2.2.3- Trimethyl-2-butyl cation

The NMR spectrum of 2,2,3-trimethyl-2-butyl cation [triptyl cation, 36, Eq. (6.25)] consists of a single proton signal at 8 11 2.90 for all the methyl groups. This indicates that all five methyl groups undergo rapid interchange through... 

The two dimers of (CH3)2C=CH2 are formed by the mechanism shown in Figure 6.16. In step 1 protonation of the double bond generates a small amount of tert-butyl cation in equilibrium with the alkene. The car bocation is an electrophile and attacks a second molecule of 2-methylpropene in step 2, forming a new carbon-carbon bond and generating a carbocation. This new carbocation loses a proton in step 3 to form a mixture of 2,4,4-trimethyl-l-pentene and 2,4,4-trimethyl-2-pentene. 

Eor ions of similar or identical stability, 1,2-shifts are also extremely rapid. Even at very low temperatures, the NMR spectrum of the 2,3,3-trimethyl-2-butyl cation 7 has only one signal, while the NMR shows two signals, one for the methyl carbons and one for the quaternary carbons. As shown in Scheme 1.1, these he almost exactly midway between those calculated for the static ion. A similar result is obtained for the 1,2-dimethylcyclopentyl cation 8, and other tertiary cations related by 1,2-shifts. Thus, even at very low temperatures, 1,2-shifts... 

The observation of alkyl cations such as the ferf-butyl cation [trimethyl-carbenium ion, (CH3)3C+] 1 and the isopropyl cation [dimethylcarbenium ion, (CH3)2CH+] 2 was a long-standing challenge. The existence of alkyl cations in systems containing alkyl halides and Lewis acids has been inferred from a variety of observations, such as vapor pressure depressions of CH3C1 and C2H5CI in the... 

Thermodynamic Information indicates that 2-butenes would be the predominant olefins released. The resulting 2-butenes presumably react in this process with a t-butyl cation to produce a trimethyl pentyl ion. Hydride transfer from isobutane or more likely an acid-soluble hydrocarbon would result In the production of a trimethylpentane. 

Propyl cation 244 2-Butyl cation 246 Cyclopentyl cation 249 2,3,3-Trimethyl-2-butyl and related cations 250... 

The 1,2-methide shift as in the 2,3,3-trimethyl-2-butyl cation [5] takes place as shown in (6) with a very small barrier, AG = 3,5 0.1 kcal mol- (Saunders and Kates, 1978), via a corner protonated transition state or intermediate [6] ... 

The isotopic perturbation method was first developed using H-nmr spectroscopy and deuterium as the perturbing isotope. The isomerization of 3-deuterio-2,3-dimethyl-2-butyl cation (Saunders et al., 1971), 2,3,3-trimethyl-2-butyl cation and cyclopentyl cation (Saunders and Vogel, 1971a) and methyl interchange in 2,3-dimethyl-2-butyl cation [10] (Saunders and Vogel, 1971b) were studied. 

Returning to the initiation mechanism with the A1R2 X system, according to Eq. (2) the first initiating species is a carbocation formed by protonation of the particular monomer used. For example, with isobutylene the first carbocation is the trimethyl carbenium ion ten-butyl cation) ... 

Superacids such as Magic Acid and fluoroantimonic acid have made it possible to prepare stable, long-lived carbocations, which are too reactive to exist as stable species in more basic solvents. Stable superacidic solutions of a large variety of carbocations, including trivalent cations (also called carbenium ions) such as t-butyl cation 1 (trimethyl-carbenium ion) and isopropyl cation 2 (dimethylcarbe-nium ion), have been obtained. Some of the carbocations, as well as related acyl cations and acidic carboxonium ions and other heteroatom stabilized carbocations, that have been prepared in superacidic solutions or even isolated from them as stable salts are shown in Fig. 1. 

Treatment of 2-methylpropene with hot aqueous sulfuric acid gives two dimers 2,4,4-trimethyl-1-pentene and 2,4,4,-trimethyl-2-pentene. This transformation is possible because 2-methylpropene can be protonated undOT the reaction conditions to furnish the 1,1-dimethylethyl (tert-butyl) cation. This species can attack the electron-rich double bond of 2-methylpropene with formation of a new carbon-carbon bond. Electrophihc addition proceeds according to the Markovnikov rule to generate the more stable carbocalion. Subsequent deprotonation from either of two adjacent carbons furnishes a mixture of the two observed products. 

Some benzylic mono- and di-cations have been studied by the NMR/DFT/IGLO technique. Of the stable dications, the trimethyl species (76 R = Me) was found to be the major resonance contributor to the structure of (75), and the same was found to be true for the trimethoxy derivative. In the related monocations, for (77) the major resonance contributor was (78), and this was also the case for the pentamethyl and 2,5-dimethyl-4-r-butyl compounds. The dication (79) and the trication (80),... 

The effect of various surfactants, the cationics-eetyl trimethyl ammonium bromide (CTAB), and cetyl pyridinium chloride (CPC), the anionic-sodium lauryl sulfate (SLS), and the nonionic-polysorbate 80 (Tween 80), on the solubility and ionization constants of some sparingly soluble weak acids of pharmaceutical interest was studied (Gerakis et al., 1993). Benzoic acid (and its 3-methyl-, 3-nitro-, and 4-tert-butyl-derivatives), acetylsalicylic acid, naproxen, and iopanoic acid were chosen as model drugs. The cationics, CTAB and CPC, were found to considerably increase th< ionization constant of the weak acidS Ka ranged from-0.21 to-3.57), while the anionic, SLS, showed a negligible effect and the nonionic, Tween 80, generally decreased the ionization constants Solubility of the acids increased in aqueous micellar and in acidiLed micellar solutions. 

The IR spectrum of the unsubstituted cydopropenylium ion exhibits only four bands at 3105, 1276, 908 and 736 cm , as expected from the Djij-symmetry. The band at 1276 cm is due to an E ring deformation vibration of the central three-membered ring, which is generally observed as a strong band in the range of 1400 to 1490 cm for the aryl- and alkyl-substituted derivatives (Table 2). The frequency of this band for the trimethyl-, tri-zc-rt-butyl, triphcnyl, and unsubstituted cydopropenylium ions exhibits a linear correlation with the index of the cation s thermodynamic stability such as For heteroatom-substituted derivatives... 

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