Substitution Reactions Teach Us Some Important Lessons

7 SUBSTITUTION REACTIONS TEACH US SOME IMPORTANT LESSONS 

S] -l and Sn2 reactions produce almost the same products. In both reactions, a leaving group is replaced by a nucleophile. In fact, the only difference in products between SnI and Sn2 reactions arises when you have a stereocenter where the leaving group is. In this situation, the Sn2 mechanism will invert the stereocenter, while the S. l mechanism will produce a racemic mixture. That s the only difference—the configuration of one stereocenter. And if there is no stereocenter, then there is no difference in outcome at all between S l or Sn2. It seems like a lot of work to go through to determine the configuration of one stereocenter (which matters only some of the time). 

So the obvious question is, why did we go through all of that trouble to learn how to determine whether a reaction is SnI or SN2 There are many answers to this 

First we learned the important concept that everything is located in the mechanisms. By understanding the mechanisms completely, everything else can be justified based on the mechanisms. All of the factors that influence the reaction can be understood by carefully examining the mechanism. This is true for every reaction you will see from now on. Now you have had some practice thinking this way. 

Finally we learned that if we analyze the first factor (substrate), we will find two effects at play electronics and sterics. We saw that S 2 reactions require primary or secondary substrates because of sterics—it is too crowded for the nucleophile to attack a tertiary substrate. On the other hand, SnI reactions did not have a problem with sterics, but electronics w as a bigger issue. Tertiary was the best, because the alkyl groups were needed to stabilize the carbocation. 

Don t get me wrong— it is very important to be able to predict whether a stereocenter gets inverted or not when a substitution reaction takes place. That alone would have been enough of a reason to learn all of the factors in this chapter. But I also want you to keep your eye on some of the bigger picture issues. They will help you as you move through the course. 

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