Factor 3—The Leaving Group

In many cases we can determine from the substrate and the nucleophile whether we will get SnI or Sn2. If we have a 1° substrate and a strong nucleophile, then the reaction will go via an Sn2 mechanism, with inversion of configuration. If we have a 3° substrate and a weak nucleophile, then the reaction will go via an SnI mechanism, with racemization. What do we do if the substrate is 2 and the nucleophile is moderate We move on to factor 3—the leaving group. 

Let s go through all three categories once more. Excellent leaving groups are those that do not have a negative charge at all. Examples are 

Finally, there are bad leaving groups. These are nucleophiles with a negative charge that is not stabilized. The charge is not on a halogen, and there is no resonance that spreads the charge out. Examples include 

Now let s see what effect the leaving group has on the rates of SnI and Sn2. Once again, we turn to the mechanisms to understand the leaving group s effect on the rates of reaction. 

Both reactions are faster when the leaving group is better. The trends are the same, but SnI is more sensitive to this factor. 

ANSWER This compound has a sulfur atom with lone pairs. A lone pair on a sulfur atom will be strongly nucleophihc, even without a negative charge, because sulfur is large and highly polarizable. Strong nucleophiles favor Sn2 reactions. 

PROBLEMS Identify whether each of the following nucleophiles will favor Sn2 or SnI. 

We have already seen that the rate of this step is very sensitive to the stabihty of the carbocation, so it should make sense that it is also sensitive to the stability of the leaving group. The leaving group must be highly stabilized in order for an SnI process to be effective. 

The most commonly used leaving groups are halides and sulfonate ions  

EXERCISE 9.18 Identify the leaving group in the following compound  

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There are four factors that impact the competition between the n2 mechanism and Sfgl (1) the substrate, (2) the nucleophile, (3) the leaving group, and (4) the solvent. 

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