Carbonium cation

What has been said about sulfonic esters applies also to sulfuric, phosphoric and nitric esters and of course the hydrolysis of halides must necessarily always yield a carbonium cation. 

The terminal primary alcohol group must carry a cationogenic substituent, i. e., a substituent the removal of which by alkali takes place in such a way that C6 becomes a carbonium cation. 

Novak et al. <1998JA1643> devised a novel approach to amino-substituted tctrazolo[ 1,5-tf Jpyridine which provides a really unique pathway (Scheme 34). These authors studied the possibility of formation of nitrenium ions from the pivaloylhydroxylamine 143 and found that if azide anion is present in the main reaction route is the formation of tetrazolo[l,5-tf]pyridine 146. The authors concluded that the first intermediate is the formation of the carbonium cation 144 which captures the azide anion to yield 2-azidopyridine 145, that is, the valence bond isomer of the product 146. 

Since no other stereoisomer but that of iditol, mannitol or sorbitol can exist in the as-fused ring series of l,4 3,6-dianhydrohexitols and since amination experiments on the ditosyl derivatives of each isomer have given three different amino derivatives, it is reasonable to assume that the two diamines which have been isolated are of the same configuration as their parent dianhydrohexitols. These two diamines LXXIV and LXXV on treatment with nitrous acid suffered deamination as expected but instead of obtaining in one case isomannide and in the other isosorbide, only one dianhydrohexitol was isolated and that was dian-hydro-L-iditol. The deamination must of course lead to the transitory carbonium cation LXXVIII and this on hydroxylation can take on any of the configurations, L-iditol, D-mannitol or D-sorbitol. That it preferentially takes on the configuration of L-iditol indicates that this is in some ways a more stable structure than the others. This behavior is paralleled... 

O atoms, the formation of a carbonium cation due to cleavage of the C—O bond, and the stabilization of the product by heterolysis of a water molecule—there is still some discussion about the structure of the carbonium cation. This may be either cyclic or acyclic, depending on the primary site of protonation. Possibly, this discussion can be settled by assuming a partial protonation of both oxygen atoms caused by the structured nature of water (2), as shown in Figure 2. 

Corma and co-workers152 have performed a detailed theoretical study (B3PW91/6-31G level) of the mechanism of the reactions between carbenium ions and alkanes (ethyl cation with ethane and propane and isopropyl cation with ethane, propane, and isopentane) including complete geometry optimization and characterization of the reactants, products, reaction intermediates, and transition states involved. Reaction enthalpies and activation energies for the various elemental steps and the equilibrium constants and reaction rate constants were also calculated. It was concluded that the interaction of a carbenium ion and an alkane always results in the formation of a carbonium cation, which is the intermediate not only in alkylation but also in other hydrocarbon transformations (hydride transfer, disproportionation, dehydrogenation). 

Tri-0-acetyl-2-amino-2-deoxy-a-D-glucosyl bromide hydrobromide (LXVa)169 170 is more stable than the V-acyl analogs, possibly because heterolysis of the C—Br bond with the generation of a carbonium cation proceeds less readily as a result of the presence of the neighboring—NH3 center. /3-u-Glycoside formation proceeds normally with alcohols,169 and treatment with an equivalent of sodium ethoxide under anhydrous conditions liberates170 the free base (LXVb). 

During the deamination of 2-amino-2-deoxy-D-gluconic acid, it is probable that the carboxyl group is involved in a participation reaction with the carbonium cation CVIII from the side of the molecule opposite to the departing diazo group, to give an intermediate CIX with the d-manno con-... 

If instead of considering group III elements such as boron or aluminium, or electron depleted carbon species such as carbonium cations, one considers electrically neutral carbon species that are hybridised into the sp configuration, then a further complication arises. For example, if two carbons atoms are hybridised into the sp configuration, then each one will have a spare pz orbital remaining,... 

The commonest example would be the carbonium cation. There is another category in which the electrophilic species has a full complement of electrons yet does not bear a full positive charge, but still attracts electrons. Suggest an example that fits into this group. 

Phenonium ion The bridged carbonium cation that results from the departure of a leaving group that is P to the aromatic ring. 

It is not essential to the theory to assume that a carbonium cation is actually liberated. Indeed, as we shall see later, it would be incorrect to do so. The essential point is that scission takes place between the ester oxygen and the alcohol carbon in sulfonic esters but not in carboxylic esters. 

Pursuing this idea further, it is seen that anhydride formation in the sugars resolves itself into an exchange of anions on a carbonium cation,... 

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