SN1 vs SN2 — How the Substrate Decides the Mechanism
Nucleophilic substitution at saturated carbon looks like one reaction and is really two, with a continuum in between. Deciding which pathway a given combination of substrate, nucleophile, solvent and leaving group will take is one of the most reliably examined judgements in organic chemistry, because the answer determines the rate law, the stereochemistry of the product, and whether a skeletal rearrangement is possible at all. This article works through each factor and shows how to reach a decision quickly and defend it.
The two mechanisms, and their rate laws
Rate = k[R–X][Nu⁻] — second order overall
Rate = k[R–X] — first order, independent of the nucleophile
That is the cleanest experimental test there is: double the nucleophile concentration and see whether the rate moves. Everything else here is about predicting the answer in advance.
Worked example 1 — reading kinetic data. For a reaction of R–Br with NaN₃ the following initial rates are measured at 298 K:
[RBr] = 0.10 M, [N₃⁻] = 0.10 M → rate = 1.2 × 10⁻⁴ M s⁻¹
[RBr] = 0.10 M, [N₃⁻] = 0.20 M → rate = 2.4 × 10⁻⁴ M s⁻¹
[RBr] = 0.20 M, [N₃⁻] = 0.10 M → rate = 2.4 × 10⁻⁴ M s⁻¹
Doubling either concentration doubles the rate, so the reaction is first order in each and second order overall: rate = k[RBr][N₃⁻]. The mechanism is SN2.
k = rate ÷ ([RBr][N₃⁻]) = 1.2 × 10⁻⁴ ÷ (0.10 × 0.10) = 1.2 × 10⁻² M⁻¹ s⁻¹. Note the units: a second-order rate constant must carry M⁻¹ s⁻¹, and checking that is a fast way to catch a misassigned order.
Factor 1 — the substrate, and why it dominates
The two mechanisms respond to alkyl substitution in opposite directions, which is what makes the substrate the single most decisive factor.
| Substrate | SN2 relative rate | SN1 (solvolysis) relative rate | Verdict |
|---|---|---|---|
| Methyl, CH₃X | ~30 | ~1 | SN2 only |
| Primary, CH₃CH₂X | 1 (reference) | ~1 | SN2 strongly preferred |
| Secondary, (CH₃)₂CHX | ~0.02 | ~10 | Either — conditions decide |
| Tertiary, (CH₃)₃CX | negligible | ~10⁶ | SN1 only |
| Neopentyl, (CH₃)₃CCH₂X | ~10⁻⁵ | slow, with rearrangement | Neither is easy |
| Allyl / benzyl | fast | fast | Both accelerated |
Why SN2 slows down with substitution: its transition state is crowded — five groups around one carbon — so every extra alkyl group raises the barrier sterically, not electronically. Why SN1 speeds up: its intermediate is a carbocation, stabilised by hyperconjugation and induction (3° > 2° > 1° > methyl), and by the Hammond postulate the transition state for the endothermic ionisation resembles that cation.
Neopentyl is the instructive case. It is primary, so it should be a good SN2 substrate, yet it reacts about a hundred thousand times more slowly than ethyl. The cause is β-branching: the bulky tert-butyl group blocks the backside trajectory even though the reacting carbon itself is unhindered. It cannot go SN1 either, since that needs a primary cation — instead, under solvolytic conditions, ionisation is accompanied by a 1,2-methyl shift to the far more stable tertiary cation, and the product is rearranged.
Allyl and benzyl are accelerated in both mechanisms: the cation is resonance-stabilised, and the adjacent π system also stabilises the SN2 transition state. Vinyl and aryl halides do neither — the C–X bond is strengthened by sp² hybridisation and conjugation, the backside is shielded, and the cation would be very unstable.
Factor 2 — the nucleophile
The nucleophile appears in the SN2 rate law and not in the SN1 one, so a strong, concentrated nucleophile pushes towards SN2 while a weak one — often the solvent itself, in which case the reaction is called solvolysis — leaves SN1 as the only option.
Nucleophilicity is kinetic and is not the same as basicity, which is thermodynamic. Across a period it does follow basicity: R₃C⁻ > R₂N⁻ > RO⁻ > F⁻. Down a group it depends on the solvent — in polar protic solvents larger anions are less tightly solvated and more nucleophilic (I⁻ > Br⁻ > Cl⁻ > F⁻), while in polar aprotic solvents there is no hydrogen bonding to the anion and the order reverts to basicity (F⁻ > Cl⁻ > Br⁻ > I⁻). Worth carrying too: the α-effect, whereby nucleophiles with a lone pair adjacent to the attacking atom (HO–O⁻, H₂N–NH₂) are far more nucleophilic than their basicity predicts.
Factor 3 — the solvent
| Solvent class | Examples | Effect |
|---|---|---|
| Polar protic | H₂O, MeOH, EtOH, HCOOH, AcOH | Favours SN1: solvates and stabilises both the carbocation and the departing anion, but also hydrogen-bonds to the nucleophile and blunts it |
| Polar aprotic | DMSO, DMF, acetone, acetonitrile, HMPA | Favours SN2: solvates the cation of the salt but leaves the anion relatively bare and highly reactive |
| Non-polar | hexane, benzene, CCl₄ | Neither — ionic species are not stabilised at all |
The effect is large, not marginal: moving a halide displacement from methanol to DMSO can raise the SN2 rate by several orders of magnitude, purely because the nucleophile is no longer wrapped in a hydrogen-bonded shell. Conversely, tert-butyl chloride solvolyses far faster in water than in ethanol, tracking the solvent's ionising power.
Factor 4 — the leaving group
Both mechanisms break the C–X bond, so both need the same thing: a stable, weakly basic anion. The order therefore tracks the strength of the conjugate acid.
This is why alcohols do not substitute directly but do so readily after tosylation, or after protonation in acid, which turns HO⁻ into H₂O. Tosylation is also the way to run a substitution on an alcohol with stereochemical control, since forming the tosylate never breaks the C–O bond.
Factor 5 — the stereochemical outcome, which is the strongest evidence
SN2 gives clean inversion — Walden inversion. The nucleophile must attack anti to the leaving group, along the back lobe of the C–X antibonding orbital, so the other three groups are pushed through the plane like an umbrella turning inside out.
SN1 gives racemisation, usually incomplete. The carbocation is planar and sp², so attack can come from either face. Pure statistics would give a 50:50 racemate, but real results typically show a 5–20% excess of the inverted product, because in the intimate ion pair the departing anion still briefly shields the face it left from. Partial rather than complete racemisation is itself evidence for that ion-pair picture.
Worked example 2 — assigning a mechanism from stereochemistry. (R)-2-bromooctane is treated with sodium hydroxide in aqueous acetone. The product is (S)-octan-2-ol, obtained with essentially complete inversion, and the rate is found to depend on [NaOH].
Both observations point the same way: second-order kinetics and complete inversion mean SN2 at a secondary centre, driven by a strong nucleophile.
The R-to-S change is a genuine consequence of inversion here only because the Cahn–Ingold–Prelog priority pattern is unchanged (Br > hexyl > methyl > H becomes OH > hexyl > methyl > H). Always re-assign priorities in the product rather than assuming the letter flips.
Now dissolve the same substrate in pure water or formic acid with no added nucleophile. The rate becomes first order and the product is largely racemic with a small excess of inversion: the mechanism has shifted to SN1. Nothing about the substrate changed — a borderline secondary centre simply follows whichever pathway the conditions favour.
Worked example 3 — retention of configuration, which is neither. Hydrolysis of an optically active 2-bromopropanoate anion under dilute conditions gives lactate with retention of configuration, which neither mechanism allows on its own.
The explanation is neighbouring group participation. The carboxylate oxygen attacks the adjacent carbon intramolecularly, displacing bromide with inversion and forming a strained three-membered α-lactone. Water then opens that ring at the same carbon, with a second inversion. Two inversions give net retention.
The kinetics confirm it: first order like SN1, but far faster than the substrate justifies — a rate enhancement called anchimeric assistance. Under concentrated hydroxide, direct SN2 outcompetes the intramolecular route and normal inversion returns. The same principle explains the reactivity of sulfur and nitrogen mustards.
The traps that catch people
- Reading "1" and "2" as the number of steps. They denote molecularity of the rate-determining step. SN1 has two steps; SN2 has one.
- Assuming inversion always flips the R/S letter. The descriptor depends on CIP priorities, which can reorder when the leaving group is replaced.
- Expecting exactly 50:50 racemisation from SN1. Ion pairing usually gives a modest excess of inverted product.
- Proposing a primary or vinyl carbocation. If your mechanism needs one, the mechanism is wrong — look for a rearrangement or a concerted pathway instead.
- Forgetting elimination. A strong, bulky base such as tert-butoxide gives E2, not substitution; heat pushes tertiary substrates towards E1 alongside SN1.
- Treating basicity and nucleophilicity as the same property. Iodide is a weak base and an excellent nucleophile in protic solvents.
- Expecting rearrangement in SN2. There is no intermediate, so there is nothing to rearrange. A rearranged product is proof of a cationic pathway.
Decision summary
| Feature | SN2 | SN1 |
|---|---|---|
| Rate law | k[RX][Nu] | k[RX] |
| Steps | One, concerted | Two, via carbocation |
| Best substrate | Methyl > 1° > 2° | 3° > 2°, allylic, benzylic |
| Nucleophile | Strong, concentrated | Weak; often the solvent |
| Solvent | Polar aprotic (DMSO, DMF, acetone) | Polar protic (H₂O, ROH, HCOOH) |
| Stereochemistry | Complete inversion | Racemisation, slight net inversion |
| Rearrangement | Never | Possible and diagnostic |
| Competing reaction | E2 | E1 |
The working routine is four questions: how substituted is the carbon, how strong is the nucleophile, is the solvent protic or aprotic, and is the leaving group good? Methyl and primary always mean SN2; tertiary always means SN1 or elimination; secondary is genuinely borderline, and the nucleophile and solvent settle it.
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