Carbonium ions systems

As has already been mentioned, carbonium ions are usually written without indicating the presence of negative ions, even though such anions are essential and the environment often affects the type of product obtained. Also, it has been suggested that the acid-carbonium ion system can act as a reservoir for carbonium ions (Whitmore, 18) ... 

From the general principles of carbonium ion systems, a host of applications may be made to important reactions of the catalytic cracking system. Some of these follow ... 

Such a cleavage, known as the beta-scission, is based on the principles developed by F. C. Whitmore in his early work on the carbonium-ion systems. This beta rule is the rule of least rearrangement since it involves the shifting of electrons only ... 

The ease of the reaction suggests that it proceeds via a stable carbonium ion system such as that represented in (XlXb). 

For a symmetrical system, the deuterium atom has to be introduced in such a manner that the degeneracy is lifted as demonstrated for 2-phenylmalonaldehyde [45-47]. This could be deuteration at the aldehyde carbon. This is called isotopic perturbation of chemical equilibrium and was originally used to study carbonium ion systems [48]. 

Because many aralkyl olefins form fairly stable, colored carbonium ions which allow the study of olefin-carbinol-carbonium ion equilibria, they have been subjected to careful scrutiny (84,85,90,142,152). We have already (II-D,E) shown how the olefin-carbonium ion system may be used to derive the acidity function from (Hb-Oh,o)... 

Whereas the differentiation of trivalent carbenium and pentacoor-dinated carbonium ions serves a useful purpose in defining them as limiting cases, it should be clear that in carbocationic systems there always exist varying degrees of delocalization. This can involve participation by neighboring -donor atoms, 7r-donor gronps, or [Pg.148]

Lower alkanes such as methane and ethane have been polycondensed ia superacid solutions at 50°C, yielding higher Hquid alkanes (73). The proposed mechanism for the oligocondensation of methane requires the involvement of protonated alkanes (pentacoordinated carbonium ions) and oxidative removal of hydrogen by the superacid system. 

Studies of solvolysis of similar polyfluonnated polycyclic aromatic systems, such as 2,3-(tetrafluorobenzo)bicyclo[2 2 2]octadienes and related compounds, proved the ionic mechanism of this rearrangement [55, 36, 37] (equation 9) Possible nonclassical carbonium ion involvement has been discussed [5S, 39, 40, 41]... 

The third mode of termination which occurs in some carbonium ion polymerizations involves rearrangement of the active carbonium ion into an inactive one which cannot continue the propagation. These reactions can be avoided to a great extent by working at sufficiently low temperatures, and on the whole, they only contribute significantly to the termination reaction in a few systems. 

Dacarbazine is activated by photodecomposition (chemical breakdown caused by radiant energy) and by enzymatic N-demethylation. Formation of a methyl carbonium ion results in methylation of DNA and RNA and inhibition of nucleic acid and protein synthesis. Cells in all phases of the cell cycle are susceptible to dacarbazine. The drug is not appreciably protein bound, and it does not enter the central nervous system. 

Redox initiation is commonly employed in aqueous emulsion polymerization. Initiator efficiencies obtained with redox initiation systems in aqueous media are generally low. One of the reasons for this is the susceptibility of the initially formed radicals to undergo further redox chemistry. For example, potential propagating radicals may be oxidized to carbonium ions (Scheme 3.44). The problem is aggravated by the low solubility of the monomers (e.g. M VIA. S) in the aqueous phase. 

The difference in behaviour between pentyl and butyl cation systems (Figs. 3 and 4) has also been encountered in trapping experiments with carbonium ions, primarily formed from alkanes and SbFs, by CO (Hogeveen and Roobeek, 1972). In the case of n-butane the secondary butyloxocarbonium ion is the main product, whereas in the case of n-pentane only the tertiary pentyloxocarbonium ion is found. 

The equilibrium (1) at the electrode surface will lie to the right, i.e. the reduction of O will occur if the electrode potential is set at a value more cathodic than E. Conversely, the oxidation of R would require the potential to be more anodic than F/ . Since the potential range in certain solvents can extend from — 3-0 V to + 3-5 V, the driving force for an oxidation or a reduction is of the order of 3 eV or 260 kJ moR and experience shows that this is sufficient for the oxidation and reduction of most organic compounds, including many which are resistant to chemical redox reagents. For example, the electrochemical oxidation of alkanes and alkenes to carbonium ions is possible in several systems... 

Although at first glance addition to the central carbon and formation of what seems like an allylic carbonium ion would clearly be preferred over terminal addition and a vinyl cation, a closer examination shows this not to be the case. Since the two double bonds in allenes are perpendicular to each other, addition of an electrophile to the central carbon results in an empty p orbital, which is perpendicular to the remaining rr system and hence not resonance stabilized (and probably inductively destabilized) until a 90° rotation occurs around the newly formed single bond. Hence, allylic stabilization may not be significant in the transition state. In fact, electrophilic additions to allene itself occur without exception at the terminal carbon (54). 

In normal carbonium-ion chemistry, reaction proceeds from a precursor with a tetrahedral carbon capable of asymmetry hence, the stereochemistry of displacement in an aliphatic system can be ascertained by observation of the fate of the chiral center from reactant to product. An ethylenic system, of course, has no such chiral center, and hence there can be no change in optical configuration as the reaction proceeds. However, the stereochemistry of vinylic displacement and hence the symmetry and geometry of the intermediate can be... 

It is well known that allylic substrates are more reactive under solvolytic conditions than their saturated counterparts because of the delocalization of the positive charge in the developing carbonium ion over the tt system and the overlap of the empty p orbital with the double bond in the intermediate ion. 

The vinyl cation analog of an allylic carbonium ion is an allenyl cation 242, where the empty p orbital on the unsaturated carbon overlaps with the perpendicular n bond of the allenyl system. Allenyl cation 242 is of course a resonance form of the well known alkynylcarbonium ion,... 

In the proper system, under appropriate conditions, a new intermediate which is simultaneously a carbene and a carbonium ion (i.e., a carbenonium ion) may be envisioned. A particularly attractive system would be 263 and 264, as promotion of an electron from the tr system in vinyl cation 263 a onto the... 

An alternative explanation for the enhanced rates made its appearance. It was proposed that cr-participation in certain nonclassical ions provided a more satisfactory interpretation. This stimulated a detailed study of the norbornyl system, considered to provide the best available case for such nonclassical carbonium ion intermediates. The results failed to confirm the presence of significant -participation and supported the conclusion that the phenomena must be largely, if not entirely, steric in origin. 

As noted previously, a wide variety of aromatic systems serve as nuclei for arylacetic acid antiinflammatory agents. It is thus to be expected that fused heterocycles can also serve the same function. Synthesis of one such agent (64) begins with condensation of indole-3-ethanol (60) with ethyl 3-oxo-caproate (61) in the presence of tosic acid, leading directly to the pyranoindole 63. The reaction may be rationalized by assuming formation of hemiketal 62, as the first step. Cyclization of the carbonium ion... 

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