SN1 and SN2 Nucleophilic Substitution Reactions HL Only
Quick Notes
- Nucleophilic substitution occurs when a nucleophile replaces a leaving group (usually a halide).
- Primary halogenoalkanes undergo SN2 mechanisms (one-step, concerted).
- Tertiary halogenoalkanes undergo SN1 mechanisms (two-step, via carbocation).
- Secondary halogenoalkanes can follow either mechanism.
- SN2 is stereospecific: inversion of configuration (backside attack).
- Energy profiles:
- SN1 pathway has two peaks (intermediate).
- SN2 pathway has one peak (transition state).
- Rate equations:
- SN1: rate = k[halogenoalkane]
- SN2: rate = k[halogenoalkane][nucleophile]
Full Notes
Nucleophilic substitution reactions occur when a nucleophile (electron pair donor) replaces a leaving group (a halide ion) in a halogenoalkane.
There are two possible mechanisms that can occur (called SN1 and SN2).
Primary halogenoalkanes tend to undergo SN2, while tertiary tend to follow SN1. Secondary halogenoalkanes often follow both SN1 and SN2 pathways.
SN2 Mechanism – Substitution Nucleophilic Bimolecular
SN2 mechanisms occur in one step (both reactants are involved in the same step).
- Step 1:Curly arrow from nucleophile to δ⁺ carbon.
- Step 2:Curly arrow from C–X bond to halogen (X⁻ leaves).
- Step 3:New bond forms between nucleophile and carbon.
The nucleophile attacks the carbon at the same time as the leaving group (halide) departs.
A transition state is formed with partial bonds – both the nucleophile and the leaving group are briefly attached.
SN2 mechanisms are generally favoured by primary halogenoalkanes, where the central carbon is less hindered (low steric hinderance) by other carbon atoms.
SN1 Mechanism – Substitution Nucleophilic Unimolecular
SN1 mechanisms occur in two steps.
- Step 1:The halide leaves first, forming a carbocation (slow step – rate-determining).
- Step 2:The nucleophile then attacks the positively charged carbon.
- Step 3:A new bond is formed between the carbon and the nucleophile.
SN1 mechanisms are generally favoured by tertiary halogenoalkanes, where the carbocation is stabilised by alkyl groups via the positive inductive effect.
Primary halogenoalkanes can’t follow SN1 mechanisms because the positive inductive effect isn’t strong enough to stabilise a carbocation. Tertiary halogenoalkanes can’t follow SN2 mechanisms because steric hindrance blocks nucleophilic attack.
Secondary Halogenoalkanes
Secondary halogenoalkanes can proceed via SN1 and SN2 pathways (often both). Reaction conditions can be changed to try and encourage more SN1 or more SN2.
For Example: Polar protic solvents favour SN1 and aprotic solvents favour SN2. The temperature and strength of nucleophile also influence likelihood of SN1 or SN2.
SN1 vs SN2 Comparison Table
| Feature | SN1 | SN2 |
|---|---|---|
| Number of steps | Two-step via carbocation | One-step concerted |
| Favoured by | Tertiary halogenoalkanes | Primary halogenoalkanes |
| Intermediate | Carbocation | Transition state |
| Stereochemistry | Racemic mixture (optically inactive) | Inversion of configuration (optically active) |
| Rate equation | rate = k[halogenoalkane] | rate = k[halogenoalkane][nucleophile] |
| Energy profile | Two peaks, one intermediate | One peak, no intermediate |
Stereoisomers and Reaction Mechanisms
There is a link between optical activity of the product and the mechanism that occured:
- SN1: Racemic mixture (optically inactive) due to planar carbocation intermediate.
- SN2: Inversion of configuration (optically active) due to backside attack.
SN1 Reactions:
SN1 reactions proceed via a carbocation intermediate, which is planar.
A Nucleophile can attack this intermediate from either side with equal probability, forming the two possible stereoisomers (specifically optical isomers) in equal amounts.
This gives a racemic mixture.
SN2 Reactions:
SN2 reactions involve a single-step mechanism where the nucleophile attacks from the opposite side to the leaving group.
The incoming nucleophile ends up bonding in the opposite position to the leaving group, causing an inversion of the configuration.
This produces only one stereoisomer (specifically an optical isomer), meaning the product mixture is optically active.
Linked Course Questions
What differences would be expected between the energy profiles for SN1 and SN2 reactions?
SN1: Two peaks with an intermediate (carbocation). First step has higher activation energy and is rate-determining.
SN2: One peak, single transition state, no intermediate.
What are the rate equations for these SN1 and SN2 reactions?
SN1: rate = k[halogenoalkane] (depends only on halogenoalkane concentration).
SN2: rate = k[halogenoalkane][nucleophile] (depends on both reactants).
How useful are mechanistic models such as SN1 and SN2?
SN1 and SN2 provide simplified models to explain and predict nucleophilic substitution reactions. They help with understanding reaction rates, stereochemistry, and conditions. Real systems may involve mixed mechanisms or solvent effects, but the models remain powerful tools for understanding reactivity.
Summary
- SN1: two-step, carbocation intermediate, racemic mixture, rate depends on halogenoalkane only.
- SN2: one-step, concerted, inversion of configuration, rate depends on both halogenoalkane and nucleophile.
- Primary halogenoalkanes favour SN2, tertiary favour SN1, secondary can follow either.
- Energy profiles: SN1 has two peaks, SN2 has one.