Cations carbonium ions

Winstein Robinson (1958) used this concept to account for the kinetics of the salt effects on solvolysis reactions. They considered that carbonium ions (cations) and carbanions could exist as contact ion-pairs, solvated ion-pairs and as free ions and that all these forms participated in the reactions and were in equilibrium with each other. These equilibria can be represented, thus ... 

So far as vinyl monomers are concerned, ionic propagation proceeds with carbonium ions (cationic polymerization) or carbanions (anionic polymerization) at the chain ends. The study of the initiation process of radiation-induced ionic polymerization seeks to elucidate how these ions are formed from the primary ionic intermediates. Possible reactions... 

The last few years brought the rapid development of acid catalyst preparations especially the synthesis of solids with high acid strength - the so-called superacids. At the same time the investigations of acid catalysed reactions showed that apart from classical carbocations, pentacoordinated carbonium ions, cation-radicals and even radicals may be considered as possible transition states. The mechanism of non-classical intermediate formation has not been fully explained yet and needs further investigations. 

Because the growing polymer chain and monomer will react to give the more stable of the two possible carbonium ions, cationic polymerization, like radical-chain polymerization, leads to regular head-to-tail incorporation of monomer units. 

Ionic initiator n. A substance providing either carbonium ions (cationic) or carba-nions (anionic) that attack the reactive double bonds of vinyl monomers and add on, r enerating the ion species on the propagating chain. Odian GC (2004) Principles of polymerization. John Wiley and Sons Inc., New York. 

Ionic Initiator n A substance providing either carbonium ions (cationic) or carbanions (anionic) that attack... 

One of the side reactions that can complicate cationic polymerization is the possibility of the ionic repeat unit undergoing the well-known carbonium ion rearrangement during the polymerization. The following example illustrates this situation. 

The rearrangement of carbonium ions that readily occurs according to the thermodynamic stabiUty of cations sometimes limits synthetic utility of aromatic alkylation. For instance, the alkylation of ben2ene with / -propyl bromide gives mostly isopropylben2ene (cumene) much less... 

TIricooRlinate caibocations are fiequendy called carbonium ions. The terms methyl cation, butyl cation, etc., are used to describe the c >rTesixiiulir.ji tricoordinate cations. Chemical Abstracts uses as specific names methylium, ethyUum, propylium. We will use carbocation as a generic term for trivalent carbon cations. 

The cationic polymerization of cardanol under acidic conditions has been referred to earlier [170,171], NMR studies [16] indicated a carbonium ion initiated mechanism for oligomerization. PCP was found to be highly reactive with aldehydes, amines, and isocyates. Highly insoluble and infusible thermoset products could be obtained. Hexamine-cured PCP showed much superior thermal stability (Fig. 12) at temperatures above 500°C to that of the unmodified cardanol-formaldehyde resins. However, it was definitely inferior to phenolic resins at all temperatures. The difference in thermal stability between phenolic and PCP resins could be understood from the presence of the libile hydrocarbon segment in PCP. 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

The reaction between carbonium ions and carbon monoxide affording oxocarbonium ions (acyl cations) is a key step in the well-known Koch reaction for making carboxylic acids from alkenes, carbon monoxide and water ... 

In previous studies (Hogeveen, 1970) the reactivity of long-lived carbonium ions towards molecular hydrogen has been investigated and interesting differences between secondary and tertiary alkyl cations have been observed. Tliis reaction is too slow, however, to be extended to other types of carbonium ions. The reactivity of carbonium ions towards carbon monoxide is much higher (about six powers of ten) than towards molecular hydrogen, which enabled us to determine not only the rate of reaction (3) for some tertiary and secondary alkyl cations, but also the rate of carbonylation of more stabilized carbonium ions. 

As mentioned in the Introduction, rearrangements of the intermediate alkyl cation in the Koch synthesis may compete with the carbonylation. Under the kinetically controlled conditions prevailing in the Koch synthesis of carboxylic acids, the rearrangements occur only from a less stable to a more stable carbonium ion, e.g. from a secondary to a tertiary ion. The reverse rearrangements—from a more stable to a less stable... 

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. 

In Sections II and III it was shown that secondary and tertiary alkyl cations can be formed by decarbonylation of the corresponding oxo-carbonium ions. This has been found impossible in the case of primary alkyl cations (Hogeveen and Roobeek, 1970) the oxocarbonium ions 13 and 14 were unchanged after one hour at 100°C k < 1-3 x 10 sec ), whereas ion 15 is fragmented by a j3-fission under these circumstances ... 

Vinyl cations, la, are carbonium ions where the electron-deficient carbon is part of an unsaturation, with the formally positive carbon bonded to only two substituents, as compared to trisubstituted normal carbonium ions, lb. Although... 

Vinyl cations also have been invoked as intermediates in the addition of carbonium ions generated in strong acid to acetylene (50-53). Sasaki et al (50) observed 1-adamantyl methyl ketone, 25, as the sole product in the reaction of acetylene with 1-bromoadamantane in concentrated H2SO4 at 5°. Bott (51), on the other hand, reported a mixture consisting of 75% 1-adamantylacetaldehyde,... 

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). 

The formation of any vinyl products in electrophilic additions to RCH=C=CH2 and RCH=C=CHR is surprising, since central protonation should yield a secondary carbonium ion compared to terminal protonation and formation of a vinyl cation. Perhaps a secondary carbonium ion destabilized by... 

Two reasons may be offered for the enhanced /3-deuterium isotope effect in vinyl cations as compared with carbonium ions (193). As pointed out by Noyce and Schiavelli (21), in the transition state of a vinyl cation, the isotopically substituted C—H bond is ideally suited for overlap with the developing vacant p orbital, as the dihedral angle between the empty p orbital and C—H bonds is zero in the intermediate, as shown in structure 239. Shiner and co-workers (195)... 

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