Tert-Butyl cation stability

Neither methyl nor ethyl fluoride gave the corresponding cations when treated with SbFs. At low temperatures, methyl fluoride gave chiefly the methylated sulfur dioxide salt, (CH3OSO) ShF while ethyl fluoride rapidly formed the rert-butyl and ferf-hexyl cations by addition of the initially formed ethyl cation to ethylene molecules also formed ° At room temperature, methyl fluoride also gave the tert-butyl cation. In accord with the stability order, hydride ion is abstracted from alkanes by super acid most readily from tertiary and least readily from primary positions. 

Replacing an a-alkyl snbstituent by an a-aryl group is expected to stabilize the cationic center by the p-Jt resonance that characterizes the benzyl carbocations. In order to analyze such interaction in detail, the cumyl cation was crystallized with hexafluoroantimonate by Laube et al. (Fig. 13) A simple analysis of cumyl cation suggests the potential contributions of aromatic delocalization (Scheme 7.3), which should be manifested in the X-ray structure in terms of a shortened cationic carbon—aromatic carbon bond distance (C Cat). Similarly, one should also consider the potential role of o-CH hyperconjugation, primarily observable in terms of shortened CH3 distances. Notably, it was found experimentally that the Cai distance is indeed shortened to a value of 1.41 A, which is between those of typical sp -sp single bonds (1.51 A) and sp -sp double bonds (1.32 A). In the meantime, a C -CH3 distance of 1.49 A is longer than that observed in the tert-butyl cation 1 (1.44 A), and very close to the normal value for an sp -sp single bond. 

The assumption that tertiary alkyl cations are not stable in solvents other than super-acids is widespread and was apparently well founded on many experiments by different workers over many years [20, 24]. For this reason the stability of our polymerised solutions was astonishing and it seemed at first unlikely that the cation of the electrolyte could be a simple tertiary ion the tert-butyl cation in the experiment with tert-butyl bromide and the ions 2-4 in the polymerised solutions. This was because we did not know then that Cesca,... 

Flectrophilic addition of polychloroalkanes such as, e.g., chloroform or 1,1,2,2-tetrachloroethane to Cjq with AICI3 in a 100-fold excess gives the monoadduct with a 1,4-addition pattern (Scheme 8.12) [93, 94], The reaction proceeds via a CjqR cation (19, Scheme 8.12) that is stabilized by the coordination of a chlorine atom to the cationic center. The cation is trapped by Cl to give the product 20. The chloroalkyl fullerenes can be readily hydrolyzed to form the corresponding fullerenol 21. This fullerenol can be utilized as a proper precursor for the cation, which is easily obtained by adding triflic acid. The stability of CjqR is similar to tertiary alkyl cations such as the tert-butyl-cation [95],... 

Gas-phase mass spectrometric studies891-894 also indicate exceptional stability of the 2-norbomyl cation relative to other potentially related secondary cations. A study by Kebarle and co-workers895 also suggests that the 2-norbornyl cation is more stable than the tert-butyl cation in the gas phase (based on hydride transfer equilibria from their respective hydrocarbons). 

When -butane 1 or isobutane 2 was reacted with HSO3F—SbF5 (Magic Acid), tert-butyl cation 4 was formed exclusively [Eq. (5.2)] as evidenced by a sharp singlet at 4.5 ppm (from TMS) in the 1H NMR spectrum. In excess Magic Acid, the stability of the ion is remarkable and the NMR spectrum of the solution remains unchanged even after having been heated to 110°C. 

Superelectrophilic activation has also been proposed to be involved, based upon the reactivity of carbocations with molecular hydrogen (a a-donor).16 This chemistry is probably even involved in an enzymatic system that converts CO2 to methane. It was found that A. A -menthyl tetrahy-dromethanopterin (11) undergoes an enzyme-catalyzed reaction with H2 by hydride transfer to the pro-R position and releases a proton to give the reduced product 12 (eq 15). Despite the low nucleophilicity of H2, cations like the tert-butyl cation (13) are sufficiently electrophilic to react with H2 via 2 electron-3 center bond interaction (eq 16). However, due to stabilization (and thus delocalization) by adjacent nitrogen atoms, cations like the guanidinium ion system (14) do not react with H2 (eq 17). 

Alkyl cations like the tert-butyl cation (2) and 2-propyl cation (89) are significantly stabilized by hyperconjugative C-H and C-C a -back donation into the empty carbocationic p-orbitals. Protosolvation involving a -bonds can diminish this hyperconjugative stabilization and thus lead to super-electrophilic carbocationic species. 

Why are intermediates (8) and (9) more stable than intermediates (8 ) and (9/) This can be explained by the inductive effect (I effect) and the hyperconjugation effect. The methyl group has an electron donation ability through the a bond. So, the tert-butyl cation and the terf-butyl radical can be stabilized by the inductive effect of the methyl group (Figure 1.4). Normally, the inductive effect is increased in the following order ... 

Arnett and Hofelich measured heats of reaction of a variety of alcohols with SbF5/FS03H in sulfuryl chloride fluoride to form their respective carbocations at constant temperature (-40 °C). In this superacid medium there were no ion-pair complications and hence reliable calorimetric data were obtained for various cyclopropyl and phenyl substituted cations. The heats of reaction for the formation of tricyclopropylcarbinyl cation (-59.2 kcalmoT ), trityl cation ( 9.0 kcalmoT ) and tert-butyl cation (-35.5 kcalmol ) show that the relative order of the stabilization of the cationic center is cyclopropyl >... 

Further compelling evidence indicating additional stabilization of the 2-norbornyl cation comes from Arnett et al. s measured heats of isomerization of secondary cations into tertiary cations.The measured heat of isomerization (A// ) of 4-methyl-2-norbornyl cation (143) to the 2-methyl-2-norbornyl cation (142) is -6.6kcalmoF [Eq. (5.20)]. In contrast, the related isomerization of the 2-butyl cation (58) to the tert-butyl cation (50) involves a difference of -14.2kcalmoTi [Eq. (5.21)] ... 

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... 

Fig. 22.10(c) shows the following surprising results (i) The predicted energy of adsorption (70 kj mol i at 0 K) is of the same order of magnitude as estimates based on experiments for related molecules (50-63 kJ moh ). (ii) With respect to isobutene in the gas phase separated from the zeolite, the tert-butyl cation is much less stable (-17 kJ moTi) than the isobutoxide (-48 kJ moh ). The reason is that dispersion contributes substantially less to the stabilization of the tert-butyl cation than to the stabilization of the adsorption complex or the isobutoxide. As result, the proton transfer energy increases from 24 kJ moH (DFT) to 59 kJ moh (MP2/DFT) and it seems very unlikely that the fert-butyl cation will be detected in zeolites, even as a short-lived species. 

Hyperconjugation occurs only if the cr bond orbital and the empty p orbital have the proper orientation. The proper orientation is easily achieved because there is free rotation about a carbon-carbon a bond (Section 2.10). In the case of the tert-butyl cation, nine C—Ho- bond orbitals can potentially overlap with the empty p orbital of the positively charged carbon. The isopropyl cation has six such orbitals, and the ethyl cation has three. Therefore, there is greater stabilization through hyperconjugation in the tertiary tert-butyl cation than in the secondary isopropyl cation and greater stabilization in the secondary isopropyl cation than in the primary ethyl cation. 

Let us discuss the SnI mechanism in more detail. Dissociation of the C-Cl bond in 2-chloro-2-methylpropane in the first step yields a tertiary carbocation (in this case a tert-butyl cation) which as a stable species lives long enough to collide with the OH" nucleophile and to form the product. It must be pointed out that the stability of the carbocation depends also on the enviromnent. For instance, the cations are more stable in polar than in nonpolar solvents. 

The stability of carbocations increases for alkyl cations with the number of alkyl groups that surround the positive charge and thereby stabilize it by their inductive effects. Thus, a methyl carbocation CH3 is the most unstable and reactive one while the tert-butyl cation [(CH3)3C]+ is the most stable and least reactive. This stability order is also the reason why carbocations frequently undergo isomerization and rearrangement reactions after formation, a reactivity that is very important for all isomerization reactions in refineries (here branched hydrocarbons are highly desired due to their higher octane number - see Chapters 6.9 and 6.10). 

There is direct evidence, from IR and NMR spectra, that the re/T-butyl cation is quantitatively formed when tert-butyl chloride reacts with AICI3 in anhydrous liquid HCl. In the case of alkenes, Markovnikov s rule (p. 984) is followed. Carbocation formation is particularly easy from some reagents, because of the stability of the cations. Triphenyhnethyl chloride and 1-chloroadamantane alkylate activated... 

Sometimes acylium ions lose carbon monoxide to generate an ordinary carbonium ion. It will be recalled that free acyl radicals exhibit similar behavior at high temperatures. Whether or not the loss of carbon monoxide takes place seems to depend on the stability of the resulting carbonium ion and on the speed with which the acylium ion is removed by competing reactions. Thus no decarbonylation is observed in Friedel-Crafts reactions of benzoyl chloride, the phenyl cation being rather unstable. But attempts to make pivaloyl benzene by the Friedel-Crafts reaction produce tert-butyl benzene instead. With compound XLIV cyclization competes with decarbonylation, but this competition is not successful in the case of compound XLV in which the ring is deactivated.263... 

In spite of their high pAR+ values, the methyl cations have less negative reduction potentials as compared to those of the cyclic cations e.g., -1.12 V for the tropylium ion 8+ and -2.20 V for cyclopropenium ion 9+ (8). Presence of tert-butyl groups in cations 2b-d+ increased the reversibility of both reduction and oxidation waves. The most negative reduction potential in the dimethylamino derivative 20+ reflects its high electrochemical stability. 

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