Cation methylium

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. 

Di(l-azulenyl)(6-azulenyl)methyl cation (24+) represented in Figure 17 exemplifies the cyanine-cyanine hybrid (20). Di(l-azulenyl)methylium unit in 24+ acts as a cyanine terminal group. The tropylium substructure stabilizes the cationic state (24+). Reduction of 24+ should afford the neutral radical 24, which is stabilized by capto-dative substitution effect, because 24 is substituted with azulenes in the donor and acceptor positions. The anionic state (24") is also stabilized by contribution of the cyclopentadienide substructure, which should exhibit the third color change in this system. 

Carbocations having more than four cationic centers are rare species, and very few have been isolated as isolable salts. A tetrahedrally-arrayed tetracation, which was stable only at low temperature was generated by G. A. Olah et al. in 1995 (22). Recently, R. Rathore et al. prepared isolable tetra- and hexatrityl cations utilizing tetraphenylmethane and hexaphenylbenzene as platforms (25). Tetracation 254 is one of the new multi-charged methylium compounds with considerable high stability. 

The author s theory which has been used here was developed in detail to explain the polymerisations by ionising radiations of some alkyl vinyl ethers, the polymerisations of which proceed by secondary ions. Although it was shown that the theory is also perfectly serviceable for the tertiary carbenium ions considered here, it must be realised that there is a fundamental difference between these two types of carbenium ions. When one of the bonds of the carbenium ion is a C—H bond, the solvators, especially of course an ion, can get much closer to the positive centre, and they are therefore correspondingly more firmly held to which effect is added that of a smaller steric hindrance. The most researched monomer propagating by secondary cations, apart from the alkyl vinyl ethers, is, of course, styrene. Thus, Mayr s many studies with diaryl methylium cations are directly relevant to the polymerisation of styrene. 

In agreement with a simple Walsh-type analysis for a AH3 system with six valence electrons, the planar Dsh structure, 35, is a minimum on the potential energy surface for EH3 cations of all group 14 elements. However, a second Cs symmetric minimum, 36, exists for all cations but the methylium ion... 

The existence of all cationic forms A+ to E+ was proved by NMR spectroscopy. Their relative abundances change with the polymerization temperature. The proton or methylium cation transfer has a low activation energy which, together with the energy state of the respective cation, determines the structure of the generated polymer. 

The kinetic stability describes a carbenium ion s lifetime in terms of its rate of decomposition. That is, although the methylium cation is thermodynamically very unstable, it can not decompose and is therefore kinet-ically stable. However, it reacts extremely quickly with even the weakest of nucleophiles due to its thermodynamic instability. 

Formally similar scheme may also operate for other initiating systems. Thus, for polymerization of cyclic acetals initiated with triphenyl-methylium (trityl) salts, the cationation of the monomer is a fast, reversible reaction [7], The next reaction, however, due to the steric hindrance, is very slow and consequently active species are formed by a parallel path involving hydride transfer (cf., Section II.A.2) [8,9],... 

Simpler procedures are of course available for the preparation and characterisation of carbenium ions in solution, particularly for the more stable ones. Concentrated sulphuric acid was extensively used as protogenic medium before the superacid mixtures were shown to be superior, but many of the spectroscopic assignements in those earlier studies were later proved erroneous, particularly in the case of such reactive entities as the 1-phenylethylium ion Model monomers which cannot polymerise because of steric hindrance can generate fairly stable carbenium ions by interacting with Lewis or Br nsted acids in normal cationic polymerisation conditions. Thus, 1,1-diphenylethylene and its dimer, and 1,1-diphenylpropene give rise to typical visible absorption bands from which the concentration of the corresponding diphenyl-methylium ions can be accurately calculated. As for carbenium ions capable of forming stable salts, their synthesis and characterisation is obviously easy. 

The much higher stability of onium ions, compart to that of the mqority of carbocations, reflrets the fact that, formally, onium ions can be considared to be equivalent to carbocatkrns bonded to (and therefore stabilized by) nucleophilic ligands such as amines, ethers or sulfides. For instance, trimethyloxonium ion lb can be treated as a methylium cation CH la attached to methyl ether and using... 

Tricoordinate carbocations are customarily called carbonium ions for a clear discussion of terminology and a suggestion favoring the view that the term carbonium ion be reserved for pentacoor-dinate carbocations, with tricoordinate carbocations being referred to as carbenium ions, see G. A. Olah, J. Am. Chem. Soc. 94, 808 (1972). Current practice uses both terms and also terms such as methyl cation and butyl cation to describe carbonium ions. Chemical Abstracts uses as specific names methylium, ethylium, etc. We will use carbonium ion as a generic term for trivalent carbocations. We will use methyl cation, ethyl cation, etc., when referring to specific ions. 

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