Carbocation classification

Carbocations are central to hydrocarbon chemistry (/). Much of this chemistry is based on acid catalysis, which leads to generation of positive ions of carbon. The resulting intermediates are classified as carbenium and carbonium ions, as proposed by Olah (2-4). Carbonium ions are the penta- or higher coordinate carbocations that maintain 8 valence electrons via 2-electron/3-center bonding, quite different from carbenium ions that possess only 6 valence electrons. Figure 1 shows a systematic classification of carbocations. 

Figure 1. Classification of carbocations into carbonium and carbenium ions,...

The l,2-dimethylbicyclo[2.1.1]hexyl cation (perturbation CD3 vj CH3) has been studied by Schmitz and Sorensen (1980). They prefer a C(2)—C(6) partial bond description with charge delocalization to C(l), and suggest that some carbocations preclude a ready classification into classical or non-classical categories and that one should rather think in terms of a whole range of intermediate bonding situations. 

The analysis of the effect that the different substituents may have on the rate coefficients in this case is particularly hard to perform by using simple inductive effects, as described for instance by the Hammett substituent constants. Note that, despite the fact that most of the 28 carbenium ions exhibit para substitution, the general structure on top of Chart 6, shows a complex substitution pattern at the carbocation centre. In order to asses the effect of multiple substitution at this site, we first considered those compounds that have two fixed hydrogen atoms at the -position of the phenyl group, which according to Hammett classification have crp(H) = 0.0, and the third phenyl group substituted at -position with H (compound 96 in Chart 6) OCH3 (compound 97 in... 

TABLE 3.1. closo, nido, and arachno Classification of Carboranes, Carbocations, and Carbanions Containing Hypcrcarbon Atoms... 

Proposed mechanisms for C-C bond formation can be organized into the following classifications carbene [70,71], carbocation [72], oxonium ylide [73,74], and free radical [75]. Some of these mechanisms [72-74] invoke a framework-bound methoxy species as a methylating agent or intermediate in the reaction. Dybowski and coworkers reported a C NMR study that supported the formation of methoxy groups in HZSM-5 [76], but the existence of these species is still controversial [77]. Framework-bound alkoxy species clearly do form on other molecular sieve catalysts. For example, Anderson and Klinowski have shown that when methanol is heated to 573 K on the silicoaluminophosphate catalyst SAPO-5, a framework-bound methoxy species forms, which is readily seen in a C MAS spectrum obtained after heating [77]. 

In 1972 Olah proposed a classification for carbocations dividing them into trivalent ( classical ) carbenium ions and penta- and tetracoordinate ( non-classical ) carbonium ions. 

There are many other kinds of reactive intermediates, which do not fit into the previous classifications. Some are simply compounds that are unstable for various possible reasons, such as structural strain or an unusual oxidation state, and are discussed in Chapter 7. This book is concerned with the chemistry of carbocations, carbanions, radicals, carbenes, nitrenes (the nitrogen analogs of carbenes), and miscellaneous intermediates such as arynes, ortho-quinone methides, zwitterions and dipoles, anti-aromatic systems, and tetrahedral intermediates. This is not the place to describe in detail the experimental basis on which the involvement of reactive intermediates in specific reactions has been estabhshed but it is appropriate to mention briefly the sort of evidence that has been found useful in this respect. Transition states have no real hfetime, and there are no physical techniques by which they can be directly characterized. Probably one of the most direct ways in which reactive intermediates can be inferred in a particular reaction is by a kinetic study. Trapping the intermediate with an appropriate reagent can also be very valuable, particularly if it can be shown that the same products are produced in the same ratios when the same postulated intermediate is formed from different precursors. 

Methyl alcohol, which is not strictly covered by this classification, is usually grouped with the primary alcohols. This classification is similar to that for carbocations (Sec. 3.10). We will see that the chemistry of an alcohol sometimes depends on its class. 

The classification of allylic cations as 1°, 2°, and 3° is determined by the location of the positive charge in the more important contributing structure. Following are examples of 2° and 3° allylic carbocations. 

The most recent definition of lipids was provided by a group of lipid chemists who formed the consortium of lipid metabolites and pathways strategy (Lipid MAPS). They defined lipids based on the origin of the lipid structures as hydrophobic or amphipathic small molecules that may originate entirely or, in part, by carbanion-based condensations of thioesters (fatty acids, polyketides, etc.) and/or by carbocation-based condensations of isoprene units (prenols, sterols, etc.). In this book, this definition, its classification (see the following), and its recommended nomenclature are largely accepted. 

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