Elimination reactions Zaitsev product

Zaitsev s rule as applied to the acid catalyzed dehydration of alcohols is now more often expressed in a different way elimination reactions of alcohols yield the most highly substituted alkene as the major product Because as was discussed in Section 5 6 the most highly substituted alkene is also normally the most stable one Zaitsev s rule is sometimes expressed as a preference for predominant formation of the most stable alkene that could arise by elimination... 

Zaitsevs rule (Section 5 10) When two or more alkenes are capable of being formed by an elimination reaction the one with the more highly substituted double bond (the more sta ble alkene) is the major product Zwitterion (Section 27 3) The form in which neutral amino acids actually exist The ammo group is in its protonated form and the carboxyl group is present as a carboxylate... 

According to Zaitsev s rule, formulated in 1875 by the Russian chemist Alexander Zaitsev, base-induced elimination reactions generally (although not always) give the more stable alkene product—that is, the alkene with more alkyl substituents on the double-bond carbons. In the following two cases, for example, the more highly substituted alkene product predominates. 

A final piece of evidence involves the stereochemistry of elimination. (Jnlike the E2 reaction, where anti periplanar geometry is required, there is no geometric requirement on the El reaction because the halide and the hydrogen are lost in separate steps. We might therefore expect to obtain the more stable (Zaitsev s rule) product from El reaction, which is just what w e find. To return to a familiar example, menthyl chloride loses HC1 under El conditions in a polar solvent to give a mixture of alkenes in w hich the Zaitsev product, 3-menthene, predominates (Figure 11.22). 

Practically everything we ve said in previous chapters has been stated without any proof. We said in Section 6.8, for instance, that Markovnikov s rule is followed in alkene electrophilic addition reactions and that treatment of 1-butene with HC1 yields 2-chJorobutane rather than 1-chlorobutane. Similarly, we said in Section 11.7 that Zaitsev s rule is followed in elimination reactions and that treatment of 2-chlorobutane with NaOH yields 2-butene rather than 1-butene. But how do we know that these statements are correct The answer to these and many thousands of similar questions is that the structures of the reaction products have been determined experimentally. 

Zaitsev s rule (Section 11.7) A rule stating that E2 elimination reactions normally yield the more highly substituted alkene as major product. 

So if we look back at the reaction above, we find that the two possible prodncts are monosubstituted and disubstitnted donble bonds. Whenever we have an elimination reaction where more than one possible donble bond can be formed, we have names for the different products based on which one is more substitnted and which one is less substituted. The more substituted product is called the Zaitsev product, and the... 

Consider the elimination reaction below, which uses a strong base. The product will be a double bond. This reaction will produce two Zaitsev products. One will be cis and one will be trans. Draw these products, and identify which is cis and which is trans. 

Zaitsev s rule states that the major product in the formation of alkenes by elimination reactions will be the more highly substituted alkene, or the alkene with more substituents on the carbon atoms of the double bond. 

It is possible to predict which product will be formed in larger amounts in these reactions. Most E2 elimination reactions follow Zaitsev s rule ... 

Another problem that occurs with eliminations is the regiochemistry of the reaction. As we saw in Chapter 9, most eliminations follow Zaitsev s rule and produce the more highly substituted alkene as the major product. However, a significant amount of the less highly substituted product is also formed. In addition, mixtures of ds and trans isomers are produced when possible, further complicating the product mixture. Because separating a mixture of such isomers is usually a difficult task, elimination reactions are often not the best way to prepare alkenes. (Other methods will be described in subsequent chapters.) However, if only one product can be formed, or if one is expected to greatly predominate in the reaction mixture, then these elimination reactions can be quite useful. 

Zaitsev s rule (Section 9.4) The major product of an elimination reaction is the alkene with more alkyl groups on the carbons of the double bond (the more highly substituted product). Most El and E2 reactions follow this rule. 

Figure 25.11 contrasts the products formed by E2 elimination reactions using an alkyl halide and an amine as starting materials. Treatment of the alkyl halide (2-bromopentane) with base forms the more substituted alkene as the major product, following the Zaitsev rule. In contrast, the three-step Hofmann sequence of an amine (2-pentanamine) forms the less substituted alkene as major product. 

Zaitsev rule (Section 8.5) In a P elimination reaction, a rule that states that the major product is the alkene with the most substituted double bond. 

If a substrate molecule has more than one P-H atom, the elimination reaction may lead to more than one alkene. The orientation is called Zaitsev orientation if the resulting double bond has the iargest numbers of substituents possible this means that the proton is abstracted from the most highiy substituted carbon. If the elimination reaction runs the other way, the orientation is called Hofmann orientation. In general, the Zaitsev product is thermodynamically more stable than the Hofmann product, due to hyperconjugation (Scheme 16). 

But-2-ene will be formed in preference to but-l-ene, as the secondary carbonium ion is significantly more stable than the primary carbonium ion, i.e. the former is the thermodynamic product. Thus, in El reactions, Zaitsev elimination is preferred. However, it does not apply if the predicted product would suffer steric strain. 

When X=Br. The elimination reaction forms predominantly the most highly substituted alkene in a Saytzev (or Zaitsev) elimination. The H-C bond is broken at the same time as the C-Br bond, and the transition state resembles the alkene product. As a result, the more highly substituted alkene is formed faster, because this is the more stable alkene product (see Section 6.1). 

Notice in the equation shown above and in Example 13.7, the major product is the more highly substituted alkene. In 1875 the Russian chemist Alexander Zaitsev developed a rule to describe such reactions. Zaitsev s rule states that in an elimination reaction, the alkene with the greatest number of alkyl groups on the double bonded carbon (the more highly substituted alkene) is the major product of the reaction. 

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