Carbenium tert-butyl cation

Carbocations are a class of reactive intermediates that have been studied for 100 years, since the colored solution formed when triphenylmethanol was dissolved in sulfuric acid was characterized as containing the triphenylmethyl cation. In the early literature, cations such as Ph3C and the tert-butyl cation were referred to as carbonium ions. Following suggestions of Olah, such cations where the positive carbon has a coordination number of 3 are now termed carbenium ions with carbonium ions reserved for cases such as nonclassical ions where the coordination number is 5 or greater. Carbocation is the generic name for an ion with a positive charge on carbon. 

Olah and Hogeveen and co-workers The carbon-carbon cleavage in neopentane (7) yielding methane and the tert-butyl cation 4 occurs by a mechanism different from the /3-scission of carbenium ions [Eq. (5.56)]. 

In dication 188, the NBO charge at the carbenium ion center is +0.69 and at the acyl carbon is +1.09. The tert-butyl cation has been found to have NBO charge at its carbenium center of +0.67, suggesting a modest superelectrophilic activation of the carbenium ion center in 188, compared with the tert -butyl cation. When charges on the methyl groups are also considered, structure 188 is similar to the protosolvated tert -butyl cation 196. 

Figure 3.53 shows an addition of a carboxybc acid to isobutene, which takes place via the tert-butyl cation. This reaction is a method for forming ferf-butyl esters. Because the acid shown in Figure 3.53 is a /i-hydroxycarboxylic acid whose alcohol group adds to an additional isobutene molecule, this also shows an addition of a primary alcohol to isobutene, which takes place via the ferf-butyl cation. Because neither an ordinary carboxylic acid nor, of course, an alcohol is sufficiently acidic to protonate the alkene to give a carbenium ion, catalytic amounts of a mineral or sulfonic acid are also required here. 

At present, there is no evidence for sacrificial hyperconjugation in neutral hydrocarbons. The concept of hyperconjugation is also applied to CARBENIUM ions and radicals, where the interaction is now between o-bonds and an unfilled or partially filled n or p-ORBiTAL. A contributing structure illustrating this for the tert-butyl cation is ... 

An obvious candidate for a stable noncyclic carbenium ion is the tert-butyl cation observed in superacidic media. Even if the proton affinity of isobutene (Table 22.1) does not make it very likely that tert-butyl cations will exist in zeolites, several quantum chemical studies have localized stationary points for tert-butyl cations in zeolite and found that they are less stable than the adsorption complex, but are similar in stability to surface butoxides. Because of technical limitations vibrational analysis, which could prove that this cation is a local minimum on the potential energy surface, that is a metastable species, have only recently been made. Within a periodic DFT study of isobutene/H-FER a complete vibrational analysis for all atoms in the unit cell was made [48], and as part of a hybrid QM/MNDO study on an embedded cluster model of isobutene/H-MOR a vibrational analysis was made with a limited number of atoms [49]. Both reached the... 

Figure 6.10.2 shows the simplified main reaction cycle in refinery alkylation. An olefin is added to the ferf-butyl cation to give the corresponding Cg carbocation. This Cg carbocation may isomerize via hydride and methyl shifis to form a more stable carbenium ion and subsequently undergoes, again, hydride transfer from isobutane. This latter step forms the saturated hydrocarbon and regenerates the tert-butyl cation to perpetuate the catalytic cyde. 

An ESCA study by Olah and coworkers " succeeded in observing the ESCA spectrum of the 2-norbornyl cation (126) and compared it with those of the related 2-methyl-2-norbomyl cation (142) and other trivalent carbenium ions such as the cyclopentyl (75), 1-methyl-1-cyclopentyl (141), and tert-butyl (50) cations. The Is electron spectrum of the 2-norbomyl cation shows no high binding energy carbenium center and a maximum separation of <1.5 eV is observed between the two equivalent cyclopropyl -type carbons... 

Proton Traps. 2,6-Di-tert-butyl-4-methylpyridine (DBMP) and 2,6-di-tert-butyIpyridine (DtBP) are hindered bases incapable of reacting with electrophiles other than protonic acids. Consequently, they can be successfully used for the trapping of protons during their transfer to monomer (174. 175). At the same time they can disseminate between the two major initiation mechanisms encountered in cationic polymerization, that is, protonic initiation or carbenium initiation (176). 

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