E1 reactions can be stereoselective

For some eliminations only one product is possible. For others, there may be a choice of two (or more) alkene products that differ either in the location or stereochemistry of the double bond. We shall now move on to discuss the factors that control the stereochemistry (geometry—CIS or trans) and regiochemistry (that is, where the double bond is) of the alkenes, starting with El reactions. 

E alkenes (and transition states leading to E alkenes) are usually lower in energy than Z alkenes (and the transition states leading to them) for steric reasons the substituents can get further apart from one another. A reaction that can choose which it forms is therefore likely to favour the formation of E alkenes. For alkenes formed by El elimination, this is exactly what happens the less hindered E alkene is favoured. Here is an example. 

The geometry of the product is determined at the moment that the proton is lost from the intermediate carbocation. The new n bond can only form if the vacant p orbital of the carbo-cation and the breaking C—H bond are aligned parallel. In the example shown there are two possible conformations of the carbocation with parallel orientations, but one is more stable than the other because it suffers less steric hindrance. The same is true of the transition states on the route to the alkenes—the one leading to the E alkene is lower in energy, and more E alkene than Z alkene is formed. The process is stereoselective because the reaction chooses to form predominantly one of two possible stereoisomeric products. 

We can use the same ideas when we think about El eliminations that can give more than one regioisomeric alkene. Here is an example. The major product is the alkene that has the more substituents because this alkene is the more stable of the two possible products. 

This is quite a general principle. But why should it be true The reason for this is related to the reason why more substituted carbocations are more stable. In Chapter 15 we said that the carbocation is stabilized when its empty p orbital can interact with the filled orbitals of parallel C—H and C—C bonds. The same is true of the k system of the double bond—it is stabilized when the empty k antibonding orbital can interact with the filled orbitals of parallel C—H and C—C bonds. The more C—C or C—H bonds there are, the more stable the alkene. 

---