Difficulty: Intermediate | Prerequisites: SN2 mechanism (Part 1 of these notes), carbocation stability, Lewis acid-base theory, stereochemistry (R/S, chirality, racemic mixtures).
The SN1 mechanism is the second major pathway for nucleophilic substitution at carbon. Where SN2 is a concerted, single-step process, SN1 proceeds through a carbocation intermediate in two (or three) distinct steps. This mechanism dominates with tertiary substrates and weak nucleophiles in polar protic solvents. Understanding SN1 is essential for predicting when racemisation occurs, recognising carbocation rearrangements, and choosing between SN1 and SN2 on exams. You should be comfortable with SN2 before working through this material.
In an SN1 reaction, the leaving group departs first to form a carbocation intermediate, then the nucleophile attacks the carbocation in a separate, fast step. The rate depends only on substrate concentration (first-order kinetics). Because the carbocation is planar, the nucleophile can attack from either face, producing a racemic mixture when the electrophilic carbon is a stereocentre.
SN1 reaction
Substitution, Nucleophilic, Unimolecular. A two-step (or three-step) mechanism in which the leaving group departs first to generate a carbocation, which is then attacked by the nucleophile. Think of it as "leave first, attack second."
Carbocation
A positively charged carbon with only three bonds and an empty p orbital. It is sp² hybridised, trigonal planar, and a very strong electrophile (Lewis acid). Stability order: 3° > 2° > 1° > methyl.
Hyperconjugation
The stabilisation of a carbocation by overlap of adjacent C–H or C–C sigma bonds with the empty p orbital on the positively charged carbon. More alkyl groups provide more hyperconjugative stabilisation, which is why 3° carbocations are more stable than 1°.
Racemic mixture (racemate)
A 50:50 mixture of both enantiomers of a chiral product. Written as (±) or (R,S). In SN1, a racemic mixture forms because the planar carbocation can be attacked from either face with roughly equal probability.
Hammond postulate
States that the transition state of an elementary step resembles the stable species (reactant or product) closest to it in energy. For endothermic steps like carbocation formation in SN1, the transition state resembles the carbocation product. This means factors that stabilise the carbocation also stabilise the transition state and lower the activation energy.
The leaving group departs with the bonding electron pair, generating a carbocation and a free leaving group anion. This is the slow, rate-limiting step with the highest activation energy (ΔG‡).
The leaving group takes both bonding electrons with it. The carbon left behind now has only six electrons and an empty p orbital, making it sp² hybridised and trigonal planar.
The nucleophile (often the solvent itself) donates a lone pair to the carbocation. This step is very fast because the carbocation is such a powerful electrophile. The activation energy for this step is small.
When water is the nucleophile, the initial product is a protonated alcohol (oxonium ion).
If the nucleophile is a neutral molecule like water, the product carries a positive charge and must lose a proton to give the neutral final product. Another molecule of solvent acts as the base. Proton transfers are generally very fast.
The overall energy diagram for SN1 shows three transition states and two intermediates, with the first transition state being the highest (rate-limiting).
The key stereochemical outcome of SN1 is racemisation. When a chiral substrate undergoes SN1, the product is a racemic mixture (equal amounts of R and S enantiomers).
This happens because the carbocation intermediate is trigonal planar (sp²) and achiral. The empty p orbital sits perpendicular to the plane of the three remaining substituents. The nucleophile can approach from either face of this plane with roughly equal probability, so both enantiomers of the product form in approximately equal amounts.
In practice, SN1 reactions sometimes give a slight excess of inversion product over retention. This occurs because the departing leaving group can briefly block one face of the carbocation (an effect called "ion pairing"), but for most exam purposes, the answer is racemisation.
The rate of SN1 reactions is directly linked to the stability of the carbocation intermediate. More stable carbocations form more readily (lower ΔG‡ for the rate-limiting step).
Stability order by substitution: 3° > 2° > 1° > methyl.
This trend is explained by hyperconjugation: adjacent C–H and C–C sigma bonds can donate electron density into the empty p orbital, partially compensating the positive charge. More alkyl groups mean more hyperconjugative donors.
Carbocations are also stabilised by resonance delocalisation. An allylic carbocation (adjacent to a C=C double bond) or a benzylic carbocation (adjacent to an aromatic ring) can spread the positive charge over multiple atoms, making it much more stable than an isolated primary or even secondary carbocation.
Very unstable carbocations (methyl, primary, vinyl, aryl) do not form under normal conditions, so SN1 does not occur at these substrates.
The rate-limiting step of SN1 (step 1) is endothermic: the transition state is closer in energy to the carbocation product than to the starting alkyl halide. According to the Hammond postulate, the transition state therefore resembles the carbocation in structure.
This is extremely useful because it means we can use carbocation stability to predict SN1 rates. Anything that stabilises the carbocation (more substitution, resonance, electron-donating groups) also stabilises the transition state and speeds up the reaction.
SN1 rate law:
rate = k[substrate]
First-order overall: the rate depends only on the concentration of the substrate. The nucleophile does not appear in the rate law because it is not involved in the rate-limiting step.
Carbocation stability order:
3° > 2° > 1° > methyl (by hyperconjugation)
Resonance-stabilised carbocations (allylic, benzylic) can be more stable than their substitution class alone would suggest.
SN1 reactions are common in biological systems where water is the solvent and nucleophiles are weak. Many enzymatic reactions proceed through carbocation intermediates, particularly in terpene biosynthesis where carbocations drive the cyclisation of long hydrocarbon chains into complex ring systems.
The hydrolysis of tertiary alkyl halides in aqueous solution is a classic SN1 process and underpins how certain organic pollutants break down in the environment.
Students often think SN1 gives complete retention of configuration. It does not. The carbocation is planar, so the nucleophile attacks both faces, giving a racemic mixture.
A frequent error is assuming that a strong nucleophile will force a tertiary substrate into SN2. It will not. Tertiary substrates are too sterically hindered for SN2 regardless of nucleophile strength; they react by SN1 (or E1/E2).
Students sometimes draw the nucleophile attacking in the rate-limiting step of SN1. This is wrong. The rate-limiting step is leaving group departure, and it involves only the substrate.
Another common mistake is confusing thermodynamics (ΔG°) with kinetics (ΔG‡). A reaction can be thermodynamically favourable (negative ΔG°) but kinetically slow (large ΔG‡), or vice versa.
⚠️ Drawing the full three-step SN1 mechanism (with curved arrows for each step) is a frequent exam question.
⚠️ You will be asked to predict the stereochemical outcome. For SN1 at a stereocentre, the answer is a racemic mixture.
⚠️ Carbocation rearrangements (hydride shifts, methyl shifts) can occur during SN1 and produce unexpected products. If the initial carbocation can rearrange to a more stable one, it usually will.
⚠️ Comparing SN1 vs SN2 is a standard exam question. Know how substrate structure, nucleophile, solvent, and leaving group determine which pathway dominates.
True or False: The nucleophile appears in the SN1 rate law. (False. SN1 is first order: rate = k[substrate] only.)
Fill in the blank: The carbocation intermediate in SN1 is ____ hybridised and ____ in shape. (sp², trigonal planar)
True or False: SN1 at a chiral centre produces a single enantiomer. (False. It produces a racemic mixture.)
Fill in the blank: Carbocation stability increases with more ____ groups due to ____. (alkyl, hyperconjugation)
True or False: The rate-limiting step in SN1 is the nucleophilic attack. (False. It is leaving group departure.)
Q: (S)-1-bromo-1-phenylethane is dissolved in methanol. Predict the product(s) and their stereochemistry.
A: SN1 occurs (benzylic substrate, weak nucleophile, polar protic solvent). The carbocation is resonance-stabilised by the phenyl ring. Methanol attacks from both faces, giving a racemic mixture of (R)- and (S)-1-methoxy-1-phenylethane (after deprotonation).
Q: Why does a tertiary alkyl halide react faster by SN1 than a secondary alkyl halide?
A: A 3° carbocation is more stable than a 2° carbocation due to greater hyperconjugative stabilisation. By the Hammond postulate, the transition state for 3° carbocation formation is also lower in energy, so the activation energy (ΔG‡) is smaller and the rate is faster.
Q: Write the three steps of the SN1 mechanism for the reaction of (CH₃)₃CCl with water.
A: Step 1 (slow): (CH₃)₃CCl → (CH₃)₃C⁺ + Cl⁻. Step 2 (fast): (CH₃)₃C⁺ + H₂O → (CH₃)₃C–OH₂⁺. Step 3 (fast): (CH₃)₃C–OH₂⁺ + H₂O → (CH₃)₃C–OH + H₃O⁺.
Q: Rank the following in order of increasing SN1 reactivity: 2-chlorobutane, 2-chloro-2-methylpropane, chloromethane.
A: Chloromethane (methyl, no SN1) < 2-chlorobutane (2°) < 2-chloro-2-methylpropane (3°). SN1 rate tracks carbocation stability.
SN1 competes with E1 elimination under the same conditions (weak base/nucleophile, polar protic solvent, 3° substrate). E1 also proceeds through a carbocation intermediate, so the same substrate and solvent preferences apply.
Carbocation rearrangements (covered in more detail in later chapters) are directly relevant here because any carbocation formed in SN1 can potentially rearrange to a more stable cation before the nucleophile attacks.
The factors that determine SN1 vs SN2 are covered in Part 3 of these study notes.
SN1, nucleophilic substitution unimolecular, carbocation, carbocation stability, hyperconjugation, resonance stabilisation, racemic mixture, racemisation, first-order kinetics, rate-determining step, leaving group departure, Hammond postulate, trigonal planar, sp2 hybridisation, allylic carbocation, benzylic carbocation, ion pairing, polar protic solvent, solvolysis, tert-butyl chloride hydrolysis, carbocation rearrangement