The SN2 Reaction

Let’s look at this substitution reaction.

Notice how we start with one enantiomer of starting material and end up with one enantiomer of product. 

 When we look at the rate of this reaction, we find that it depends on a few things:

1. The concentration of the nucleophile and the electrophile. If we double the concentration of either, we double the rate. If we double the concentration of both, we quadruple the rate. 
2. Furthermore, if we examine the rate of this reaction with 1-butyl chloride instead of 2-butyl chloride, it’s at least 20 times faster.

What mechanism could account for all of these observations?

A backside attack mechanism. 

Our best theory is that the reaction occurs in one step, and the nucleophile attacks the backside of the C–Br bond. This leads to a transition state where there are five bonds to carbon (unstable) and then, like an umbrella in the wind, the three groups on carbon fold back while the leaving group leaves. This process is called “inversion”. 

This is a good theory explains 3 things:

1. the dependence of the rate on the concentration of both nucleophile and electrophile
2.  the stereochemistry (inversion!)
3. the fact that the rate proceeds in the order primary alkyl halides > secondary >> tertiary. The nucleophile has to hit that tiny little antibonding orbital. For a tertiary carbon, this is like trying to score on a really fat goalie. 

We call this mechanism the SN2 mechanism.

• Substitution, because that’s the type of reaction
• Nucleophilic, because a nucleophile is attacking the electrophile.
• 2, because the reaction is bimolecular (depends on concentration of the nucleophile and the electrophile).

This reaction is one of the “stalwarts” of organic chemistry. We’ll see it again and again and again.
Tomorrow: let’s wrap up the week.

Thanks for reading! James

P.S. Further reading: The SN2 Mechanism