SN1 Reaction Mechanism

The SN1 reaction mechanism is a 2-step substitution reaction. Just as in SN2 reactions, the “1” in SN1 does not refer to the number of steps of the reaction, but rather to the rate law, which only depends on the concentration of the substrate. Therefore, the rate law of an SN1 reaction is completely independent of the concentration of the nucleophile.

 

Here is the SN1 reaction mechanism:

 

This image shows the 2 steps of the Sn1 reaction mechanism. It also illustrates the Sn1 rate law.

As you can see above, the 2 steps of the SN1 reaction mechanism are (1) the formation of a carbocation and (2) the nucleophilic attack on the substrate. This reaction does NOT follow the “bowling ball mechanism” of an SN2 mechanism, but rather, it is more of a cause and effect mechanism.

 

Cause: The leaving group on the substrate leaves, putting a positive charge on the substrate.
Effect: The electron-rich nucleophile is attracted to the newly-formed positive charge, resulting in nucleophilic attack at this location. A racemic mixture, or mixture of equal parts R and S enantiomers, is formed, so the final product lacks chirality.

 

Keep it Simple
Why does a racemic mixture always form in an SN1 reaction mechanism? This is because the mechanism goes through an sp2-hybridized intermediate, so it loses all chirality it initially might have had.
Let’s look at this concept more closely by looking at the SN1 reaction of the chiral molecule below.

 

Picture56

 

The first step will be the loss of bromine, the leaving group.

 

Note that a racemic mixture is formed during an Sn1 reaction mechanism.

When we form the carbocation, the hybridization changes from sp3-hybridized to sp2-hybridized, which has major consequences for stereochemistry. The newly formed sp2-hybridridized molecule is trigonal planar, meaning the bonds are flat in the plane of the page. In a trigonal planar carbocation, the nucleophile has a 50% chance of attacking the “front” of the molecule and a 50% chance of attacking the “back” of the molecule. Since the location of the nucleophile’s attack determines whether the molecule formed is R or S, a 50/50 mixture of R and S enantiomers will be formed.

In summary, the equal likelihood of the nucleophile attacking either side of the sp2-hybridized, trigonal planar molecule results in an achiral product.

 

Since carbocations are only stable when they are secondary or tertiary, SN1 reactions only occur on secondary or tertiary carbons. SN1 reactions cannot occur on a primary carbon because a primary carbocation isn’t stable enough to be formed.

 

Picture58

 

Because of the formation of a carbocation intermediate in the mechanism, SN1 reactions are prone to rearrangements, like hydride shifts and methyl shifts. Here’s an example of a hydride shift occurring after the formation of a carbocation in step 1 of the SN1 mechanism.

 

Picture59

 

When does an SN1 Reaction Occur?

SN1 reactions primarily occur on tertiary carbons, and sometimes secondary carbons, that have leaving groups attached to them. This is because the carbocation that is formed will be tertiary or sometimes secondary (and therefore fairly stable) when the leaving group leaves.

SN1 reactions do not need a particularly strong nucleophile because the positive charge formed in the first step will attract nucleophiles even as weak as water. Unlike the SN2 reaction in which the nucleophiles instigate the reaction, in an SN1 reaction, the nucleophile attacks only after a carbocation is formed. Therefore, the nucleophile in an SN1 reaction is typically weaker.

 

Summary Points for SN1 Reactions

  • Forms a carbocation in step 1, which is subject to hydride shifts and methyl shifts.
  • Occurs at tertiary and sometimes secondary carbons because carbocations can only stably exist on tertiary and sometimes secondary carbons.
  • Often uses weak nucleophiles. The presence of a positive charge on the substrate can attract even weak nucleophiles.
  • Forms a racemic mixture. Even if starting reactant is chiral, the end product will be achiral (assuming the stereogenic center is the site of reaction).

 

Keep it Simple
Notice that the keys points about an SN1 reaction are all dictated by the carbocation that is formed at the end of step 1. For example:
1. SN1 reactions only occur at tertiary and secondary carbons because those are the only places a carbocation can stably form.
2. SN1 reactions require only weak nucleophiles because the carbocation can attract even partially negatively charge species.
3. A racemic mixture is formed in an SN1 reaction because of the sp2-hybridized, trigonal planar intermediate formed by the carbocation.