Carbonium ions distance

When alkylating isobutane, chain tennination forms primarily, but not entirely, 2,2,4-trimethylpentane the alkylate from chain termination very closely resembles isobutene alkylate. The similarity of alkylate compositions, particularly their C0 fractions, originating from various olefins and the distance from thermodynamic equilibrium composition indicates that alkylate molecules, once formed, are relatively stable under alkylation conditions and undergo little isomerization. Undesirable side products, e.g., dimethylhexanes and residue, are probably formed by buter e isomerization and polymerization (rather than by isomerization of alkylate or by isomerization of the C3 carbonium Ion which subsequently becomes alkylate). 

Calculations of the effectiveness of overlap indicate slight stabilisation of a C(3) carbonium ion by the sz-electrons even at the normal C<3) C(5) distance of 2.5 A, with maximum stabilisation (by 4-5 kcal/mole) at a distance of 2.1 A, when resonance energy and structural strain are balanced [33]. It seems likely that this corresponds approximately to the geometry of the mesomeric cation. 

A potentially anti-homoaromatic species (79, see Table 16) has been generated by protonation of l,6-methano-[10]annulene at the 2-position 224>. As expected, 1,3 overlap is not significant and the compound can be formulated as a classical carbonium ion. In fact the abnormally large H(2a) to H(2a) coupling constant of —24.9 Hz indicates that the 1,3 distance has been maximized in order to avoid the expected destabilization, and thus there is no evidence for a paramagnetic ring current in this system. 

Fig. 10. Models proposed for proton translocation (Model A) single proton translocation with carbonium ion formation. (Model B) concerted double proton translocation with retroretinal formation, (c) Tunneling potential energy barriers for formation of prelumirho-dopsin. represents the energy barrier and cIq the width or translocation distance. 

When alkanes are dissolved in superacids, they are transformed into various products [52]. In protic media, the first step of the reaction is the protonation to form in the case of methane, the methonium ion, CHs. For the theoretical description of such a structure see refs. [53]. The ground state of this ion has symmetry which may be regarded as a complex of CHi cation with Hi molecule (H-H distance has been found to be 0.936 A which suggests formation of a bond). The ground state is only insignificantly different ca. 1 kcal moP ) from the excited state of C4v symmetry. Even closer are two transition states of other symmetries [53c], This finding corresponds to extremely fast hydrogen atom movements in CH5 and other similar carbonium ions therefore, fast isomerization, e.g., epimerization of alkane molecules, is expected to proceed in... 

AE° + dQZae /ed that is in agreement with the directly measured experimental value of the corresponding carbonium ion. It would appear that the crystallographic distance closely approximates the distance in solution for NRBA salts. 

---