In an SN2 (substitution nucleophilic bimolecular) reaction, the leaving group is displaced by the incoming nucleophile. The preference for the attack of the nucleophile from the backside of the leaving group can be explained by two key factors: steric hindrance and orbital interactions.
Steric Hindrance: In an SN2 reaction, the nucleophile approaches the carbon center where the leaving group is attached. By attacking from the backside, the nucleophile avoids significant steric hindrance from the bulky leaving group and other substituents. This approach minimizes repulsive interactions between electron clouds and allows for a more efficient collision.
Orbital Interactions: In SN2 reactions, the nucleophile donates a pair of electrons to the empty antibonding orbital of the carbon-leaving group bond. This interaction leads to the formation of a new bond and the breaking of the carbon-leaving group bond. The antibonding orbital, also known as the LUMO (lowest unoccupied molecular orbital), has higher energy compared to the bonding orbital. Therefore, the electron density in the antibonding orbital is lower than that in the bonding orbital.
When the nucleophile attacks from the backside, the approaching nucleophile's electron pair aligns with the antibonding orbital of the carbon-leaving group bond. As a result, the overlap between the nucleophile's electron pair and the antibonding orbital is poor, leading to weaker orbital interactions. This results in a higher energy transition state and a slower reaction rate.
On the other hand, if the nucleophile were to attack from the front side, there would be better overlap between the nucleophile's electron pair and the antibonding orbital. This would lead to stronger orbital interactions and a lower energy transition state. However, in such a frontside attack, steric hindrance from the leaving group and other substituents would be significant, making it less favorable.
In summary, the preference for attack from the backside in SN2 reactions is due to the minimization of steric hindrance and the weaker orbital interactions when the nucleophile attacks from the front side. This backside attack allows for efficient collision and the formation of a new bond while minimizing repulsive interactions.