Cycloalkanes protonation

Protonation of alkenes, cycloalkanes and alkanes lead to the formation of carbe-nium ions, carbonium ions and protonated cyclopropane rings (also called cyclo-... 

We turn to the chemical behavior of cycloalkane holes. Several classes of reactions were observed for these holes (1) fast irreversible electron-transfer reactions with solutes that have low adiabatic IPs (ionization potentials) and vertical IPs (such as polycyclic aromatic molecules) (2) slow reversible electron-transfer reactions with solutes that have low adiabatic and high vertical IPs (3) fast proton-transfer reactions (4) slow proton-transfer reactions that occur through the formation of metastable complexes and (5) very slow reactions with high-IP, low-PA (proton affinity) solutes. 

Indeed, the enthalpies of formation of the cycloalkylmagnesium bromides were calculated from the enthalpies of formation of the cycloalkanes themselves by way of the protonation reaction (equation 15). 

These interactions may usefully be described as an acid-base type interaction, in which the cyclopropane ring acts as a base (electron donor) and the proton or cationic center acts as the acid (electron acceptor). One of the factors that controls the basicity of a hydrocarbon is the energy of the highest occupied molecular orbital (HOMO).60 The 6-31 G HOMO energies of some cycloalkanes and cycloalkenes are given in Table 4.61... 

It can be seen that the HOMO energy of cyclopropane is higher than that of cyclobutane or cyclohexane, and that the much more reactive bicyclo[1.1.0]butane has a much higher HOMO energy, which is close to that of propene. Another important factor is the polarizability, which reflects how easily the electron density may be shifted in the presence of an electric field (such as that developed by a proton). Here again, cyclopropanes have significantly higher polarizability than other cycloalkanes.52... 

The reactivity of 1 in substitution reactions is markedly different from that of other cycloalkanes. An electrophilic substitution of 1 is followed by opening of the three-membered ring to a 2-propyl cation. Therefore, protonation of 1 as the simplest electrophilic attack has been extensively investigated, both experimentally218 220 and computationally221"223. 

The shifts of the protons of alkanes and cycloalkanes fall in the range 0.9-1.5 ppm with C—H protons coming at the low-field end of this range and —CH3 protons coming at the high-field end (see Table 9-4). 

Although proton spectra are not very useful for identification purposes, 13C nmr spectra are very useful. Chain-branching and ring-substitution normally cause quite large chemical-shift changes, and it is not uncommon to observe 13C shifts in cycloalkanes spanning 35 ppm. Some special features of application of 13C nmr spectra to conformational analysis of cycloalkanes are described in Section 12-3D. 

Cracking and disproportionation in the reaction of hexane could be suppressed by the addition of cycloalkanes (cyclohexane, methylcyclopentane, cyclopentane).101 Furthermore, 3-methylpentane and methylcyclopentane also reduced the induction period. These data indicate that reactions are initiated by an oxidative formation of alkene intermediates. These maybe transformed into alkenyl cations, which undergo cracking and disproportionation. When there is intensive contact between the phases ensuring effective hydride transfer, protonated alkenes give isomerization products. 

Similar reactions have been investigated with a wide variety of alkanes.599 642 Cycloalkanes in particular give cyclic carboxonium ions along with protonated ketones. The reaction of cyclopentane is shown in Scheme 5.64. 

The enamine chemistry in Figure 12.19 can help to overcome this reactivity problem. The crucial enamine C is so electron-rich that it reacts electrophilically with a C=0 group of the adjacent cycloalkane dione moiety without the need for prior protonation to an oxocarbenium ion. This is just like the electrophilic reaction of the aldehyde on the enamine A in Figure 12.18. 

Cycloalkanes and Saturated Hetero-cyclics The chemical shifts of the CH2 groups in monocyclic alkanes are given in Table 4.7. The striking feature here is the strong shift to the right of cyclopropane, analogous to the shift of its proton absorptions. 

While examples corresponding to (a), (b) and the deprotonation case of (c) can be counted literally in thousands, the effect of substrate protonation in anodic oxidation is less well documented. However, amines and other nitrogen compounds have been thoroughly investigated on this point (Adams, 1969) and found to behave normally, but some recent work on anodic reactions in superacidic media has revealed a theoretically interesting exception to the rule. This concerns the anodic oxidation of alkanes and cycloalkanes in fluoro-sulphuric acid (Table 4, no. 9) with varying concentrations of added base, potassium fluorosulphate and/or acetic acid (Bertram et al., 1971, 1973). 

Similar reactions, illustrated in detail in Figs. 7 to 9, were suggested to account for the formation of the tritiated products in the protonation of 0-C4H8, C-C5H10, and C-C6H12 with HeT+ ions. The conclusions reached in the study of the gas-phase triton transfer to cycloalkanes from the HeT+ ions can be summarized as follows ... 

These conclusions, and especially the strong evidence for the existence of gaseous cycloalkanium ions, can usefully be compared with the considerable body of information on the protonated cycloparaffins, obtained from inass-spectrometric studies, kinetic investigations on the reactions of radiolytically formed ions with cycloalkanes, and the solution chemistry of cycloparaffins in strong Bronsted acids. 

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