Carbenium chemistry

Pt-alumina is a profoundly subtle and possibly unique system for bifunctional reforming catalysis. The Pt crystallite sizes which gamma alumina can stabilise contain high proportions of sites upon which selective catalysis can take place in addition, the acidity of gamma alumina can be promoted and controlled for selective carbenium chemistry in-situ. Further enhancement of Pt selectivity has been achieved by the addition of a second element, notably Re, Ir and Sn. Both ensemble and electronic models have been put forward to explain the effects, and have been vigorously discussed in the open literature. Geometric effects tend to be preferred for Pt-Re and Pt-Sn, but inevitably in such multivariate systems, there is still room for debate. The commercial success of these catalyst systems depends on their ability... 

Protonic initiation is also the end result of a large number of other initiating systems. Strong acids are generated in situ by a variety of different chemistries (6). These include initiation by carbenium ions, eg, trityl or diazonium salts (151) by an electric current in the presence of a quartenary ammonium salt (152) by halonium, triaryl sulfonium, and triaryl selenonium salts with uv irradiation (153—155) by mercuric perchlorate, nitrosyl hexafluorophosphate, or nitryl hexafluorophosphate (156) and by interaction of free radicals with certain metal salts (157). Reports of "new" initiating systems are often the result of such secondary reactions. Other reports suggest standard polymerization processes with perhaps novel anions. These latter include (Tf)4Al (158) heteropoly acids, eg, tungstophosphate anion (159,160) transition-metal-based systems, eg, Pt (161) or rare earths (162) and numerous systems based on tri flic acid (158,163—166). Coordination polymerization of THF may be in a different class (167). 

The carbonium ion s charge is not stable and the acid sites on the catalyst are not strong enough to form many carbonium ions. Nearly all the cat cracking chemistry is carbenium ion chemistry. 

Effects of solvent polarity, counter-anion nucleophilidty, temperature, and monomer concentration on the carbenium ion polymerization chemistry have been extensively studied29,36 38,49. Based on previous knowledge26"29 Me3Al was chosen because with this coinitiator undesired chain transfer to monomer processes are absent. Preliminary experiments showed that Et3Al coinitiator did not yield PaMeSt, possibly because the nuc-leophilicity of the counter-anion Et3AlQe is too high and thus termination by hydrida-tion is faster than propagation36. ... 

A wide variety of NMR methods are being applied to understand solid acids including zeolites and metal halides. Proton NMR is useful for characterizing Brpnsted sites in zeolites. Many nuclei are suitable for the study of probe molecules adsorbed directly or formed in situ as either intermediates or products. Adsorbates on metal halide powders display a rich carbenium ion chemistry. The interpretation of NMR experiments on solid acids has been greatly improved by Ae integration of theoretical chemistry and experiment. 

There is good reason to believe that the potential of NMR studies of carbenium ions on solid metal halitks exceeds that of corresponding studies in superacid solutions. Of course the advantages of working in solids include Ae possibility of very low temperatures and the mass transport restrictions of frozen media. Thus, Mehre and Yannoni were able to characterize the sec-butyl cation in frozen SbF5 by NMR [20] and Schleyer and coworkers have obtained infrared evidence of the allyl cation in the same medium [21]. So far, we have been successful in every case in which we have tried to duplicate known solution carbenium ion chemistry on... 

Carbonyl reactions are extremely important in chemistry and biochemistry, yet they are often given short shrift in textbooks on physical organic chemistry, partly because the subject was historically developed by the study of nucleophilic substitution at saturated carbon, and partly because carbonyl reactions are often more difhcult to study. They are generally reversible under usual conditions and involve complicated multistep mechanisms and general acid/base catalysis. In thinking about carbonyl reactions, 1 find it helpful to consider the carbonyl group as a (very) stabilized carbenium ion, with an O substituent. Then one can immediately draw on everything one has learned about carbenium ion reactivity and see that the reactivity order for carbonyl compounds ... 

The difference between carbonyl chemistry and (simple) carbocation chemistry is a result of much greater stability of the carbonyl group relative to a simple carbenium... 

The chemistry of the heavy carbenium ion analogs has been the subject of several preceding reviews, of which the latest one by Miiller P is the most comprehensive. In this section we will deal briefly with the most essential achievements made in the 1990s, and will emphasize the latest progress in the field, particularly concerning the synthesis and structural studies of the stable noncoordinated ( free ) cations of the type RR R"E+. 

The chemistry of acid catalyzed reactions is deeply documented and will not be considered here. It is sufficient to say that they are based in carbenium ion reactions, which yield lower molecular weight compounds and branched paraffins (since tertiary carbenium ions are the most stable). The result of the cracking of a paraffin of n-carbon atoms is another paraffin with (n-x) carbon atoms and an olefin of x-carbon atoms. 

Song, W., Nicholas, J.B. and Haw, J.F. (2001). Acid-base chemistry of a carbenium ion in a zeolite under equilibrium conditions verification of a theoretical explanation of carbenium ion stability. J. Am. Chem. Soc. 123, 121-129... 

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. 

In the older papers the inaccurate old word carbonium ion, which originated from organic chemistry, has not been replaced by the semantically correct term carbenium ion. The appearance of this term marks the time when Olah s discovery of quinquevalent carbon in super-acid solution, the true carbonium ion, made the change of nomenclature essential and urgent. 

However, in addition to these advances, the paper contains two further innovations which are important outside the domain of polymer chemistry. One is the demonstration that in adequately pure systems aliphatic carbenium tetrahaloaluminates, R3C+A1X4" in solution are stable electrolytes. The other is the direct demonstration that A1X3 and isobutene form a stable, reversible, complex. 

We wish to emphasise that the formation of esters (E) from alkenes (M) and acids (HA), the catalysis of the reactions of E by HA or MtXn, and the activation of E, such as organic chlorides, by the co-ordination of a Lewis acid, such as A1C13, are all very familiar chapters in conventional organic chemistry. It follows that the pseudo-cationic theory is nothing more than a generalisation of conventional organic-chemical ideas and a revival of some pre-Whitmore interpretations which had become occulted by the usefulness and novelty of the carbenium ion concept. 

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