SolutionsSN2: kinetics, mechanism and stereochemistry
1. The rate law
In an SN2 reaction the halide concentration is tripled and the nucleophile concentration is halved. How does the rate change?
v = k[RX][Nu]: the rate is multiplied by 3 × ½ = 1.5.
2. Hybridization
What is the hybridization of the attacked carbon in the substrate, in the transition state and in the product?
sp3 in the substrate, sp2 (with a p orbital forming partial bonds with the nucleophile and the leaving group) in the transition state and sp3 again in the product.
3. Backside attack
Why does the nucleophile attack from the side opposite the leaving group?
Because in this way it interacts with the small, rear lobe of the carbon sp3 orbital (the C–X antibonding orbital) and avoids repulsion with the electrons of the leaving group. It is the only approach that leads to an effective collision.
4. Units
In what units is the rate constant k of an SN2 reaction expressed if the rate is given in mol/(L·s)?
k = v/([RX][Nu]): L/(mol·s).