SN1 vs SN2 — Nucleophilic Substitution
Watch SN2 flip a molecule inside out in one step, and SN1 go through a flat carbocation — in 3D.
About this simulation
The two nucleophilic substitution mechanisms, animated in 3D. In SN2 the hydroxide ion attacks from the side opposite the bromine and the molecule turns inside out like an umbrella in one concerted step, through a five-coordinate transition state. In SN1 the bromide leaves first, the carbon flattens into a planar carbocation, and the nucleophile can attack either face. Students choose the mechanism and the substrate (methyl, 1°, 2° or 3°), can hold the animation at each step, and read the rate law, the number of steps, the stereochemistry (inversion or racemisation) and how stable the carbocation would be — so why methyl halides go SN2 and tertiary ones SN1 is something they watch.
How to use it
- Set 'Mechanism' to SN2 and 'Substrate' to Methyl: OH⁻ approaches from the side opposite Br, and the three groups flip over like an umbrella in the wind.
- Use 'Hold at' Step 2 to stop at the pentacoordinate transition state, with partial bonds to both OH and Br, and watch it turn.
- Switch 'Mechanism' to SN1 and 'Substrate' to 3°: Br leaves first, the carbon flattens into a carbocation, and OH⁻ can attack from either face.
- Change 'Substrate' from Methyl to 3° under each mechanism and read the verdict at the top, such as 'Strongly favoured' or 'Does not occur', and the 'carbocation stability' meter.
- Slow the action with 'Animation speed' and compare the lines under the verdict: 'rate = k[RX][Nu⁻] 2nd order · 1 step' and 'Inversion (Walden)' for SN2, 'rate = k[RX] 1st order · 2 steps' and 'Racemic (~50:50)' for SN1.
What to notice
- SN2 is a single concerted step: the new C–O bond forms as the C–Br bond breaks, so the rate depends on both the haloalkane and the nucleophile.
- Backside attack in SN2 turns the molecule inside out, giving inversion of configuration (Walden inversion).
- SN1 happens in two steps through a flat carbocation; the nucleophile can attack either face, so an optically active substrate gives a largely racemic product.
- Bulky groups block the backside attack of SN2 on a tertiary carbon, while the same groups stabilise the carbocation that SN1 needs, so reactivity orders run in opposite directions.
The formulas
- SN2: rate = k [RX] [Nu⁻]
- Second-order rate law: the rate depends on the concentrations of both the haloalkane RX and the nucleophile Nu⁻.
- SN1: rate = k [RX]
- First-order rate law: only the haloalkane takes part in the slow ionisation step.
- (CH₃)₃C–Br → (CH₃)₃C⁺ + Br⁻ (slow); (CH₃)₃C⁺ + OH⁻ → (CH₃)₃C–OH (fast)
- The two steps of SN1 hydrolysis of tert-butyl bromide through a carbocation.
Questions
What is the difference between SN1 and SN2 reactions?
SN2 is a one-step bimolecular reaction: the nucleophile attacks from the back as the leaving group departs, the rate depends on both reactants and the configuration is inverted. SN1 is a two-step unimolecular reaction: the haloalkane first ionises to a carbocation, then the nucleophile attacks, so the rate depends only on the haloalkane and the product is largely racemic. The simulation shows both in 3D.
Why do tertiary alkyl halides prefer SN1 while primary alkyl halides prefer SN2?
A tertiary carbocation is stabilised by three alkyl groups through their electron-releasing effect and hyperconjugation, so ionisation is easy, while the same bulky groups crowd the back of the carbon and block SN2. A primary carbon is open for backside attack but would form a very unstable carbocation. Compare the carbocation stability meter and the verdict at the top for 1° and 3° substrates.
Why does an SN2 reaction cause inversion of configuration?
In SN2 the nucleophile attacks the carbon from the side directly opposite the leaving group. In the transition state the carbon has five partial bonds, with the other three groups in a flat plane. As the leaving group departs, those three groups are pushed to the other side, like an umbrella turning inside out. Hold the SN2 animation at Step 3 to see the flip.
Why does an SN1 reaction of an optically active haloalkane give a racemic mixture?
In SN1 the leaving group departs first, leaving a carbocation in which the carbon is sp² hybridised and planar. The nucleophile can attack this flat ion equally from either face, giving both enantiomers. The product is therefore largely racemic, though often with slight inversion because the departing ion partly shields one face. In the simulation, SN1 shows a faint second nucleophile on the opposite face.
How does the solvent affect SN1 and SN2 reactions?
Polar protic solvents such as water and alcohols stabilise the carbocation and the leaving anion by solvation, so they favour SN1. They also hydrogen-bond to the nucleophile and weaken it. Polar aprotic solvents such as acetone or DMSO solvate the nucleophile poorly, leaving it reactive, so they favour SN2. Strong nucleophiles at high concentration also push the reaction towards SN2.