Substrate structure may allow

In El mechanisms, once the leaving group has departed almost anything will serve as a base to remove a proton from the intermediate carbocation. Weakly basic solvent molecules (water or alcohols), for example, are quite sufficient, and you will often see the proton just falling off in reaction mechanisms with the assumption that there is a weak base somewhere to capture it. We showed the loss of a proton like this in the last example, and in the chart on this page. 

In very rare cases, such as the superacid solutions we described in Chapter 15 (p. 335), the cation is stable because counterions such as BFj and SbFg are not only non-nucleophilic but also so non-basic that they won t even accept a proton. This fact tells us that despite this common way of writing the E1 mechanism, some sort of weak base is necessary even for El. 

Polar solvents also favour El reactions because they stabilize the intermediate carbocation. El eliminations from alcohols in aqueous or alcohol solution are particularly common and very useful. An acid catalyst is used to promote loss of water, and in dilute H2SO4, H3PO4, or HCl the absence of good nucleophiles ensures that substitution does not compete. With phosphoric acid, for example, the secondary alcohol cyclohexanol gives cyclohexene. 

But the best El eliminations of all are with tertiary alcohols. The alcohols can be made using the methods of Chapter 9 nucleophilic attack by an organometaUic on a carbonyl compound. Nucleophilic addition, followed by El elimination, is an excellent way of making this substituted cyclohexene, for example. Note that the proton required in the first step is recovered in the last— the reaction requires only catalytic amounts of acid. 

Cedrol is important in the perfumery industry—it has a cedar wood fragrance. Corey s synthesis includes this step—the acid (toluenesulfonic acid, see p. 227) catalyses both the El elimination and the hydrolysis of the acetal. 

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In such cases a local-equilibrium structure may be obtained theoretically by minimization of the free energy of the system under the constraint of a fixed alloy composition in the surface region [8,17-24]. Although this approach is very similar to the one used for bulk systems, it should be modified due to the specific features introduced by the surface. First of all, since the structure of the underlying bulk system is fixed, it acts as the source of an external field for the surface alloy, creating, for instance, epitaxial strain. Secondly, since the surface is an open system, it allows the formation of a great variety of different structures, which may not have any connection at all to the crystal structure of the substrate. Finally, the surface is a spatially inhomogeneous system, and thus different alloy components have their own... 

The elucidation of so many structures has allowed a successful classification of protein structures [7,8] it has laid the basis for certain predictive methods [9,10], and it has given insight into the possible evolutionary origins of proteins [11-13]. Our understanding of biological function in terms of structure has not increased so fast. This has turned out to be a more difficult problem. In the case of enzymes, a description of several different states of the protein complexed with substrate, intermediates and products together with necessary co-factors and activators is required. Often these different states can only be achieved by co-crystallisation, and even then it may be difficult to trap the necessary conformation. To date, it is... 

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