SN1 vs SN2 – Stereochemical Behaviour
SN1 and SN2 are two important nucleophilic substitution mechanisms. Their stereochemical behaviour is different because the nucleophile approaches the substrate differently in the two mechanisms.
SN2 → Back-side attack → Inversion of configuration
SN2: Walden inversion
SN2 = Substitution Nucleophilic Bimolecular
SN2 reaction occurs in a single concerted step. The nucleophile attacks the carbon atom from the side opposite to the leaving group.
↓
Back-side attack
↓
R–Nu + X⁻
Back-side Attack
The nucleophile cannot effectively attack from the same side as the leaving group. Therefore, it approaches from the opposite side.
Back-side attack
Walden Inversion
Because of the back-side attack, the three-dimensional arrangement around the carbon atom is inverted. This is called Walden inversion.
SN1 = Substitution Nucleophilic Unimolecular
SN1 reaction occurs in two main steps and involves formation of a carbocation intermediate.
Step 1 – Formation of Carbocation
This is generally the slow, rate-determining step.
Step 2 – Nucleophilic Attack
The carbocation is approximately trigonal planar. Therefore, the nucleophile can attack from either side.
Stereochemical Result
Attack from both sides can produce both configurations. Therefore, an optically active substrate generally gives a mixture of products, often described as racemisation.
| Feature | SN1 | SN2 |
|---|---|---|
| Number of steps | Two or more steps | Single step |
| Intermediate | Carbocation | No intermediate |
| Attack | Can occur from either face | Back-side attack |
| Intermediate geometry | Planar | No carbocation |
| Stereochemical result | Racemisation / mixture | Inversion of configuration |
| Special term | Racemisation | Walden inversion |
Walden inversion is the inversion of spatial configuration at a stereogenic carbon atom during an SN2 reaction.
⬇
Nucleophile attacks from back side
⬇
Leaving group departs
⬇
Product with inverted configuration
In SN1, the leaving group first leaves and produces a carbocation. The carbocation is approximately planar.
↓
R⁺ + X⁻
↓
Planar carbocation
Since the carbocation is planar, the nucleophile can approach from either face.
Attack from back → Product B
The two configurations can therefore be formed. Hence, an optically active substrate generally undergoes racemisation in an SN1 reaction.
SN1 is favoured by:
- Stable carbocation formation
- 3° alkyl halides generally react readily
- Polar protic solvents
- Good leaving groups
- Weak nucleophiles can participate
SN2 is favoured by:
- Less sterically hindered substrates
- Methyl and primary halides generally react readily
- Strong nucleophiles
- Polar aprotic solvents
- Good leaving groups
SN2 → Steric hindrance is very important
Q. What is the stereochemical result of an SN2 reaction?
Answer:
In an SN2 reaction, the nucleophile attacks the substrate from the back side. This causes inversion of configuration at the stereogenic carbon atom.
Q. Why does SN1 reaction generally lead to racemisation?
Answer:
- SN1 reaction forms a carbocation intermediate.
- The carbocation is approximately planar.
- The nucleophile can attack from either face.
Therefore, both configurations may be formed, resulting in racemisation or a mixture of stereoisomers.
Q. Compare the stereochemical outcomes of SN1 and SN2 reactions.
| SN1 | SN2 |
|---|---|
| Forms a planar carbocation | No carbocation intermediate |
| Attack from either face | Back-side attack |
| Usually gives racemisation | Gives inversion |
| Two-step mechanism | One-step mechanism |
Q. Explain the mechanisms of SN1 and SN2 reactions and compare their stereochemical outcomes.
SN1 Mechanism
The leaving group first departs, producing a carbocation.
The planar carbocation is then attacked by the nucleophile from either side, producing both possible configurations.
↙ ↘
Nu attack Nu attack
Hence SN1 generally gives racemisation.
SN2 Mechanism
The nucleophile attacks from the side opposite to the leaving group while the leaving group leaves simultaneously.
Back-side attack
This causes inversion of configuration, known as Walden inversion.
SN2 → Walden inversion
Q. Explain SN1 and SN2 mechanisms in detail and compare their stereochemical inversion.
SN1 Mechanism
SN1 stands for substitution nucleophilic unimolecular. It proceeds through a carbocation intermediate.
Step 1 – Ionisation
The carbocation formed is approximately trigonal planar.
Step 2 – Nucleophilic Attack
The nucleophile can attack either face of the planar carbocation. Therefore, both configurations may be obtained.
Planar carbocation
↓
Attack from either face
↓
Racemisation / mixture
SN2 Mechanism
SN2 stands for substitution nucleophilic bimolecular. It occurs in one concerted step.
↓
Back-side attack
↓
R–Nu + X⁻
The nucleophile attacks opposite to the leaving group. This produces inversion of configuration.
Back-side attack
↓
Walden inversion
Final Comparison
| Property | SN1 | SN2 |
|---|---|---|
| Intermediate | Carbocation | None |
| Attack | Either face | Back side |
| Stereochemistry | Racemisation | Inversion |
| Mechanism | Stepwise | Concerted |
SN2 → Back-side attack → Walden inversion
1. SN1 stands for:
✅ Answer
A
2. SN2 reaction occurs in:
✅ Answer
B
3. SN1 reaction involves formation of:
✅ Answer
B
4. The SN2 nucleophile attacks from:
✅ Answer
B) Back side
5. Stereochemical inversion in SN2 is called:
✅ Answer
B
6. The carbocation intermediate in SN1 is approximately:
✅ Answer
B
7. SN1 generally results in:
✅ Answer
B
8. SN2 stereochemistry results mainly from:
✅ Answer
B
9. Which mechanism has no carbocation intermediate?
✅ Answer
B
10. SN1 is favoured by:
✅ Answer
A
11. SN2 is generally fastest for:
✅ Answer
B
12. Which mechanism is bimolecular in its rate-determining event?
✅ Answer
B
13. The stereochemical product of a pure SN2 substitution at a chiral centre is associated with:
✅ Answer
A
14. Why can SN1 give attack from both sides?
✅ Answer
A
15. The key stereochemical feature of SN2 is:
✅ Answer
B
16. Which reaction is more sensitive to steric hindrance?
✅ Answer
B) SN2
17. Which mechanism commonly uses a planar carbocation intermediate?
✅ Answer
A) SN1
18. In SN2, the leaving group and nucleophile:
✅ Answer
B
19. Which combination is correct?
✅ Answer
C
20. Walden inversion is associated with:
✅ Answer
B) SN2
🎯 Quick Revision
SN2: Substitution Nucleophilic Bimolecular
SN1: Carbocation intermediate
SN2: No intermediate
SN1: Planar carbocation → attack from either face
SN2: Back-side attack
SN1: Generally racemisation
SN2: Walden inversion
