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Tertiary alkyl cation

II. Reactivity and Stabilization of Tertiary Alkyl Cations in. Reactivity and Stabilization of Secondary Alkyl Cations. ... [Pg.29]

In previous studies (Hogeveen, 1970) the reactivity of long-lived carbonium ions towards molecular hydrogen has been investigated and interesting differences between secondary and tertiary alkyl cations have been observed. Tliis reaction is too slow, however, to be extended to other types of carbonium ions. The reactivity of carbonium ions towards carbon monoxide is much higher (about six powers of ten) than towards molecular hydrogen, which enabled us to determine not only the rate of reaction (3) for some tertiary and secondary alkyl cations, but also the rate of carbonylation of more stabilized carbonium ions. [Pg.31]

Calculation of the second-order rate constant of carbonylation, kg, and the equilibrium constant, K = [t-C4H9CO+]/[t-C4H ][CO] = A c/fcD> requires knowledge of the concentration of CO. The constant a in Henry s law Pco = [CO] was determined to be 5-3 litre mole atm in HF—SbFs (equimolar) and 53 litre mole atm in FHSOs—SbFs (equimolar) at 20°C. From the ratio [t-C4HBCO+]/[t-C4HJ"] at a known CO pressure, values for k and K were obtained. The data are listed in Table 1, which includes the values for the rate and equilibrium constants of two other tertiary alkyl cations, namely the t-pentyl and the t-adamantyl ions (Hogeveen et al., 1970). [Pg.32]

The equilibrium constant K is the same for R =t-C4HJ and t-CsHi. As also the rate constants of carbonylation and decarbonylation are about equal for these two ions, it is concluded that both the thermodynamics and the kinetics of the carbonylation reaction are independent of the structure of R+, if R+ is an acyclic tertiary alkyl cation. This agrees with former findings (Brouwer, 1968) on the relative stabilities of such ions. [Pg.33]

Fig. 1. Free-enthalpy diagram of the carbonylation-decarbonylation of tertiary alkyl cations at 20 0 in FHSOa—SbFs (concentrations expressed in mole litre ). Underlined numbers directly from experimental data. Fig. 1. Free-enthalpy diagram of the carbonylation-decarbonylation of tertiary alkyl cations at 20 0 in FHSOa—SbFs (concentrations expressed in mole litre ). Underlined numbers directly from experimental data.
This difference in stabilization isreflectedinboththerateof carboiiylation and decarbonylation (Table 1). The free-enthalpy profiles for the car-bonylation of tertiary alkyl cations are shown in Fig. 1. [Pg.34]

The value for K was determined to be 10 litre mole at 20°C, which is of the same order of magnitude as those for tertiary alkyl cations ((0-07 — 2) X 10 litre mole . Section II) and dramatically different from those for secondary alkyl cations (about 10 ° litre mole , calculated from Figs. 2 and 3). These data show that the 2-norbomyl ion is only 1-6 kcal mole less stabilized than, for example, the tertiary butyl cation and about 8 kcal mole" more stabilized than secondary alkyl cations. Another thermodynamic argument for the high stability of the 2-norbornyl ion in solution is found in the work of Amett and Larsen (1968)... [Pg.41]

In Sections II and III it was shown that secondary and tertiary alkyl cations can be formed by decarbonylation of the corresponding oxo-carbonium ions. This has been found impossible in the case of primary alkyl cations (Hogeveen and Roobeek, 1970) the oxocarbonium ions 13 and 14 were unchanged after one hour at 100°C k < 1-3 x 10 sec ), whereas ion 15 is fragmented by a j3-fission under these circumstances ... [Pg.43]

Although a value for the rate constant of carbonylation of primary alkyl cations has so far not been obtained experimentally, it can be shown that the reaction must be diffusion-controlled. The difference in stabilization between secondary and tertiary alkyl cations in solution (9 + 1 kcal mole Section III, C) shows up as a difference in rate of carbonylation of a factor of 10 (Section III, A). As the difference in stabilization between primary and secondary alkyl cations is certainly larger than that between secondary and tertiary alkyl cations (various estimates have been summarized by Brouwer and Hogeveen, 1972), the rate constant of carbonylation of primary alkyl cations— and even more so for the methyl cation—will exceed that of secondary alkyl cations by more than a factor of 10, so that k> 10 litre mole sec, which means that the reaction is diffusion controlled. [Pg.45]

It should be emphasized that, in contrast to the cases of secondary alkyl cations (Olah et al., 1964 Saunders et al., 1968) and tertiary alkyl cations (Olah et al., 1964 Brouwer and Mackor, 1964), the evidence for the existence of primary alkyl cations as distinct species has been only indirect, because they have escaped direct spectroscopic observation so far. [Pg.45]

In Sections II and III the quantitative aspects have been summarized of the reversible carbonylation of secondary and tertiary alkyl cations as studied under thermodynamically controlled conditions. In Section IV the results have been reviewed of the irreversible carbonylation of the much less stable primary alkyl and vinyl cations as studied under kinetically controlled conditions. No kinetic details had been obtained in the latter case owing to the short-hved character of the ions. [Pg.46]

The mechanism involves a simple 1,2 shift. The ion (52, where all four R groups are Me) has been trapped by the addition of tetrahydrothiophene. It may seem odd that a migration takes place when the positive charge is already at a tertiary position, but carbocations stabilized by an oxygen atom are even more stable than tertiary alkyl cations (p. 323). There is also the driving force supplied by the fact that the new carbocation can immediately stabilize itself by losing a proton. [Pg.1397]

A large number of tertiary alkyl cations have been directly observed by NMR spectroscopy and in some cases by X-ray crystallography (9). An unique subgroup of these cations are those formed in medium-size ring systems, in which a p-H-bridging structure has been proposed (10, 11, 12), and some examples are shown below ... [Pg.284]

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,... [Pg.319]

Brouwer and Mackor (1964) found that concentrated and stable solutions of a series of tertiary alkyl cations can be prepared in HF-SbFs and their proton magnetic resonance spectra were recorded. The t-butyl, t-pentyl and t-hexyl cations were observed in this solvent system. The spectra were identical with those obtained previously in SbFs and FSOsH-SbFs solvent systems. [Pg.333]

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],... [Pg.263]

Table 1.3 provides rate constants for the decay of selected carbocations and oxocar-bocations in H2O, TFE, and HFIP. As a general comment, water, methanol, and ethanol are highly reactive solvents where many carbocations that are written as free cations in standard textbooks have very short lifetimes. The diphenylmethyl cation, with two conjugating phenyl groups, has a lifetime in water of only 1 ns. Cations such as the benzyl cation, simple tertiary alkyl cations such as tert-butyl, and oxocarbocations derived from aldehydes and simple glycosides, if they exist at all, have aqueous lifetimes in the picosecond range, and do not form and react in water as free ions. This topic is discussed in more detail in Chapter 2 in this volume. [Pg.21]

R H) is much faster than alkylation, so that alkylation products are also derived from the new alkanes and carbocations formed in the exchange reaction. Furthermore, the carbo-cations present are subject to rearrangement (Chapter 18), giving rise to new carbocations. Products result from all the hydrocarbons and carbocations present in the system. As expected from their relative stabilities, secondary alkyl cations alkylate alkanes more Teadily than tertiary alkyl cations (the r-butyl cation does not alkylate methane or ethane). Stable primary alkyl cations are not available, but alkylation has been achieved with complexes formed between CH3F or C2H5F and SbFs-212 The mechanism of alkylation can be formulated (similar to that shown in hydrogen exchange with super acids, 2-1) as... [Pg.601]

Why do tertiary alkyl compounds ionize so much more rapidly than either secondary or primary compounds The reason is that tertiary alkyl cations are more stable than either secondary or primary cations and therefore are formed more easily. You will appreciate this better by looking at the energy diagram of Figure 8-4, which shows the profile of energy changes for hydrolysis of an alkyl compound, RX, by the SN1 mechanism. The rate of... [Pg.226]

The correlation of Fig. 6 is dominated by carbocations which undergo deprotonation to form aromatic products. The positive deviations of tertiary alkyl cations have already been mentioned (p. 43). As discussed by Richard7 these... [Pg.89]

Higher protonated alcohols cleave to stable tertiary alkyl cations. For protonated primary and secondary alcohols, the initially formed primary and secondary carboca-tions rapidly rearrange to the more stable tertiary carbenium ions under the conditions of the reaction. For example, protonated ra-butyl alcohol 7 cleaves to ra-butyl cation which rapidly rearranges to tot-butyl cation (k2 ki) [Eq. (4.4)]. [Pg.315]

Fig. 14.15. Regioselectivity of the pinacol rearrangement of an unsymmetrical glycol. The more stable carbenium ion is formed under product development control. Thus, the benz-hydryl cation B is formed here, while the tertiary alkyl cation D is not formed. Fig. 14.15. Regioselectivity of the pinacol rearrangement of an unsymmetrical glycol. The more stable carbenium ion is formed under product development control. Thus, the benz-hydryl cation B is formed here, while the tertiary alkyl cation D is not formed.
Recently Saunders and Kates (1978) have been successful in measuring the rates of degenerate 1,2-hydride and 1,2-methide shifts of several simple tertiary alkyl cations employing high field (67.9 MHz) C-nmr spectroscopy. From band broadening in the fast exchange limit the free energies of activation (AC ) were determined to be 3.1 0.1 kcal mol at —138°C for [10] and 3.5 0.1 kcal mol at —136°C for [5]. [Pg.253]

Both silyl enolates and allylsilanes are excellent nucleophiles for alkylation by other stabilized carbocations such as the tertiary alkyl cations 111 or 112 (Scheme 2.42). Similarly, Michael-like additions, for example, the coupling of 113 with silyl ketene acetal 114, can be also achieved.Owing to the high electrophilicty of the enone system, this reaction proceeds smoothly in polar solvents, even in the absence of Lewis acids. [Pg.95]

Tertiary alkyl cation (3°) (CHjIjC Stabilized by three alkyl groups... [Pg.96]

The same was found to hold with a series of tertiary alkyl cations obtained from their chlorides and alcohols. ... [Pg.63]


See other pages where Tertiary alkyl cation is mentioned: [Pg.31]    [Pg.786]    [Pg.276]    [Pg.326]    [Pg.1073]    [Pg.77]    [Pg.228]    [Pg.608]    [Pg.446]    [Pg.1087]    [Pg.253]    [Pg.802]    [Pg.1087]    [Pg.1087]    [Pg.326]    [Pg.120]    [Pg.1087]    [Pg.116]   
See also in sourсe #XX -- [ Pg.96 ]




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Alkyl cation

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