Elimination Reactions Create Pi Bonds

The El elimination has the same rate determining first step, Dn, as the SnI substitution and is subject to the exact same limitations a carbocation stability usually 

The following is a ranking of alkene stability. The most substituted alkene is the most stable. The cis-disubstituted double bond is more sterically hindered and less stable than the trans-disubstituted double bond. The gew-disubstituted double bond is about the same stability as the ds-disubstituted double bond. The monosubstituted 1-hexene is the least stable of the hexenes listed. 

Any H adjacent to the carbocation can be lost, often producing a different alkene. This loss is often reversible, so the El often produces an equilibrium mixture of alkenes, the most stable alkene predominating (Zaitsev s rule). When a reaction produces a mixture of different constitutional isomers, we use the term regiochemistry to refer to product ratios in that mixture. When the El produces a 1,2-disubstituted alkene, the trans stereoisomer predominates because it is less hindered, so more stable. 

For the compound below, draw an El elimination mechanism and show both major and minor products. 

Answer The first step of the El is the loss of the leaving group by path Dn to give a carbocation, which is deprotonated by a base to form a pi bond by path Dg. Either the methyls or methylene adjacent to the carbocation can lose the H, but the more stable, more substituted product is major. 

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