Regioselectivity of E2 elimination

The regioselectivity of elimination is accommodated m the E2 mechanism by noting that a partial double bond develops at the transition state Because alkyl groups... 

Dehydration of alcohols (Sections 5 9-5 13) Dehydra tion requires an acid catalyst the order of reactivity of alcohols IS tertiary > secondary > primary Elimi nation is regioselective and proceeds in the direction that produces the most highly substituted double bond When stereoisomeric alkenes are possible the more stable one is formed in greater amounts An El (elimination unimolecular) mechanism via a carbo cation intermediate is followed with secondary and tertiary alcohols Primary alcohols react by an E2 (elimination bimolecular) mechanism Sometimes elimination is accompanied by rearrangement... 

Quaternary ammonium hydroxides un dergo elimination on being heated It is an anti elimination of the E2 type The regioselectivity of the Hofmann elimina tion IS opposite to that of the Zaitsev rule and leads to the less highly substi tuted alkene... 

When tertiary halides are treated with base, they undergo E2 elimination. The regioselectivity of elimination of tertiary halides follows the Zaitsev rule. 

Fig. 4.21. Steric base effects on the Saytzeff/Hofmann selectivity of an E2 elimination. The small base EtO can attack the H atoms in both positions / to the leaving group, i.e., it does not matter whether the H atom is bound to a primary or secondary C atom. The regioselectivity therefore results only from product development control the thermodynamically more stable.

Because of the same necessity for anf/-selectivity, trans-1,2-dibromocyclohexane does not react to form 1-bromocyclohexene in the KOfBu-promoted HBr elimination (Figure 4.23). Instead 3-bromocyclohexene is produced through a fairly regioselective E2 elimination. In a subsequent fast 1,2-elimination, it loses a second equivalent of HBr. In this way 1,3-cyclohexadiene is produced. 

Although El reactions show some stereo- and regioselectivity, the level of selectivity in E2 reactions can be much higher because of the more stringent demands on the transition state for E2 elimination. We will come back to the most useful ways of controlling the geometry of double bonds in Chapter 31. 

This regioselectivity distinguishes a Hofmann elimination from other E2 eliminations, which form the more substituted double bond by the Zaitsev rule (Section 8.5). This result is sometimes explained by the size of the leaving group, N(CH3)3. In a Hofmann elimination, the base removes a proton from the less substituted, more accessible carbon atom, because the bulky leaving group on the nearby a carbon. 

Further exploration of the regioselectivity of alkene formation in elimination reactions (ElcB anc E2). 

The regioselectivity of elimination is accommodated in the E2 mechanism by noting that a partial double bond develops at the transition state. Since alkyl groups stabilize double bonds, they also stabilize a partially formed tt bond in the transition state. The more stable aUcene therefore requires a lower energy of activation for its formation and predominates in the product mixture because it is formed faster than a less stable one. 

What is the regioselectivity of an E2 reaction In other words, what are the factors that dictate which of the two elimination products will be formed in greater yield ... 

In Section 11.1, we saw that an E2 reaction is regioselective, which means that more of one constitutional isomer is formed than the other. For example, the major product formed from the E2 elimination of 2-bromopentane is 2-pentene. 

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