El reactions can be regioselective

We can use the same ideas when we think about El eliminations that can give more than one regio-isomeric alkene. Here is an example. 

Jhe major product is the alkene that has the more substituents, because this alkene is the more stable of the two possible products. 

But why should it be true The reason for this is related to the reason why more substituted carbocations are more stable. In Chapter 17 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 n system of the double bond—it is stabilized when the empty jr antibonding orbital can interact with the filled orbitals 

The more substituted alkene is more stable, but this does not necessarily explain why it is the one that forms faster. To do that, we should look at the transition states leading to the two alkenes. Both form from the same carbocation, but which one we get depends on which proton is lost. Removal of the proton on the right (brown arrow) leads to a transition state in which there is a monosubstituted double bond partly formed. Removal of the proton on the left (orange arrow) leads 

explanation of both stereo-and regioselectivity in El reactions is based on kinetic arguments—which alkene forms faster. But it is also true that some El eliminations are reversible the aikenes may be protonated in acid to re-form carbocations, as you will see in the next chapter. This reprotonation allows the more stable product to form preferentially under thermodynam/ccontrol. In any individual case, it may not be clear which is operating. However, with E2 reactions, which follow, only kinetic control applies E2 reactions are never reversible. 

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Although El reactions show some stereo- and regioselectivity, the level of selectivity in E2 reactions can be much higher because of the more stringent demands on the transition state for E2 elimination. We will come back to the most useful ways of controlling the geometry of double bonds in Chapter 31. 

Although El reactions show some stereo and regioselectivity, the level of selectivity in E2 reactions can be much higher because of the more stringent demands on the transition state for E2 elimination. In an E2 elimination, the new n bond is formed by overlap of the C—H o bond with the C—X a antibonding orbital. The two orbitals have to lie in the same plane for best overlap, and now there are two conformations that allow this. One has H and X syn-periplanar, the other anti-periplanar. The anti-periplanar conformation is more stable because it is staggered (the syn-periplanar conformation is eclipsed) but, more importantly, only in the anti-periplanar conformation are the bonds (and therefore the orbitals) truly parallel. 

An El reaction is regioselective. The major product is the most stable alkene, which is generally the most substituted alkene. An El reaction is stereoselective. The major product is the alkene with the bulkiest groups on opposite sides of the double bond. The carbocation formed in the first step can undergo both syn and anti elimination therefore, the two groups to be eliminated in a cyclic compound do not have to be trans or both in axial positions. Alkyl substitution increases the stability of a carbocation and decreases the stability of a carbanion. 

Both El and E2 reactions are regioselective, favoring formation of the more stable (Zaitsev) product alkene (as long as Lv and H can be oriented anti and coplanar). 

When more than one alkene can be formed, the El reaction, like the E2 reaction, is regioselective. The major product is the more stable alkene. 

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