Positional Orientation of Elimination Zaitsevs Rule

Many compounds can eliminate in more than one way, to give mixtures of products. In many cases, we can predict which elimination product will predominate. In the example shown in Mechanism Box 6-9, the carbocation can lose a proton on either of two adjacent carbon atoms. 

The first product has a trisubstituted double bond, with three substituents (circled) on the doubly bonded carbons. It has the general formula R2C=CHR. The second product has a disubstituted double bond, with general formula R2C=CH2 (or R—CH = CH — R ). In most El and E2 eliminations where there are two or more possible elimination products, the product with the most substituted double bond will predominate. This general principle is called Zaitsev s rule, and reactions that give the most substituted alkene are said to follow Zaitsev orientation. 

ZAITSEV S RULE In elimination reactions, the most substituted alkene usually predominates. 

This order of preference is the same as the order of stability of alkenes. We consider the stability of alkenes in more detail in Section 7-7, but for now, it is enough just to know that more substituted alkenes are more stable. In Chapter 7, we will study some unusual reactions where Zaitsev s rule does not apply. 

When 1-bromo-l-methylcyclohexane is heated in ethanol for an extended period of dme, three products result one ether and two alkenes. Predict the products of this reaction, and propose a mechanism for their formation. Predict which of the two alkenes is the major elimination product. 

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