Concerted E2-type mechanism

The mechanism by which proton acids catalyze the dehydration of primary and secondary alcohols in water is not perfectly well understood (1). There is universal agreement that the dehydration of tertiary alcohols can be explained by an El mechanism (1,2) involving either a II complex ( ) or a symmetrically solvated carbonium ion (4) as the key reaction intermediate. Although an occasional text ( ) also describes the dehydration of primary alcohols by an El mechanism, authoritative reviews (1/4) conclude that a concerted E2 type mechanism is more probable. The dehydration behavior of secondary alcohols is presumed to be similar to primary alcohols (4). Discussions of the gas phase dehydration of alcohols by heterogeneous Lewis acid catalysts admit more possibilities. In their authoritative review Kut, et al. (1) consider E1-, E2-, and ElcB-like mechanisms, as well as the possible role of diethyl ether as a reaction intermediate, but they reach no conclusion concerning the relative importance of these mechanisms in the formation of olefins from alcohols. 

However, recent studies suggested that in non-aqueous organic media, the reaction appears to follow the concerted E2-type mechanism with some ElcB character, but probably not involving sulfonylamines. The relative order of nucleophilicity towards the sulfonyl sulfur atom is similar to that obtained at a carbonyl carbon atom and is shown in Figure 1. 

The carbanion can be destroyed in two ways, k2 or k, and two limiting types of behavior are expected for ElcB mechanisms. If k2 3> fe-i[BH], then the carbanion always decomposes to the alkene product and the rate law simplifies to feobs = fei. In other words, the rate is only dependent on carbanion formation and the rate law has a form that is identical to what would be obtained in a concerted E2 reaction. This has been referred to as an EIcBirr mechanism, where IRR indicates that deprotonation is irreversible. On the other hand, if k2 fc i[BH], then carbanion formation rarely leads to the product and the rate law simplifies to the following ... 

The oxidant may aid the elimination in a concerted or E2 type of mechanism, as illustrated in Eq. (7) for such examples, the oxidant is not bonded to the substrate, except possibly in the transition state. Other oxidants, for example chromic acid, have been shown to form intermediate esters such as 1 (although other mechanisms have been proposed7), which subsequently decompose by a related, bimolecular elimination [Eq. (2)] here the leaving group is the reduced form of the oxidant, and the C-H bond must necessarily break with the liberation of a proton. As in Eq. (7), the capture of electrons by the oxidant is the driving force of the reaction, so that the breaking of the C-H bond occurs simultaneously in the rate-determining step (Scheme 1). 

Siddan and Narayan also employed 7-AI2O3 and Th02 for the dehydration of a number of model alcohols and observed that if the basicity of the alumina was increased by Na -ion doping, 7-elimination was enhanced using both neopentyl and pinacolyl alcohol. It appeared that as the alumina became less acidic and more basic, there was a shift from El/E2-like behaviour to an ElcB-type mechanism, which manifested itself in a concerted 7-elimination (Scheme 10 for neopentyl alcohol A,B = acid, base sites respectively reproduced by permission from J. Catal, 1979, 59, 405). This tendency was also observed by use of erythrof f/ireo)-3-methylpentan-2-ol. 

Two basic mechanisms can account, in general, for the interpretation of /3-elimination reactions. The E2 type pathway is a one-step process occurring in a concerted fashion, whereas in a two-step El mechanism a carbocationic intermediate is involved. A third mechanistic possibility, the ElcB elimination, may occur when a molecule possesses a poor leaving group and a highly acidic... 

While frarcs-periplanar relationships are important to E2 elimination reactions, it is important to remember that, as illustrated in Schemes 6.6 and 6.7, E2 elimination reaction mechanisms do not have to occur in a concerted manner. After deprotonation, if the relevant orbitals do not line up, elimination will not occur until they do. Furthermore, recall that rotation around an acyclic single bond, as illustrated in Figures 6.4 and 6.5, occurs readily. Therefore, elimination reactions should not be removed from consideration if a molecule is drawn in a conformation that makes these reactions appear unfavorable. When looking at any type of nucleophilic reaction, initial identification of relevant trans-periplanar relationships will aid in the identification of potential side products and their respective mechanisms of formation. 

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