SN2 Reaction Mechanism

The SN2 reaction mechanism is a 1-step mechanism that substitutes one atom/group for another. In one step, the nucleophile attacks the substrate (or the electrophile), simultaneously causing the leaving group to leave the molecule. Here is the mechanism for an SN2 reaction:

Picture47

The rate law is dependent on both the concentration of the nucleophile and the electrophile/substrate.

 

Keep it Simple
Why is there a “2” in SN2 reactions if it’s a 1-step mechanism? It’s because this “2” doesn’t refer to the number of steps in the reaction at all. Instead, it refers to the fact that the rate law depends on the concentration of 2 reactants, both the nucleophile and the substrate.

Rate of SN2= k [Nucleophile][Substrate]

 

The nucleophile (the –OH) attacks the substrate on the carbon next to the leaving group (the bromine), causing the leaving group to leave. Think about it like 3 bowling balls:

 

This image shows a simplified version of the SN2 reaction mechanism, using a bowling ball analogy.

 

In order to displace the bromine, the –OH must attack from the exact opposite side of the bromine, which is known as backside attack. This leads to an important point: all SN2 reactions take place via backside attack. Think of the bowling ball analogy to understand why SN2 reactions must occur via backside attack.

Backside attack is important because it results in inversion of stereochemistry at the carbon of the attack. Therefore, all SN2 reactions result in inversion of stereochemistry at the carbon of the attack. This is particularly important if this carbon is a stereogenic center.

Let’s look at an example.

 

Picture49

Looking at the example above, the far-right carbon on the substrate, a stereogenic center, is the site of the nucleophilic attack (see why in the Keep it Simple below). To undergo the SN2 reaction mechanism, the -OH must attack the backside of the carbon indicated by the arrow below.

Picture50

 

The –OH assumes the position it attacks, while the deuterium and the hydrogen flip upward because of the attack of the –OH. This has inverted the stereochemistry. The mechanism for this reaction is shown below.

 

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Keep it Simple
A common question students ask is “How do I know which carbon the nucleophile attacks?” To answer this, let’s examine the molecule below:

 

Picture52

 

Because of induction, the electronegative chlorine has a partial negative charge, while the neighboring carbon has a partial positive charge.

 

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Because nucleophiles are electron-rich and often negatively charged, they will be attracted towards a positive charge, like the partial positive charge on the specified carbon atom. You can think of this like a magnet being attracted to another, oppositely polarized magnet.

It’s also important to note that a leaving group must be present at the carbon for a substitution reaction to occur.

In summary, it is the attraction of opposite charges that brings the nucleophile and the electrophile together. If a good leaving group is also present at this carbon, then a reaction can occur.

 

 

When does the SN2 reaction mechanism occur?

SN2 reactions occur with (1) strong nucleophiles at (2) very non-hindered carbons on the substrate (mostly primary and sometimes secondary carbons). We elaborate on these 2 factors below.

(1) The nucleophile itself must be strong as it is the instigator of the attack; it must actively go out and attack the substrate itself.

(2) If there are lots of other groups in the way (like with a tertiary carbon), the nucleophile won’t be able to make that “bowling ball” contact that is necessary for an SN2 reaction as the nucleophile will be blocked from its desired backside attack. Remember, an SN2 reaction always reacts via backside attack.

 

Picture54

 

As you can see from the green arrows above, the nucleophile must invasively attack the molecule as it must attack the carbon attached to the leaving group. Therefore, steric hindrance effects (like in a tertiary carbon) prevent the nucleophile from attacking where it wants.

 

Summary Points for SN2 Reactions

  • The rate law depends on the concentrations of both the substrate and the nucleophile.
  • Inversion of stereochemistry occurs due to the nature of the SN2 backside attack.
  • Occurs with strong nucleophiles because the nucleophile is the instigator of the reaction. The nucleophile must be strong enough to go out and attack a substrate.
  • Occurs at unhindered carbons, like primary or sometimes secondary carbons. This is because the nucleophile must invasively attack the substrate, which it would not be able to do if it was blocked from backside attack because of the steric hindrance of a tertiary carbon.