SN1 — Substitution & Elimination

SN1 reaches the same kind of product as SN2 — one group on carbon swapped for another — by a completely different route, and every observable consequence differs: the rate law, the stereochemistry, which substrates work, which solvents help. Learning SN1 is largely a matter of learning how each of those contrasts with SN2, and why.

The key difference from SN2

In SN2 the nucleophile attacks and expels the leaving group in one motion. In SN1 the leaving group departs first, unassisted, before any nucleophile is involved. That ionization step is slow and rate-determining, and it leaves behind a carbocation: flat, sp²-hybridized, with an empty p orbital and only six valence electrons.

The nucleophile arrives afterwards, in a fast second step. A third step — deprotonation — is often needed if the nucleophile was neutral, as when water attacks and must lose a proton to give the alcohol.

Because the nucleophile appears only after the slow step, it does not appear in the rate law at all: rate = k[substrate]. Unimolecular, hence the 1. Doubling the nucleophile concentration does nothing to the rate, which is the cleanest experimental distinction between the two mechanisms.

Why the cation is flat — and why that matters

The same carbocation twice, turned ninety degrees between the two views. From above it looks like an ordinary trigonal centre; edge-on you can see there is nothing there — no fourth bond, no pucker, nothing above the plane or below it. That is the whole stereochemical argument for SN1 in one picture: both faces are equally open, so both get attacked about equally often, and a single enantiomer of starting material comes out racemic.

Two steps, and everything follows from what sits between them. The carbocation is sp², trigonal planar, with an empty p orbital sticking out equally above and below — so a nucleophile arriving later has no reason to prefer one face. Whatever handedness the starting material had is gone by the time step 2 happens. Note also that the nucleophile appears nowhere in step 1, which is exactly why the rate does not depend on it.That flatness is the whole stereochemical story. SN2 inverts every molecule because the nucleophile has only one way in. SN1 loses the information entirely at the cation, so a single enantiomer of starting material gives a roughly 50:50 racemic mixture — the signature outcome of this mechanism.

A carbocation carbon has only three groups and no lone pair, so it is sp² and trigonal planar, with the empty p orbital projecting equally above and below that plane. Nothing blocks either face.

If the original carbon was a stereocenter, attack from both faces gives a racemic mixture — roughly 50:50, and optically inactive. This total loss of stereochemical information is the diagnostic signature of SN1, exactly as clean inversion is the signature of SN2. In practice the mixture often shows slight excess inversion (typically 55:45 up to 70:30), because the departing leaving group lingers briefly as an ion pair and partially shields the face it left from. That detail is evidence for the mechanism rather than an exception to it.

What controls the rate: carbocation stability

The SN1 rate is the rate of step 1, and step 1 is just making a carbocation — so the whole question is how tolerable that cation is. Alkyl groups next to the empty orbital donate electron density into it and spread the charge, so each one helps. Read this ladder against the SN2 one in the previous section: they run in opposite directions, and that opposition is what makes substrate class such a reliable first filter.Each alkyl group next door feeds electron density into the empty orbital, and a positive charge that is shared is a positive charge better tolerated. The ordering is the exact OPPOSITE of SN2’s, which is why substrate class alone usually tells you which of the two mechanisms you are looking at.

Since ionization is rate-determining, anything that stabilizes the carbocation accelerates the whole reaction. The ordering is 3° > 2° > 1° > methyl, and the differences are enormous — tertiary substrates solvolyze around 10¹² times faster than methyl.

The reason has a name and a concrete picture worth carrying for the rest of the course: hyperconjugation. An alkyl group attached to the cationic carbon brings its own C–H sigma bonds. When one of those bonds lines up parallel with the empty p orbital, the electron pair in it spills partially into that orbital — not a full bond, just a sharing that spreads positive charge off the cationic carbon and onto the neighbouring C–H bonds. More alkyl groups means more C–H bonds positioned to do this, more spreading, and a more stable cation.

Note that this is the exact opposite preference from SN2, which wants an uncrowded carbon. The same structural feature — alkyl substitution — accelerates one mechanism and shuts down the other, which is why substrate class is the first thing you check in the decision framework.

Resonance beats alkyl substitution

Hyperconjugation is a modest effect compared with genuine delocalization. An allylic cation (next to a C=C) or a benzylic cation (next to a ring) spreads its charge over several atoms by resonance, and a primary benzylic cation is roughly as stable as a tertiary alkyl one. This is why benzyl chloride undergoes SN1 readily despite being formally primary.

At the other extreme, some cations cannot form at all: vinyl and aryl cations are prohibitively unstable, so vinyl and aryl halides are inert to SN1 as well as to SN2. Between them, those two facts explain why aryl halides are essentially unreactive toward simple substitution and need entirely different chemistry.

Worked example — tert-butyl bromide in water

Step 1 (slow): the C–Br bond ionizes, giving the tertiary cation (CH₃)₃C⁺ and bromide. Only one arrow, from the C–Br bond onto bromine.

Step 2 (fast): water attacks the empty p orbital from either face, giving the protonated alcohol (CH₃)₃C–OH₂⁺.

Step 3 (fast): another water molecule removes a proton, giving tert-butanol.

Three steps, one of which matters for the rate. When the solvent is also the nucleophile, as here, the reaction is called solvolysis — which is the normal way SN1 is run, since a weak nucleophile is required anyway.

Rearrangements: the complication

A carbocation is a real intermediate with a real lifetime, and if a more stable cation is one step away it will often get there before the nucleophile arrives. A hydride shift moves an H with its bonding pair from the adjacent carbon; a methyl shift moves a CH₃ the same way. Both convert a secondary cation into a tertiary one in a single fast step.

The consequence is that SN1 can give a product whose skeleton differs from anything you would predict by just swapping the leaving group. Treating 3-methylbutan-2-ol with HBr gives mainly 2-bromo-2-methylbutane, not the 2-bromo-3-methylbutane you would expect — the secondary cation shifted a hydride to become tertiary before bromide attacked.

Whenever you draw a carbocation, check for a rearrangement. Look at each carbon adjacent to the cationic center: if moving an H or a CH₃ from it would produce a more stable cation, assume the shift happens. Forgetting this is one of the most common sources of wrong SN1 and E1 products, and it is also the single clearest piece of evidence that a free carbocation exists at all — SN2, with no intermediate, never rearranges.

Solvent

SN1 is favoured by polar protic solvents — water, alcohols, acetic acid — for two reasons at once. They stabilize the developing positive charge in the transition state, and their hydrogen bonds stabilize the departing anion. Both effects lower the barrier to the slow step.

This is the opposite of SN2's preference, and it is a useful practical lever: the same secondary substrate can be pushed toward SN2 by running it in DMSO with a strong nucleophile, or toward SN1 by running it in aqueous ethanol with none.

The four variables, summarized

SN1 is favoured by: a substituted substrate (3° > 2°, plus allylic and benzylic; never 1° or methyl); a weak, usually neutral nucleophile, frequently the solvent itself; a good leaving group, which matters even more here than in SN2 since it leaves without assistance; and a polar protic solvent.

What carries forward

Carbocation stability and rearrangement are the recurring themes of Modules 6, 7 and 8 — they explain Markovnikov addition, alcohol dehydration, and the surprising products of several reactions in between. And E1, the subject of two sections from now, shares SN1's first step exactly, which is why the two always occur together.