Topic J

The SN2 Mechanism

1.The SN2 Reaction Proceeds With Inversion of Configuration

When we start with a molecule with a chiral center, an SN2 reaction results in the inverstion of sterochemistry

(R→S) or (S→R)

⁠
⁠
2.The Rate Law Of The SN2 Is Second Order Overall

The rate of this reaction is dependent on both the concentration of the nucleophile and the substrate

⁠
⁠
3.The Reaction Rate Is Fastest For Small Alkyl Halides (Methyl > Primary > Secondary >> Tertiary)

The substitution pattern of the alkyl halide results in dramatic changes in the rate of the reaction

Smaller alkyl halides like methyl bromide are fast while more highly substituted tertiary alkyl bromide doesn’t proceed at all

⁠
⁠
4.The SN2 Mechanism Proceeds Through A Concerted Backside Attack Of The Nucleophile Upon The Alkyl Halide

This reaction is called a “backside attack” because the nucleophile approached the alkyl halide 180 degrees from the C-Br bond, and as the C-nucleophile bond forms, the c-leaving group bond breaks

⁠
⁠
⁠

And in an analogy you’ll no doubt hear many times, then, like an umbrella in a strong wind, the three groups flip over as the leaving group leaves, resulting in inversion of configuration. Note that inversion happens at carbons without stereocenters too – it’s just that we can’t observe it because there’s no way to detect the change in configuration.

Steric Hindrance is Like a Fat Goalie

⁠

Want to print your doc?
This is not the way.

Try clicking the ⋯ next to your doc name or using a keyboard shortcut (

CtrlP

) instead.