Evidence for the El mechanism

What is the evidence for the El mechanism The elimination reactions that... 

Although the formation of the Zaitsev products is typical in El processes, it cannot be taken as evidence for the carbocationic mechanism Rearrangement of the intermediate carbocation and consecutive alkene isomerization (double bond migration) induced by the acidic catalyst may occur. These processes strongly affect product distribution and eventually give rise to the formation of a very complex mixture of isomeric alkenes. 

Much evidence has been obtained in support of the El mechanism. For example, El reactions show first-order kinetics, consistent with a rate-limiting spontaneous dissociation process, l- urthermore, El reactions show- no deuterium isotope effect because rupture of the C—H (or C—D) bond occurs after the rate-limiting step rather than during it. Thus, we can t measure a rate difference between a deuterated and nondeuterated substrate. 

We have told you what sorts of starting materials and conditions favour El or E2 reactions, but we haven t told you how we know this. El and E2 differ in the order of their rate equations with respect to th.e base, so one way of finding out if a reaction is El or E 2 is to plot a graph of the variation of rate with base concentration. But this can be difficult with El reactions because the base (which need be only very weak) is usually the solvent. More detailed evidence for the differences between reaction mechanisms comes from studying the rates of elimination in substrates that differ only in that one or more of the protons have been replaced by deuterium atoms. These differences are known as kinetic isotope effects. 

Saldana SN, Barker EL (2004) Temperature and 3,4-methylenedioxymethamphetamine alter human serotonin transporter-mediated dopamine uptake. Neurosci Lett 354 209-212 Samuvel DJ, Jayanthi LD, Bhat NR, Ramamoorthy S (2005) A role for p38 mitogen-activated protein kinase in the regulation of the serotonin transporter evidence for distinct cellular mechanisms involved in transporter surface expression. J Neurosci 25 29-41 Schmitz Y, Schmauss C, Sulzer D (2002) Altered dopamine release and uptake kinetics in mice lacking D2 receptors. J Neurosci 22 8002-8009... 

Much evidence has been obtained in support of the El mechanism. For example, El reactions show first-order kinetics, consistent with a rate-Umiting spontaneous dissociation process ... 

Alcohol (a) can be used to indicate the extent of syn- and anti-elimination (see Scheme 8) and (b), through a study of H/ H exchange can indicate whether the mechanism is El, E2, or ElcB. Thus for an El mechanism, deuterium exchange can occur in all positions in the butenes and loss of a- H from [2- Hi ]butan-2-ol provides strong evidence of this mechanism. 

Figure 1 also includes previously reported values for H and E for comparison with the mechanical property maps of Cuy et al. [12]. It is clear that nanoindentation is able to sense a much wider variation in these properties than previously observed. This is at least partially because the past investigations of mechanical properties yielded mainly average values of H and E that correspond to the values for the interior enamel. Many of these previous studies did not show the extreme local variations that can be measured with nanoindentation. Only the earlier nanoindentation studies have shown any evidence for the highest E and El found by Cuy et al. [12] using nanoindentation. Willems et al. [3] reported E = 90.59 16.13 GPa and Mahoney et al. [21] reported H = 4.88 0.35 GPa. 

Mechanism of Nonoxidative Thermal Dehydrochlorination. This subject is still very controversial, with various workers being in favor of radical, ionic, or molecular (concerted) paths. Recent evidence for a radical mechanism has been provided by studies of decomposition energetics (52), the degradation behavior of PVC-polystyrene (53) or PVC-polypropylene (54) mixtures, and the effects of radical traps (54). Evidence for an ionic mechanism comes from solvent effects (55) and studies of the solution decomposition behavior of a model allylic chloride (56). Theoretical considerations (57,58) also suggest that an ionic (El) path is not unreasonable. Other model compound decompositions have been interpreted in terms of a concerted process (59), but differences in solvent effects led the authors to conclude that PVC degrades via a different route (59). 

Evidence for the stepwise El mechanism includes the observation of a first-order rate equation. 

---