In a SN2 reaction, what is the required angle for the reaction to occur?
a) 90
b)120
c) 180
d) None of the above
e) a and b
The correct answer and explanation is :
The correct answer is:
c) 180
Explanation:
An SN2 (Substitution Nucleophilic Bimolecular) reaction occurs in a single, concerted step where the nucleophile directly attacks the electrophilic carbon, displacing the leaving group in a backside attack mechanism. This backside attack is crucial for the reaction to proceed and results in inversion of configuration at the carbon center.
Why 180° is Required?
- Backside Attack Mechanism:
- In an SN2 reaction, the nucleophile must approach the carbon center from the side opposite to the leaving group.
- For this to happen efficiently, the angle between the nucleophile and the leaving group must be 180°, meaning they are positioned exactly opposite each other.
- Orbital Overlap Consideration:
- The carbon undergoing substitution is usually sp³ hybridized, with a tetrahedral geometry.
- The optimal orbital overlap occurs when the nucleophile attacks along the axis directly opposite the leaving group. This ensures a smooth transition state and effective bond formation.
- Steric and Electronic Factors:
- If the angle were anything other than 180°, the overlap between the nucleophile’s lone pair and the carbon’s antibonding orbital would be inefficient, leading to a slower or failed reaction.
- A smaller angle, such as 90° or 120°, would cause steric hindrance and reduced orbital alignment, making the reaction highly unfavorable.
Conclusion
For an SN2 reaction to proceed efficiently, the nucleophile must attack from exactly the opposite direction of the leaving group. This 180° approach ensures the best orbital alignment and results in a smooth transition state. Thus, the correct answer is (c) 180°.