Ethyl cation bridged

The first attempt to detect a bridged ethyl cation in solution was that of Roberts and Yancey in 1952. These workers showed that deamination of ethylamine-l- C in aqueous solution yields ethylene and 38% ethanol that contains 1.5% ethanol-2-Clearly the bridged ion is not an important intermediate in this reaction. It has also been shown that solvolyses of ethyl-l- C toluene-p-sulfonate in acetic acid, formic acid, and 75% dioxane-water and of specifically deuterated ethyl toluene-p-sulfonate in trifluoroacetic acid and 96 % sulfuric acid " proceed without appreciable rearrangement. It seems that bimolecular solvolysis is the predominating reaction in all these solvents. However, solvolysis of deuterium-labeled ethyl toluene-p-sulfonate in fluorosulfuric acid yields a product with 30-40 % rearrangement, " and it is possible that a bridged ion is an intermediate in this reaction. 

Structures for the norbornyl cation. A. The classical carbenium ion form. Two views of the computed structure are given along with the LUMO. Note the extensive contribution of the C1-C6 bond to the LUMO. B. The bridged non-classical structure. A structure is given, along with the LUMO and HOMO. Note the similarity of these MOs to tho.se of the bridged ethyl cation (Figure 1.25). 

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.)...

Fig. 5.17 The ethyl cation problem at various levels. At the three Hartree-Fock levels the classical cation is a minimum, but at the post-Hartree-Fock (MP2/6-31G ) level only the symmetrical bridged ion is a minimum. The HF/6-31G results are calculations by the author (ZPE ignored), the other three levels are taken from ref. [75]...

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... 

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