SN1 vs SN2

There is a hierarchy of 3 main factors that determines whether a reaction will undergo an SN1 vs SN2 reaction, 1 of which we have not yet discussed. Note that each of the factors is discussed below as though one is only trying to choose between an SN1 reaction and an SN2 reaction.

  1. The location of the leaving group: If the leaving group is on a tertiary carbon, the reaction will tend to undergo SN1 substitution. If the leaving group is on a primary carbon, the reaction will tend to undergo SN2 substitution. If the leaving group is on a secondary carbon, one must look at the remaining factors.
  2. The strength of the nucleophile: If the nucleophile is very strong due to a negative charge or high polarizability (e.g. I-), the reaction will tend to undergo SN2 reactions. If the nucleophile is weaker (e.g. H2O), then it will tend to undergo SN1 reactions.
  3. The solvent: This factor has not yet been covered. The solvent is the majority liquid in a solution. In the reaction below, CH3OH is the solvent.

 

A typical reaction used to demonstrate Sn1 vs Sn2 reactions

 

The type of solvent used plays a factor in whether the mechanism will be SN1 vs SN2. Polar protic solvents (solvents with an –OH) typically favor an SN1 mechanism, while polar aprotic solvents typically favor an SN2 mechanism. We’ll go over each in more detail below.

Polar protic solvents, solvents with an –OH, favor SN1 reactions. They are called polar protic because they are polar (due to the presence of an electronegative atom like oxygen on the molecule), and they are protic because they have an acidic proton that can be used to hydrogen bond with other molecules. They favor SN1 reactions because they are able to hydrogen bond with the carbocation that is formed after step 1 of the mechanism, which stabilizes the positive charge.

 

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Polar aprotic solvents do not contain any –OH groups, so it cannot undergo hydrogen bonding. Because of this, it is not a favorable solvent for a charged species, so polar aprotic solvents favor SN2 reactions.

Below is a list of common polar protic and aprotic solvents:

 

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