1.2- Elimination reactions, characteristics reactivity order

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. 

For efficiency and reasons of economy THF is recommended. Representative dehalogenations in THF are shown in Table 1. Most alkyl halides are reduced at 25 C to alkanes, except for bicyclic halides, such as eA o-2-bromonorbomane. In most cases, the reaction exhibits the typical characteristics of an 5n2 substitution reaction in the reactivity order I > Br > Cl primary > secondary > tertiary. In all cases, no al-kene, or only traces of it, is formed as a side product as a result of elimination. This method provides a convenient synthetic procedure for hydrodehalogenation of alkyl halides. 

The discussion of elimination reactions considers the classical E2, El, and Elcb eliminations that involve removal of a hydrogen and a leaving group. We focus on the kinetic and stereochemical characteristics of elimination reactions as key indicators of the reaction mechanism and examine how substituents influence the mechanism and product composition of the reactions, paying particular attention to the nature of transition structures in order to discern how substituent effects influence reactivity. We also briefly consider reactions involving trisubstituted silyl or stannyl groups. Thermal and concerted eliminations are discussed elsewhere. 

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