In A-Level Chemistry, SN1 and SN2 are often introduced as two separate mechanisms. At H3 level, the more interesting question is not simply what happens in each mechanism? but which pathway will a real substrate actually take?
A molecule does not recognise labels such as “SN1 substrate” or “SN2 substrate”. Instead, the reaction follows the pathway with the more favourable activation barrier under the particular conditions. Substrate structure, nucleophile, solvent and leaving group therefore have to be considered together.
Try these five questions before reading the chapter. Focus on what controls the rate of each mechanism and how molecular structure affects the preferred pathway.
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1. SN1 and SN2 are competing reaction pathways
Both mechanisms replace a leaving group by a nucleophile, but they reach the substitution product in fundamentally different ways. That difference changes the kinetics, energy profile, substrate preference and stereochemistry.
| Feature | SN2 | SN1 |
|---|---|---|
| Number of elementary steps | One | Usually two major steps |
| Key event | Nucleophilic attack occurs as the leaving group departs | Leaving group departs first to generate a carbocation |
| Rate law | rate = k[RX][Nu] | rate = k[RX] |
| Intermediate | None | Carbocation |
| Stereochemical signature | Inversion at the reacting centre | Substantial racemisation when a stereogenic carbocation is involved |
The competition is really a competition between activation barriers. Whichever pathway has the more accessible activation barrier under the reaction conditions contributes more strongly to the observed products.
The SN2 rate law
Both the substrate and nucleophile are involved in the single elementary step. Increasing either concentration therefore increases the frequency with which the required reacting arrangement is reached.
The SN1 rate law
The slow step is ionisation of the substrate. The nucleophile attacks only after the carbocation has formed, so changing nucleophile concentration does not normally change the rate of this rate-determining step.
2. The energy profiles reveal why the mechanisms behave differently
The number of peaks in an energy profile corresponds to the number of transition states. An SN2 reaction has one elementary step and therefore one transition state. A simple SN1 reaction has a carbocation intermediate and therefore two major transition states.
The profiles below are deliberately drawn as exothermic examples so that the products are visibly lower in energy than the reactants. The actual overall ΔH depends on the particular reaction; it is the shape of the profile that distinguishes the mechanisms.
SN2: one transition state and no intermediate
At the transition state, the nucleophile–carbon bond is partly forming while the carbon–leaving-group bond is partly breaking. There is no carbocation minimum between reactants and products.
SN1: two transition states separated by a carbocation intermediate
The first peak corresponds to ionisation of the substrate and formation of the carbocation. This is commonly the largest barrier. The second peak corresponds to nucleophilic attack on the carbocation.
3. Substrate structure is the first factor to examine
Before thinking about nucleophile or solvent, identify the carbon directly bonded to the leaving group. The number of carbon atoms directly attached to this carbon determines whether the substrate is methyl, primary, secondary or tertiary.
Methyl is not the same as primary. In a methyl halide, the carbon bearing X is bonded to zero other carbon atoms. In a primary halide, it is bonded to one carbon atom.
Methyl substrates: overwhelmingly SN2
A methyl substrate presents almost no steric obstruction to nucleophilic attack. At the same time, an SN1 pathway would require formation of a methyl carbocation, CH₃⁺, which is extraordinarily unstable.
The combination is decisive: very favourable accessibility for SN2 and extremely unfavourable carbocation formation for SN1.
Primary substrates: usually SN2
Primary substrates remain relatively accessible to nucleophilic attack, whereas ordinary primary alkyl carbocations are highly unstable. Consequently, uncomplicated primary halogenoalkanes generally react by SN2.
This should not be turned into an absolute rule. A carbocation that is strongly stabilised by resonance, such as an allylic or benzylic carbocation, can change the balance dramatically.
Tertiary substrates: SN2 is strongly suppressed
In an SN2 reaction the nucleophile must approach the carbon from the side opposite the leaving group. Three alkyl substituents around that carbon create severe steric crowding and greatly raise the energy of the SN2 transition state.
Tertiary substrates can, however, form relatively stable tertiary carbocations. Under suitable ionising conditions this strongly favours SN1 over SN2.
Secondary substrates: genuine competition
A secondary carbon occupies the middle ground. It is hindered enough for SN2 to become sensitive to nucleophile and solvent, yet a secondary carbocation may be sufficiently accessible under favourable SN1 conditions.
This is why secondary substrates cannot be classified reliably from structure alone. The remaining reaction conditions become the tie-breakers.
| Substrate | SN2 tendency | SN1 tendency |
|---|---|---|
| Methyl | Very strongly favoured | Essentially inaccessible through a free methyl carbocation |
| Primary | Usually favoured | Usually unfavourable unless additional stabilisation is present |
| Secondary | Possible | Possible |
| Tertiary | Strongly suppressed by steric hindrance | Often favoured under suitable conditions |
4. Carbocation stability controls the SN1 pathway
Because SN1 requires carbocation formation, anything that stabilises the developing positive charge lowers the energy required for the ionisation step.
For ordinary alkyl carbocations, increasing alkyl substitution generally increases stability through electron donation and hyperconjugative stabilisation.
Resonance stabilisation can be even more important. Allylic and benzylic carbocations distribute positive charge over more than one atom and may therefore undergo reactions that do not follow the simple alkyl classification alone.
Never use “primary means SN2” without considering resonance. The correct deeper question is whether the carbocation required for SN1 would be sufficiently stabilised.
5. The Hammond postulate explains the substrate effect in SN1
The first step of an SN1 reaction involves breaking the C–X bond and generating separated or partially separated charges. This ionisation step is normally energetically uphill relative to the starting substrate.
According to the Hammond postulate, the transition state for an endothermic elementary step resembles the higher-energy species more closely. Therefore, the transition state for carbocation formation has substantial carbocation-like character.
Stabilising the carbocation also stabilises the carbocation-like transition state. The activation energy for the first SN1 step falls, so the SN1 reaction becomes faster.
This gives a more rigorous explanation for the strong substrate effect in SN1 than simply memorising that “tertiary reacts faster”.
6. Nucleophile strength matters most when SN2 is structurally possible
The nucleophile participates directly in the SN2 transition state. Increasing its effectiveness can therefore lower the activation barrier for the bimolecular pathway.
Anionic nucleophiles such as CN⁻, RS⁻ and many halide ions can be highly effective SN2 nucleophiles under suitable conditions. Neutral nucleophiles such as H₂O and ROH are generally less reactive.
Accessible methyl or primary substrate: a good nucleophile reinforces an already favourable SN2 pathway.
Secondary substrate: nucleophile effectiveness can make a major difference to the competition.
Tertiary substrate: making the nucleophile stronger does not remove the severe steric obstruction to ordinary SN2 attack.
Nucleophilicity is not exactly the same as basicity
Basicity describes an equilibrium tendency to accept H⁺, whereas nucleophilicity is a kinetic property describing how rapidly a species attacks an electrophilic centre.
Solvent therefore matters enormously. Two nucleophiles with a particular basicity order do not necessarily show the same nucleophilicity order in every solvent.
7. Solvent can shift the SN1–SN2 balance
Solvent is more than a passive medium. It stabilises ions and can also change how available a nucleophile is for reaction.
Polar protic solvents favour ionisation
Polar protic solvents such as water and alcohols can stabilise both the developing carbocation and the leaving-group anion through ion–dipole interactions and hydrogen bonding.
This lowers the energetic cost of charge separation and can therefore make SN1 substantially more favourable when the substrate can form a reasonably stable carbocation.
Polar protic solvents can reduce anionic nucleophilicity
An anionic nucleophile can become strongly solvated by hydrogen bonding. Before it can attack a carbon centre, part of this solvation environment must be disrupted. This can slow SN2 attack.
Polar aprotic solvents often favour SN2
Polar aprotic solvents such as DMSO, DMF and acetone can solvate cations effectively but do not hydrogen-bond strongly to anionic nucleophiles. The anion therefore remains comparatively exposed and reactive.
| Solvent environment | Main effect | Typical consequence |
|---|---|---|
| Polar protic | Stabilises ions; strongly solvates many anions | Can favour SN1 and reduce SN2 nucleophilicity |
| Polar aprotic | Leaves many anionic nucleophiles comparatively reactive | Often favours SN2 when the substrate is accessible |
8. A good leaving group helps both pathways
Both SN1 and SN2 require cleavage of the bond to the leaving group. A better leaving group therefore lowers the energetic penalty associated with C–X bond breaking.
Good leaving groups are typically species that are relatively stable after departure. Weak bases therefore tend to be better leaving groups than strongly basic species.
Sulfonate groups such as tosylate are also excellent leaving groups. By contrast, OH⁻ is normally a poor leaving group unless the functional group is first activated — for example, protonation converts an –OH group into a species that can leave as neutral H₂O.
A good leaving group does not by itself tell you whether the mechanism is SN1 or SN2. It can accelerate both. Substrate structure, nucleophile and solvent must still be considered.
9. SN2 stereochemistry: inversion at the reacting carbon
The geometry required for effective SN2 orbital interaction places the incoming nucleophile opposite the leaving group. As bond formation and bond breaking occur together, the tetrahedral arrangement at the reacting carbon turns inside out.
This is known as Walden inversion.
Be precise in examination answers: SN2 gives inversion of the spatial arrangement at the reacting stereocentre. Do not automatically state that an R reactant must give an S product, because replacing the leaving group may alter the CIP priority order.
10. SN1 stereochemistry: why racemisation may be imperfect
A simple carbocation centre is approximately trigonal planar. Once it has formed, the nucleophile can in principle approach from either face. Consequently, an SN1 reaction at a stereogenic centre tends towards formation of both configurations.
A first approximation therefore predicts racemisation.
Real SN1 reactions, however, do not always produce an exactly 50:50 mixture. The departing leaving group may remain close to the carbocation as an ion pair, or the surrounding solvent cage may make one face less accessible than the other.
Hence “SN1 gives racemisation” is a useful model, not a guarantee of perfect 50:50 attack. Residual ion-pair effects can give a modest excess of one stereochemical pathway, often favouring attack from the side opposite the departing group.
11. How to decide between SN1 and SN2 systematically
Instead of memorising isolated statements, work through the factors in a fixed order.
| Question | What to look for | Implication |
|---|---|---|
| 1. What is the substrate? | Methyl, primary, secondary, tertiary or resonance-stabilised? | Sets the fundamental feasibility of SN1 and SN2 |
| 2. How effective is the nucleophile? | Anionic/reactive or weak/neutral? | A strong effective nucleophile favours SN2 where attack is accessible |
| 3. What is the solvent? | Polar protic or polar aprotic? | Can stabilise ionisation or alter nucleophile reactivity |
| 4. How good is the leaving group? | Can it depart as a relatively stable species? | Good leaving groups facilitate substitution |
| 5. What does the experiment show? | Rate law and stereochemistry | Can provide direct evidence for the operative pathway |
A useful decision hierarchy
Methyl: think SN2.
Ordinary primary: think SN2 unless there is an unusual stabilising feature.
Secondary: inspect nucleophile and solvent carefully.
Tertiary: normal SN2 is strongly disfavoured; under suitable substitution conditions, SN1 becomes much more plausible.
12. Remember that elimination may also compete
Real reaction mixtures are not always a choice between SN1 and SN2 alone. Strongly basic reagents and higher temperatures can make elimination important, particularly for secondary and tertiary substrates.
Therefore, deciding that SN2 is sterically unfavourable does not automatically prove that SN1 will dominate. E1 or E2 pathways may also have to be considered when the reagent and conditions permit elimination.
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Treating methyl and primary substrates as identical | The methyl carbon is bonded to zero other carbons; a primary reacting carbon is bonded to one. | Classify the carbon directly attached to the leaving group. |
| Assuming every primary substrate must react by SN2 | Resonance stabilisation can make carbocation formation far more favourable than the simple alkyl classification suggests. | Check for allylic, benzylic or other stabilising effects. |
| Assuming a very strong nucleophile can force a tertiary SN2 reaction | The central problem is severe steric obstruction to backside approach. | Examine substrate accessibility before considering nucleophile strength. |
| Saying a good leaving group proves SN1 | A good leaving group can assist both SN1 and SN2. | Combine leaving-group ability with substrate, solvent and nucleophile. |
| Writing “SN2 always changes R into S” | SN2 guarantees geometrical inversion, but the CIP priority ranking may change after substitution. | State that inversion occurs at the reacting stereocentre. |
| Assuming SN1 must give exactly 50% of each enantiomer | Ion pairing and solvent-cage effects may make the two faces of the carbocation temporarily nonequivalent. | Describe SN1 as tending towards racemisation, with possible non-statistical stereochemical outcomes. |
What you should be able to do
Classify the substrate
Distinguish methyl, primary, secondary and tertiary reacting centres correctly.
Use kinetic evidence
Relate rate = k[RX][Nu] to SN2 and rate = k[RX] to SN1.
Compare activation barriers
Recognise the one-transition-state SN2 profile and two-transition-state SN1 profile.
Predict solvent effects
Explain why polar protic environments often assist SN1 while polar aprotic solvents can enhance anionic nucleophiles for SN2.
Explain stereochemistry
Relate SN2 to inversion and SN1 to substantial, but not necessarily perfect, racemisation.
Combine competing factors
Judge an unfamiliar reaction by considering substrate, nucleophile, solvent and leaving group together.
These ten questions test whether you can combine structure, nucleophile, solvent, kinetics and stereochemistry rather than relying on a single memorised rule.
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Frequently asked questions
Questions students often ask about SN1 vs SN2
Once you stop treating SN1 and SN2 as isolated mechanisms, a few subtler questions naturally appear. These are useful because they expose where the simple introductory rules reach their limits.
Is bromomethane a primary halogenoalkane?
No. Bromomethane is classified separately as a methyl halide. Its carbon bonded to Br is attached to zero other carbon atoms. In a primary halogenoalkane, the carbon bonded to the halogen is attached to one other carbon atom. Both classes favour SN2 strongly, but they are not the same structural category.
Can a primary substrate ever undergo SN1?
An ordinary primary alkyl carbocation is too unstable for SN1 to be competitive under normal conditions. However, the simple primary-secondary-tertiary rule must be modified when the positive charge can be strongly stabilised, particularly by resonance. Benzylic and allylic systems therefore require separate consideration.
Does a strong nucleophile always mean SN2?
No. A strong nucleophile favours SN2 only when the electrophilic carbon is sufficiently accessible. A tertiary substrate remains severely hindered even in the presence of an excellent nucleophile. Strongly basic nucleophiles may also introduce competition from E2 elimination.
Why does SN1 not always give exactly 50:50 racemisation?
The idealised model assumes a freely solvated planar carbocation that can be attacked equally from either side. Immediately after ionisation, however, the leaving group and surrounding solvent may remain associated with the carbocation. An ion pair or solvent cage can partly block one face, so the product mixture need not be exactly 50:50.
What is the fastest way to decide between SN1 and SN2 in an examination?
Start with the substrate. Methyl and ordinary primary substrates strongly suggest SN2; tertiary substrates strongly suppress SN2; secondary substrates require closer inspection. Then consider the nucleophile and solvent, followed by leaving-group ability. If experimental information is provided, use the rate law and stereochemistry because these can give direct mechanistic evidence.
The most useful way to understand competition between SN1 and SN2 is to stop asking which mechanism a molecule has been “assigned” and instead ask which activation barrier the reaction conditions make easier to cross. Substrate structure sets the possibilities; nucleophile, solvent and leaving group then shift the balance. Once you reason from those ideas, unfamiliar SN1–SN2 questions become much more systematic.