Shield-forming mechanism

Partial but not complete loss of optical activity m S l reactions probably results from the carbocation not being completely free when it is attacked by the nucleophile Ionization of the alkyl halide gives a carbocation-hahde ion pair as depicted m Figure 8 8 The halide ion shields one side of the carbocation and the nucleophile captures the carbocation faster from the opposite side More product of inverted configuration is formed than product of retained configuration In spite of the observation that the products of S l reactions are only partially racemic the fact that these reactions are not stereospecific is more consistent with a carbocation intermediate than a concerted bimolecular mechanism... 

Severe concentration cell corrosion involves segregation of aggressive anions beneath deposits. Concentrations of sulfate and chloride, in particular, are deleterious. Acid conditions may be established beneath deposits as aggressive anions segregate to these shielded regions. Mineral acids, such as hydrochloric and sulfuric, form by hydrolysis. The mechanism of acid formation is discussed in Chap. 2. 

A model for the mechanism of the highly enantioselective AlMe-BINOL-cata-lyzed 1,3-dipolar cycloaddition reaction was proposed as illustrated in Scheme 6.13. In the first step nitrone la coordinates to the catalyst 11b to form intermediate 12. In intermediate 13, which is proposed to account for the absolute stereoselectivity of this reaction, it is apparent that one of the faces of the nitrone, the si face, is shielded by the ligand whereas the re face remains available... 

All the various forms of concentration cell corrosion described are types of indirect attack induced by the effects of surface shielding, although not all concentration cell corrosion mechanisms involve the presence of oxygen. 

In the general case, when the shielding tensor is not of axial symmetry, in place of its diagonal elements axx, ayy and kyyl > axx i the following parameters are generally used asymmetry parameter V = <3xx — ayy)/Gzz = (3/2)(cTj.j. — oyy)/h.o. The second form of rj arises from the fact that, as far as relaxation is concerned, the shielding tensor can be defined with respect to any time-independent reference (which therefore will not act as a relaxation mechanism). [Pg.96]

The flame retardant mechanism for phosphorus compounds varies with the phosphorus compound, the polymer and the combustion conditions (5). For example, some phosphorus compounds decompose to phosphoric acids and polyphosphates. A viscous surface glass forms and shields the polymer from the flame. If the phosphoric acid reacts with the polymer, e.g., to form a phosphate ester with subsequent decomposition, a dense surface char may form. These coatings serve as a physical barrier to heat transfer from the flame to the polymer and to diffusion of gases in other words, fuel (the polymer) is isolated from heat and oxygen. 

In order for an E2 mechanism to take place a base must approach the proton marked. In C this proton is shielded on both sides by R and R. In D the shielding is on only one side. Therefore, when anti elimination does take place in such systems, it should give more cis product than trans. Also, when the normal anti elimination pathway is hindered sufficiently to allow the syn pathway to compete, the anti — trans route should be diminished more than the anti — cis route. When syn elimination begins to appear, it seems clear that E, which is less eclipsed than F, should be the favored pathway and syn elimination should generally give the trans isomer. In general, deviations from the syn-anti dichotomy are greater on the trans side than on the cis. Thus, trans olefins are formed partly or mainly by syn elimination, but cis olefins are formed entirely by anti elimination. Predominant syn... 

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