Ethyl cation structure

Examine the structure of 1-phenyl-1-ethyl cation. Is it chiral Examine the LUMO. Would you expect the cation to give a racemic mixture of alcohols or the mixture that is actually obtained Explain. 

Next, examine the structure of 1-phenyl-1-ethyl cation-chloride anion, an ion pair that is initially generated. What evidence is there for cai bon-chlorine bond cleavage Examine the electrostatic potential map for the ion pair. Which face of the cation is more available for attack How could the other enantiomer form ... 

Non-classical structures are predicted to be unstable relative to classical structures (for example ethyl cation). 

The equilibrium between a and b in Eq. (2) depends on the energies of both the structures. In Table 1 the relative energies of the ethyl cation in the structures a and b, calculated with different methods, are shown. 

The comparative study of the experimental NMR spectra of Zs-1-cyclopropyl -2-(triisopropylsilyl)ethyl cation (17) and the computational model structure E-1-cyclopropyl-2-(trimethylsilyl)ethyl cation (18) demonstrates another application of calculations of 3H and 13C NMR chemical shifts and nuclear spin-spin coupling constants. In particular vicinal3./(11,11) spin-spin coupling constants are useful for... 

The experimental H, 13C (Figure 2) and 29Si- (Figure 3) NMR spectra show the exclusive formation of l-phenyl-2-(triisopropylsilyl)ethyl cation 12 and give proof of its structure and conformation (75). 

The experimental 3J(HH) spin-spin coupling constants for Ha, Hp, and Hp for the -isomer 22 were quite satisfactorily reproduced (A = 0.1 - 1 Hz) by calculations, using a finite perturbation method (FPT level (26), Perdew/IGLO-III at a MP2/6-31G(d)) geometry for the model structure (E)-1 -cyclopropyl-2-(trimethylsilyl)ethyl cation. The calculations confirm the /rexperimentally observed carbocation 22 (20, 27, 28). The (E)- 1-cyclopropy 1-2-... 

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. 

Many mechanistic implications have been discussed, but we will concentrate here only on the most important structures in the context of dihydrogen-cation complexes. Deuterium-labeled methane and methyl cations were employed to examine the scrambling and dissociation mechanisms. The protonated ethane decomposition yields the ethyl cation and dihydrogen. Under the assumption that the extra proton is associated with one carbon only, a kinetic model was devised to explain the experimental findings, such as H/D scrambling. ... 

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.)...

These points have been pursued in detail for two reasons. The first is to indicate the level of uncertainty in deriving pATas when the rate of deprotonation falls significantly short of its relaxation limit and the structure-reactivity correlation for the alkene conjugate base of the cation is insufficiently defined. The second is that the identity of the rate constants for 2-propene and 2-butene still imply a difference of 0.3 log units between 2-propyl and 2-butyl cations. In so far as this difference corresponds with the small difference in geminal interaction of the OH groups, the implication is that as measured by their HIAs the two ions have the same stability (cf. discussion on p. 25). In conclusion, the preferred pATR for the 2-propyl cation is listed below with the more secure values for the /-butyl and ethyl cations. 

Conroy-Lewis, F.M., Mole, L., Redhouse, A.D., Litster, S.A. and Spencer, J.L. (1991) Synthesis of coordinatively unsaturated diphosphine nickel(II) and palladium(II) p-agostic ethyl cations X-ray crystal structure of [Ni[tert-Bu2 P(C H 2) 2 PBu-tert2] (C2H5)][BF4]. /. Chem. Soc., Chem. Commun., 1601. 

The 1 Ai state of EH2 with E = C to Pb has been calculated in the context of theoretical studies by Trinquier which focused on the structures and isomers of the heavier analogues of the ethyl cation E2H5- 66. The author reports calculated proton affinities of ( A1) EH2 and E2H4 which are discussed below in Section IV.A.3. 

Symmetry is another factor to affect Tm. The salts with symmetric ions generally show higher Tm than those with asymmetric ones. For example, 1,3-dimethylimidazolium tetrafluoroborate showed higher Tm than 1-methylimi-dazolium or l-ethyl-3-methylimidazolium salts, as shown in Figure 3.1. In the case of tetraalkylammonium salts, their Tm also increased with increasing symmetry of the cation structure [18]. This tendency is understood to relate to the structural effect on crystallinity [19], i.e., highly symmetric ions are more efficiently packed into the crystalline structure than unsymmetric ones. Other kinds of chain structures such as polyether [20], perfluorocarbon [21], etc. [22] are obviously also effective in influencing thermal properties. 

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... 

---