Ethyl cation bridged structure

The 3- and 4-heptyl cations cannot form a 1,6-p-H-bridge. The 3-heptyl cation could in principle form a primary-secondary 1-5-p-H-bridged structure with an ethyl group at the secondary cation terminus. 

Pfeiffer and Jewett (1970), however, have made ab initio calculations on the ethyl cation and report the charge distributions in Figure 4b for the most stable ethyl ion. Their calculations agree with Hoffmann s in predicting that the classical ethyl structure is more stable than a bridged structure, but their calculated charge distribution is entirely different. 

The C—H—C bond is not linear, the angle being about 170° according to high-level MO calculations. Several bridged cycloalkyl carbocations of the type 2 have been prepared [236]. Complexes between a number of alkyl cations and alkanes have been detected in mass spectrometric experiments [235]. The nonclassical structure of the ethyl cation, 3, may be cited as another example of hydride bridging (for a discussion, see ref. 55). 

Fig. 22. CCSD (tzp, spherical) optimized structures for the bridged and classical forms of the ethyl cation used in NMR calculations. (Reprinted with permission from Ajith Perera et a . (123). Copyright 1995 American Chemical Society.)...

Halo-substituted ethyl cations are stabilized with respect to the ethyl cation [D-(Et+—H ) = 271 kcalmol-1] by an extent which depends on their structure85. Ions formed via X-loss from the neutral precursor CH3CHX2, which presumably have structure 20, are stabilized, relative to Et+, by 12 and 13 kcalmol-1 for X = Cl and Br, respectively. Ions formed via X-loss from XCH2CH2X, for which structure 22 could be anticipated, are stabilized by 10, 13 and 21 kcalmol-1 for X = Cl, Br and I, respectively85. For these latter species, however, the authors note that the observed stabilization values are more consistent with the bridged structure 21111 rather than with 22. The presence of the X substituent on a carbon atom not bearing the charge, as in 22, should produce only a limited stabilization in the case of chlorine and even a small destabilization in the case of... 

We recall that ethyl cation has a bridged, nonclassical structure, and that the classical CH3CH2+ is calculated by high level quantum chemical calculations (36) to be ca. 6 kcal mol 1 less stable. Combining this difference with the experimentally measured heats of formation of ethyl cation and of the neutral ethyl radical, we derive the "classical" ionization potential of CH3CH2 to be 8.4 eV = 193 kcal mol"1. Consider now the geminally-fluorinated ethyl radicals CH3CF2 ,... 

---