An equation such as R–X + Nu⁻ → R–Nu + X⁻ tells us which bonds have changed, but it does not reveal how the electrons moved while the reaction was taking place.
At H3 level, that hidden molecular pathway matters. Does the nucleophile attack carbon while the leaving group is still attached? Or does the leaving group depart first, allowing a carbocation to form before nucleophilic attack? These possibilities lead to the SN2 and SN1 mechanisms respectively.
Try these five questions before reading the chapter. Focus on electron movement, kinetics and molecular structure.
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Nucleophiles are electron-pair donors
A nucleophile is a species that donates an electron pair to form a new covalent bond. In Lewis acid–base terminology, a nucleophile is therefore a Lewis base.
During nucleophilic substitution, the nucleophile attacks an electron-deficient carbon bonded to a leaving group. For a polar carbon–halogen bond:
The electron-rich nucleophile is attracted towards the Cδ+ centre and can donate its electron pair to form a new bond.
Nucleophilicity describes how readily a species attacks an electrophilic centre under a particular set of conditions.
Charge and electron density
When closely related species are compared under similar conditions, an anion is often more nucleophilic than its neutral conjugate species.
OH⁻ versus H₂O: OH⁻ is normally more nucleophilic because its negative charge gives it greater available electron density.
NH₂⁻ versus NH₃: NH₂⁻ is generally the stronger electron-pair donor, although solvent and acid–base reactions also matter.
Nucleophilicity is not identical to basicity
Basicity is an equilibrium property describing the tendency to accept H⁺. Nucleophilicity is a kinetic property describing how rapidly a species attacks an electrophile.
The two properties often correlate, but solvent and steric effects can cause their trends to differ.
Solvation and polarizability
Larger ions have more readily distorted electron clouds and are more polarizable. This can increase their nucleophilicity, particularly in polar protic solvents.
Small, charge-dense anions are strongly solvated by hydrogen bonding in polar protic solvents and may therefore be less available for attack.
| Environment | Important effect | Typical consequence |
|---|---|---|
| Polar protic solvent | Strong solvation of small anions | I⁻ may be more nucleophilic than Br⁻, Cl⁻ and F⁻ |
| Polar aprotic solvent | Anions are less strongly hydrogen-bond solvated | Nucleophilicity often follows basicity more closely |
| Neutral nucleophile | Lower electron density than a comparable anion | Often slower attack, other factors being similar |
Leaving groups must be able to depart
A leaving group is an atom or group that leaves the substrate together with the bonding electron pair.
Good leaving groups are therefore species that are relatively stable after receiving the bonding electron pair.
Weak bases tend to be good leaving groups because they are relatively stable after departure.
Halide leaving groups
For many alkyl-halide substitution reactions:
The trend reflects factors including C–X bond strength, anion stability and polarizability.
Why OH⁻ is usually a poor leaving group
OH⁻ is relatively basic and is therefore normally a poor leaving group. Acidic conditions can improve leaving-group ability by protonating –OH:
The group can then leave as neutral H₂O.
The SN2 mechanism: one concerted step
SN2 means substitution nucleophilic bimolecular. Both substrate and nucleophile participate in the same elementary step.
The nucleophile approaches the carbon from the side opposite the leaving group. As the new C–Nu bond begins to form, the C–LG bond breaks and its electron pair moves onto the leaving group.
SN2 contains no carbocation intermediate. Bond formation and bond breaking occur during the same elementary step.
Backside attack
The nucleophile approaches approximately 180° from the leaving group. This is known as backside attack.
In molecular-orbital terms, the nucleophile donates electron density into the σ* antibonding orbital of the C–LG bond. Backside approach provides the appropriate orbital alignment.
The SN2 transition state
At the highest-energy point of the pathway, the C–Nu bond is partly formed while the C–LG bond is partly broken.
This configuration is a transition state, represented using the symbol ‡. It is not a reaction intermediate.
Why SN2 gives inversion
Backside attack causes the three other substituents around carbon to turn through to the opposite arrangement, rather like an umbrella turning inside out.
This is called Walden inversion.
When substitution occurs at a stereogenic carbon, SN2 is therefore stereospecific and gives inversion at that reaction centre.
Inversion of geometry does not necessarily mean that an R descriptor must become S, because replacing the leaving group may also change the CIP priority order.
Kinetics of SN2
Since substrate and nucleophile are involved in the same elementary step:
The reaction is first order in substrate and first order in nucleophile, giving second-order kinetics overall.
Steric hindrance controls SN2
Backside attack requires the nucleophile to reach the rear of the reacting carbon. Increasing crowding around this carbon makes attack progressively more difficult.
Ordinary tertiary substrates therefore undergo negligible normal SN2 substitution.
Neighbouring branching also matters
Classifying the reacting carbon as primary is not always sufficient. Branching on an adjacent carbon can also obstruct backside attack.
Neopentyl halides are a classic example: the carbon bearing the leaving group is primary, yet the adjacent highly branched carbon produces severe steric hindrance.
The SN1 mechanism: ionisation followed by attack
SN1 means substitution nucleophilic unimolecular. Unlike SN2, the mechanism occurs in separate elementary steps.
Step 1: carbocation formation
The C–LG bond breaks heterolytically and the electron pair moves onto the leaving group:
This generates a carbocation intermediate. Formation of separated charge is energetically demanding, so this is normally the slow, rate-determining step.
Step 2: nucleophilic attack
The nucleophile then donates an electron pair to the carbocation:
This is normally much faster than carbocation formation.
If the attacking nucleophile is neutral, such as H₂O or an alcohol, a later proton-transfer step may be required to give the neutral product.
The carbocation is a genuine reaction intermediate: it lies between two transition states and has a finite, although usually extremely short, lifetime.
The carbocation is trigonal planar
A classical carbocation centre is approximately trigonal planar. Its three σ bonds lie approximately in one plane, while an empty p orbital is perpendicular to that plane.
A nucleophile can therefore approach from either face.
Stereochemistry of SN1
If the original carbon was stereogenic, attack on the two different faces of the planar carbocation can produce the two opposite configurations.
The simple free-carbocation model therefore predicts loss of the original stereochemical preference.
Why SN1 need not give exactly 50:50 racemisation
The departing leaving group may remain close to the carbocation as part of an ion pair or solvent cage.
One face may consequently be partially shielded. Real SN1 reactions can therefore show partial racemisation, often with an excess of inversion, rather than an exactly 50:50 mixture.
Carbocation stability controls SN1
Anything that stabilises the developing carbocation makes ionisation easier.
For ordinary alkyl carbocations:
Alkyl groups stabilise neighbouring positive charge through effects including hyperconjugation and σ-bond polarisation.
Increasing alkyl substitution has opposite consequences for the two mechanisms: it stabilises a carbocation and favours SN1, while steric crowding suppresses SN2.
Resonance-stabilised carbocations are important exceptions
The simple 3° > 2° > 1° trend applies to ordinary alkyl carbocations. Benzylic and allylic carbocations can be unusually stable because positive charge is delocalised by resonance.
Kinetics of SN1
When substrate ionisation is rate determining:
The reaction is first order in substrate and independent of nucleophile concentration in the familiar SN1 kinetic limit.
Why the nucleophile can disappear from the SN1 rate law
The absence of nucleophile concentration from the familiar SN1 rate law does not mean that the nucleophile is absent from the mechanism.
Consider the simplified sequence:
Let ionisation have rate constant k₁, recombination k₋₁ and nucleophilic capture k₂.
Applying the steady-state approximation
Hence:
Rearranging:
Since:
substitution gives:
The familiar first-order limit
If nucleophilic capture is much faster than recombination:
the expression becomes:
The nucleophile concentration cancels mathematically. The nucleophile remains essential for product formation; it simply does not control the observed rate in this limit.
Energy profiles distinguish SN1 from SN2
Reaction-coordinate diagrams provide a particularly clear way to compare the mechanisms. Each elementary step has a transition state, represented by an energy maximum. A genuine intermediate appears as a local minimum between two transition states.
The profiles below are schematic exothermic examples, so the products are deliberately shown clearly lower in potential energy than the reactants. Exothermicity is not a defining feature of either SN1 or SN2.
SN2 energy profile
SN2 contains one elementary step. Its energy profile therefore contains one transition state and no intermediate.
SN1 energy profile
The simplest SN1 pathway has two main elementary steps. Its profile therefore contains two transition states separated by a carbocation intermediate.
The first activation barrier is usually larger because formation of the carbocation is normally the slow, rate-determining step.
How substrate structure controls the mechanism
Increasing alkyl substitution around the reacting carbon produces opposite effects on SN1 and SN2.
Greater substitution stabilises a possible carbocation and favours SN1, but simultaneously increases steric hindrance and suppresses SN2 backside attack.
| Substrate | SN2 tendency | SN1 tendency |
|---|---|---|
| Methyl | Very favourable | Extremely unfavourable |
| Primary | Usually favourable | Usually unfavourable |
| Secondary | Possible | Possible |
| Tertiary | Strongly hindered | Often favourable if ionisation is feasible |
How solvent affects SN1 and SN2
Solvent affects how strongly ions, nucleophiles and developing charge are stabilised. It can therefore change the relative activation barriers of the two pathways.
Polar protic solvents
Polar protic solvents such as water and alcohols stabilise ions through ion–dipole interactions and hydrogen bonding.
They can therefore assist SN1 ionisation by stabilising both developing positive charge and the departing anion.
Polar aprotic solvents
Polar aprotic solvents strongly solvate cations without hydrogen-bonding strongly to anionic nucleophiles.
The nucleophile therefore remains relatively reactive, which often favours SN2.
Conditions favouring SN2: relatively unhindered substrate, effective nucleophile, good leaving group and a solvent that preserves nucleophile reactivity.
Conditions favouring SN1: substrate capable of forming a stable carbocation, good leaving group and an ionising solvent that stabilises developing charge.
Carbocation rearrangements provide mechanistic evidence
Once a carbocation exists as an intermediate, it can sometimes rearrange before nucleophilic attack if a more stable carbocation can be produced.
Possible rearrangements include hydride shifts and alkyl shifts.
Observation of a rearranged carbon skeleton supports a mechanism involving a carbocation intermediate. Normal SN2 reactions do not undergo carbocation rearrangement because no carbocation intermediate exists.
SN1 versus SN2: putting the evidence together
Mechanisms should not be assigned from a single memorised rule. A stronger conclusion comes from combining structural, kinetic, stereochemical and solvent evidence.
| Feature | SN1 | SN2 |
|---|---|---|
| Pathway | Stepwise | Concerted |
| Intermediate | Carbocation | None |
| Simple rate law | rate = k[RX] | rate = k[RX][Nu] |
| Nucleophile in simple rate law? | No | Yes |
| Substrate preference | Stable carbocation formation | Low steric hindrance |
| Stereochemistry | Attack from either face | Inversion |
| Energy profile | Two main maxima and an intermediate | One maximum and no intermediate |
| Carbocation rearrangement | Possible | Not expected |
A practical way to decide between SN1 and SN2
1. Examine the substrate
Ask whether backside attack is sterically accessible and whether ionisation could produce a reasonably stable carbocation.
2. Examine the nucleophile
A high concentration of an effective nucleophile favours direct bimolecular attack and therefore supports SN2.
3. Examine the leaving group
A good leaving group facilitates both mechanisms because the C–LG bond must ultimately break.
4. Examine the solvent
Ask whether the solvent stabilises developing ions strongly or instead leaves an anionic nucleophile highly reactive.
5. Use experimental evidence
Dependence on nucleophile concentration and inversion support SN2.
First-order substrate kinetics, carbocation rearrangement or attack from either face support SN1.
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Thinking the 2 in SN2 means two steps | It refers to molecularity. | SN2 contains one concerted elementary step involving substrate and nucleophile. |
| Drawing a carbocation intermediate in SN2 | Bond formation and bond breaking occur simultaneously. | Show one transition state rather than an intermediate. |
| Forgetting backside attack | Backside approach is essential to the normal SN2 pathway. | Relate backside attack directly to inversion. |
| Drawing an SN1 carbocation as tetrahedral | A classical carbocation centre is approximately trigonal planar. | Use planarity to explain attack from either face. |
| Saying SN1 must give exactly 50:50 racemisation | Ion-pair and solvent-cage effects can make attack from the two faces unequal. | Allow for partial racemisation and excess inversion. |
| Treating nucleophilicity as identical to basicity | Basicity is an equilibrium property whereas nucleophilicity is kinetic. | Also consider solvent, polarizability and steric effects. |
| Assuming SN1 and SN2 must always be exothermic | Overall energy change depends on the particular reaction. | Treat the profiles here as schematic exothermic examples and use them primarily to identify transition states and intermediates. |
What you should be able to do
Recognise nucleophiles and leaving groups
Identify electron-pair donors and explain what makes a good leaving group.
Explain SN2
Describe backside attack, simultaneous bond changes, inversion and second-order kinetics.
Explain SN1
Describe ionisation, carbocation formation, nucleophilic attack and first-order kinetics.
Explain stereochemistry
Relate inversion to SN2 and carbocation planarity to SN1.
Interpret energy profiles
Distinguish transition states from intermediates and identify the number of elementary steps.
Predict the favoured pathway
Combine substrate, nucleophile, leaving group, solvent and kinetic evidence.
These ten questions test whether you can combine structural, kinetic, energetic and stereochemical evidence.
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Frequently asked questions
Questions students often ask about SN1 and SN2
Once the mechanisms are understood in terms of electron movement, geometry and energetics, most of the familiar rules become much easier to reason out.
Why is SN2 called bimolecular if it occurs in one step?
Bimolecular refers to the number of reacting species involved in the elementary step, not the number of steps. Both substrate and nucleophile participate in the SN2 transition state.
Why does SN2 require backside attack?
Backside approach gives the required orbital alignment between the nucleophile and the σ* antibonding orbital of the C–LG bond. It also leads directly to inversion at the reaction centre.
Does SN1 always give exactly 50:50 racemisation?
No. Ion-pair and solvent-cage effects can partially shield one face of the carbocation, so the two faces need not be attacked at exactly the same rate.
Why is the nucleophile absent from the simple SN1 rate law?
The slow process is substrate ionisation. Nucleophilic attack occurs afterwards and therefore does not control the observed rate in the usual SN1 kinetic limit.
Is a strong base always a strong nucleophile?
No. Basicity is an equilibrium property, while nucleophilicity is kinetic. Solvent, steric hindrance and polarizability can make the trends differ.
Why does SN1 have two peaks in its energy profile?
Each elementary step has its own transition state. The valley between the two peaks corresponds to the carbocation intermediate.
Do these diagrams mean SN1 and SN2 are always exothermic?
No. The profiles are drawn as schematic exothermic examples so the difference between reactant and product energy is visually clear. The actual overall energy change depends on the particular reaction.
Can a primary substrate ever participate in a carbocation pathway?
Ordinary primary alkyl carbocations are highly unstable, but benzylic and allylic carbocations can be resonance stabilised. Electronic structure must therefore be considered alongside the primary, secondary or tertiary classification.
SN1 and SN2 are best understood as two competing molecular pathways rather than two lists of rules. SN2 proceeds through one concerted backside attack, whereas SN1 first forms a carbocation before nucleophilic capture. Once structure, kinetics, solvent effects, stereochemistry and energy profiles are connected, the preferred mechanism becomes something you can reason out rather than simply memorise.