The Regiochemical Outcome of an E2 Reaction

Recall from Chapter 8 that the term regiochemistry refers to where the reaction takes place. In other words, in what region of the molecule is the reaction taking place When H and X are eliminated (where X is some leaving group), it is sometimes possible for more than one alkene to form. Consider the following example, in which two possible alkenes can be formed  

Where does the double bond form This is a question of regiochemistry. The way we distinguish between these two possibilities is by considering how many groups are attached to each double bond. Double bonds can have anywhere from 1 to 4 groups attached to them  

if we look back at the reaction above, we find that the two possible products are trisubstituted and disubstituted  

For an elimination reaction where there is more than one possible aUcene that can be formed, we have names for the different products based on which alkene is more substituted and which is less substituted. The more substituted alkene is called the Zaitsev product, and the less substituted alkene is called the Hofmann product. Usually, the Zaitsev product is the major product  

However, there are many exceptions in which the Zaitsev product (the more-substituted alkene) is not the major product. For example, if the reaction above is performed with a stericaUy hindered base (rather than using ethoxide as the base), then the major product will be the less-substituted alkene  

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In this case, the Hofmann product is the major product, because a sterically hindered base was used. This case illustrates an important concept The regiochemical outcome of an E2 reaction can often be controlled by carefully choosing the base. Below are two examples of sterically hindered bases that will be encountered frequently throughout your organic chemistry course ... 

In Chapter 7, we explored a method that would allow an E2 process in this example, which maintains control over the regiochemical outcome. The OH group can first be converted into a tosylate, which is a much better leaving group than OH (for a review of tosylates, see Section 7.8). After the OH is converted into a tosylate, the strategy outlined above can be followed. Specifically, a strong, sterically hindered base is used for the elimination reaction, followed by tfwft -Markovnikov addition of H—OH ... 

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