Zaitsev elimination mechanisms

Besides E, elimination, in some cases, an Ei elimination mechanism can be followed, and the more stable olefin is formed. Instead of Hofmann s rule, Zaitsev s rule is obeyed (the double bond goes mainly toward the most highly substituted carbon). In some reactions the direction of elimination is determined by the need to minimize steric interactions, sometimes even when the steric hindrance appears only during the transition state. Also an E2 mechanism may be followed. It should be remembered that an E2 reaction requires a proton acceptor and occurs as follows ... 

We need to consider the three mechanisms in turn. In the El process, the formation of the product from the intermediate carbocation is fast. This means that the stability of the developing alkene is the only factor that we need to consider. The most stable alkene, generally the most substituted one, will predominate. This is sometimes referred to as Zaitsev elimination and is exemplified in Figure 10.15. Either proton could be lost from the intermediate carbocation, and the major product is the trisubstituted rather than the disubstituted alkene. 

However, the E2C mechanism has been criticized, and it has been contended that all the experimental results can be explained by the normal E2 mechanism. McLennan suggested that the transition state is that shown as 18. An ion-pair mechanism has also been proposed. Although the actual mechanisms involved may be a matter of controversy, there is no doubt that a class of elimination reactions exists that is characterized by second-order attack by weak bases. " These reactions also have the following general characteristics (1) they are favored by good leaving groups (2) they are favored by polar aprotic solvents (3) the reactivity order is tertiary > secondary > primary, the opposite of the normal E2 order (p. 1319) (4) the elimination is always anti (syn elimination is not found), but in cyclohexyl systems, a diequatorial anti elimination is about as favorable as a diaxial anti elimination (unlike the normal E2 reaction, p. 1302) (5) they follow Zaitsev s rule (see below), where this does not conflict with the requirement for anti elimination. 

Here is where we get back to mechanisms. Whether we are talking about Zaitsev vs. Hoffman elimination reactions or about Markovnikov vs. anti-Markovnikov addition reactions, the explanation of the regiochemistry for every reaction is contained within the mechanism. If we completely understand the mechanism, then we will understand why the regiochemistry had to be the way it turned out. By understanding the mechanism, we eliminate the need to memorize the regiochemistry for every reaction. With every reaction you encounter, you should consider the regiochemistry of the reaction and look at the mechanism for an explanation of the regiochemistry. 

First-Order Elimination The E1 Reaction 258 Key Mechanism 6-8 The E1 Reaction 258 Mechanism 6-9 Rearrangement in an E1 Reaction 261 Summary Carbocation Reactions 262 6-18 Positional Orientation of Elimination Zaitsev s Rule 263 6-19 Second-Order Elimination The E2 Reaction 265 Key Mechanism 6-10 The E2 Reaction 266 6-20 Stereochemistry of the E2 Reaction 267... 

Characteristic features of this mechanism are that (i) the rate of the reaction does not depend on the concentration of the base and the kinetics are first order (in substrate) (ii) the reaction may not be stereospecific (iii) the elimination/substitution ratio is mostly independent of the leaving group (but in solvents of low ionization energy ion pairs are formed and then the ratio depends upon the leaving group) (iv) by-products are formed via rearrangements (v) the reaction is reversible (vi) generally the most stable alkene is formed (Zaitsev orientation see Section 5.1.2.5)... 

In this reaction two different procedures have been used. The first is the classical Hofmann degradation, which prepares the alkene by thermal decomposition of the quaternary ammonium hydroxide. Hofmann orientation is generally observed in acyclic and Zaitsev orientation in cyclohexyl substrates. The second is the treatment of quaternary ammonium halides with very strong bases, e.g. PhLi, KNH2 in liquid NH3. The formation of the alkene proceeds via an 1 mechanism, which means a syn elimination in contrast to the anti elimination which is observed in most of the classical Hofmann degradations. In some cases this type of elimination can also be accomplished by heating the salt with KOH in polyethylene glycol monomethyl ether. 

The reactant in this problem is the same as in Problem 9.38a. However, the reaction conditions are different. In Problem 9.38a, the reaction was conducted with the strong base, ethoxide anion, so the mechanism was E2 and anti elimination was preferred. In this problem, there is no strong base present, so the mechanism is S I/EI. First the Cl leaves to form a carbocation. Because the carbocation is planar, stereochemistry is lost and anti elimination is not required. So both products are formed. Product 2 should be the major elimination product because E1 reactions follow Zaitsev s rule. 

Draw structural formulas for the alkene(s) formed by treating each of the following haloalkanes with sodium ethoxide in ethanol. Assume that elimination is by an E2 mechanism. Where two alkenes are possible, use Zaitsev s rule to predict which alkene is the major product (See Examples 7.6, 7.7)... 

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