Leaving groups in E2 reactions

Effects that arise because one spatial arrangement of electrons (or orbitals or bonds) IS more stable than another are called stereoelectronic effects There is a stereoelec tromc preference for the anti coplanar arrangement of proton and leaving group in E2 reactions Although coplanarity of the p orbitals is the best geometry for the E2 process modest deviations from this ideal can be tolerated In such cases the terms used are syn periplanar and anti periplanar... 

The NR3 group in a quaternary amine, R —NRj, is a good leaving group in E2 reactions this enables the Hofmann elimination to take place. 

Effect on Reactivity. The leaving groups in elimination reactions are similar to those in nucleophilic substitution. The E2 eliminations have been performed with the following groups NRj, PR, SRj, OHR", SO2R, OSO2R,... 

The leaving groups commonly employed in E2 reactions are listed in Table 2.1. As you can see, they are essentially the same as those displaced in nucleophilic substitution reactions (see Part 2), with two exceptions. First, protonated alcohols are not listed as substrates RX in Table 2.1, because they usually react by the El mechanism (as we shall see later) rather than the E2 mechanism. Secondly, the trimethylammonium and dimethylsulfonium groups have limited importance as leaving groups in substitution reactions, although they are particularly important in elimination reactions. In fact, the reaction involving trimethylammonium is known, after its discoverer, as the Hofmann elimination... 

This last step is one of the rare examples in which the leaving group is OH. Generally, hydroxide is a poor leaving group in substitution (SN1 or SN2) or elimination (El or E2) reactions (see Section 8-7C). 

The leaving group The relative reactivity of a leaving group in an E2 elimination depends on where, in the spectrum of transition states, the transition state of the particular reaction lies. If the reaction is very E2C-like, the reactivities... 

Elimination reactions often compete with substitution. They involve elimination of the halogen and a hydrogen from adjacent carbons to form an alkene. Like substitution, they occur by two main mechanisms. The E2 mechanism is a one-step process. The nucleophile acts as a base to remove the adjacent proton. The preferred form of the transition state is planar, with the hydrogen and the leaving group in an anti conformation. The E1 mechanism has the same first step as the SN1 mechanism. The resulting carbocation then loses a proton from a carbon atom adjacent to the positive carbon to form the alkene. 

In principle, the bases Y are also nucleophiles, and, hence, they can react with the same alkyl halides and sulfonates via the SN2 mechanism. The point of reaction is the C atom that bears the leaving group. In order to carry out E2 eliminations chemoselectively, competing Sn2 reactions must be excluded. To understand the outcome of the competition (E2 elimination vs. Sn2 reaction), it is analyzed kinetically with Equations 4.1-4.3. 

Of these possible conformations, the anti-coplanar arrangement is most commonly seen in E2 reactions. The transition state for the anti-coplanar arrangement is a staggered conformation, with the base far away from the leaving group. In most cases, this transition state is lower in energy than that for the syn-coplanar elimination. 

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