Question

Write salient features of S_N2 mechanism.

Answer 1

1. Single-step mechanism with simultaneous bond breaking and bond forming.
2. The nucleophile attacks the carbon undergoing substitution from the side opposite to that of the leaving group. This is to avoid steric repulsion (repulsion due to bulkyness of the groups) and electrostatic repulsion between the incoming nucleophile and the leaving group.
3. In the transition state (T.S.), the nucleophile and leaving groups are bonded to the carbon with partial bonds and carry partial negative charge. (Thus, the total negative charge is diffused.)
4. The T.S. contains pentacoordinate carbon having three σ (sigma) bonds in one plane, making bond angles of 120° with each other and two partial covalent bonds along a line perpendicular to this plane.
5. When S_N2 reaction is brought about at chiral carbon (in an optically active substrate), the product is found to have opposite configuration compared to that of the substrate. In other words, S_N2 reaction is found to proceed with the inversion of configuration. This is like flipping of an umbrella. It is known as Walden inversion. The inversion in configuration is the result of a backside attack of the nucleophile.