primary but reacts about 10⁵ times slower than ethyl bromide, because the adjacent quaternary carbon blocks the approach.

Rate law and solvent

SN2 is bimolecular: rate = k[Nu][substrate]. Both concentrations appear, because both species are present in the single rate-determining step. Doubling either doubles the rate, and this is directly measurable — it is the experimental evidence that the mechanism is concerted.

Polar aprotic solvents (acetone, DMSO, DMF, acetonitrile) strongly favour SN2: they solvate the cation but leave the nucleophile's lone pair effectively naked and highly reactive. Polar protic solvents (water, alcohols) hydrogen-bond to the nucleophile and cage it, and that shell must be stripped away before attack — which can cost several orders of magnitude in rate. This is the caging effect from the Nucleophiles section of Module 2, and it is why the same reaction can run in minutes in DMSO and in days in methanol.

SN2 never happens at a tertiary carbon, and never at an sp² carbon. The tertiary case is steric. The sp² case — vinyl and aryl halides — is both steric and electronic: the backside of that carbon is blocked by the pi system, the C–X bond is shorter and stronger, and the carbon is less electrophilic. Proposing an SN2 on chlorobenzene is a mechanism error, not a slow reaction.

The four variables, summarized

SN2 is favoured by: an unhindered substrate (methyl > 1° > 2°, never 3°); a strong nucleophile, usually anionic; a good leaving group (I⁻ > Br⁻ > Cl⁻ ≫ F⁻, or a sulfonate); and a polar aprotic solvent. Every one of those four appears again in the decision framework at the end of this chapter, and SN2 is the mechanism that wants each of them in its strongest form.

What carries forward

SN2 is the template for concerted mechanisms generally, and its inversion is the basis of stereochemical planning in synthesis. It competes directly with E2 for the same substrates and reagents, which is the central judgement of this chapter. And backside attack into a σ* orbital reappears throughout the course — in epoxide opening, in halonium opening, and anywhere a three-membered ring is attacked in Module 8.