2-Propenyl cation

In contrast, /1-silyl-substitution is predicted by the calculations to be far more stabilizing than /1-alkyl substitution. Thus, the isodesmic equation 3 predicts for 9 a stabilization by the /1-silyl group of 38 kcalmol-1, while the /1-methyl substitution in 10 gives only a stabilization of 28 kcalmol-1 [MP3/6-31G(d)//3-21G(d)]5. The stabilization by the silyl substituent is markedly orientation-dependent. Thus, the perpendicular conformation of the /1-silylethyl cation 9p is higher in energy by 29.6 kcalmol-1 compared with the bisected conformation 9 [MP3/6-31G(d)//3-21G(d)]5. The open /1-silyl-substituted vinyl cation 11 is lower in energy by 28.6 kcalmol-1 and 20.5 kcalmol-1 [MP3/6-31G(d)//3-21G(d)] compared with the vinyl cation (equation 4, R = H) and the 1-propenyl cation (equation 4, R = Me), respectively5. 

Because of the preferred linearity of vinyl cations, the stability of cyclic vinyl cations depends on the ring strain or the ring size. Rates of solvolysis of Inflates show that the 1-cyclohexenyl cation is 6 X 10 -fold less readily generated than the 1-methyl-1-propenyl cation. The 1-cyclopentenyl cation is not formed at all. On the other hand, 1-cyclobutenyl triflate is 3700 times more reactive than 1-cyclohexenyl triflate.The stability of the cyclobutenyl cation is due to resonance involving cyclopropyl-stabilization of the positive charge. ... 

Both allenes141 and alkynes142 require special consideration with regard to mechanisms of electrophilic addition. The attack by a proton on allene might conceivably lead to the allyl cation or the 2-propenyl cation. 

As a result, protonation both in solution143 and gas phase144 occurs at a terminal carbon to give the 2-propenyl cation, not the allylic cation. 

Ground and transition state Gibbs energy relationships in cationic intermediate resulted in the formulation of the nonclassical 7-norbornenyl (bishomocyclo-propenyl) cation (6).2... 

It is strong evidence for Huckel s rule that 52 and 53 are not aromatic while the cyclo-propenyl cation (47) and the cyclopentadienyl anion (31) are, since simple resonance theory predicts no difference between 52 and 47 or 53 and 31 (the same number of equivalent canonical forms can be drawn for 52 as for 47 and for 53 as for 31). 

Propadiene adds hydrogen chloride to yield 2-chloropropene. However, the possibility exists that initial attack of a proton might lead to the 2-propenyl cation (Section 6-6), which then would react with chloride ion to form 3-chloropropene. Using the rules for application of the resonance method (Section 6-5B) and the atomic-orbital model for 1,2-propadiene (Figure 13-4), rationalize why a 2-propenyl cation might not be formed easily by addition of a proton to 1,2-propadiene and why 2-chloropropene is the observed product. 

MO treatment of the 2-propenyl cation begins with the atomic-orbital model 22 ... 

Figure 21-9 Energies and schematic representations of the it molecular orbitals of the 2-propenyl cation, 22. The calculated jr-electron energy of the cation is 2 a + 1.41/3) = 2a + 2.82/3. Orbitals with the energy a are neither bonding nor antibonding and are called nonbonding molecular orbitals (NBMO). The intermediate energy molecular orbital of the 2-propenyl cation is a NBMO because its component atomic orbitals, being on C1 and C3, are too far apart to be bonding or antibonding.

Of these, the methonium cation, CHr> , is formed in largest amounts,8 the ethyl cation, C2H5 , is next, and there is a smaller amount of the C3HS cation (2-propenyl cation Section 8-7B). These ions then react with the substance... 

The propenyl cation, Fig. 4.19b cf. Fig. 4.16. If we take the total % electronic energy of a molecule to be simply the number of electrons in a n MO times the energy level of the orbital, summed over occupied orbitals (a gross approximation, as it ignores interelectronic repulsion), then for the propenyl cation... 

We want to compare this energy with that of two electrons in a normal molecule with no special features (the propenyl cation has the special feature of a formally empty p orbital adjacent to the formal C/C double bond), and we choose neutral ethene for our reference energy (Fig. 4.15)... 

Since jl is negative, the re-electronic energy of the propenyl cation is calculated to be below that of ethene providing an extra, formally empty p orbital for the electron pair causes the energy to drop. Actually, resonance energy is usually presented as a positive quantity, e.g. 100 kJ mol-1 . We can interpret this as 100 kJ mol-1 below a reference system. To avoid a negative quantity in SHM calculations like these, we can use IjSI instead of />. 

The SHM predicts the propenyl cation, radical and anion to have the same resonance energy (stabilization energy). Actually, we expect the resonance energy to decrease as we add ji electrons why should this be the case ... 

Addition of various hydrogen halides to propyne has been investigated by Griesbaum et al. (1965) (la-c of Table 2). The product distribution parallels that obtained from allene under identical conditions and the most likely mechanism involves formation of a propenyl cation which may react with halide ions to give the expected addition products or cyclodimerize to 1,3-dihalocyclobutane (see discussion in section II, Bib). 

Alkenyl (vinyl), aryl, and alkynyl carbocations are particularly unstable with respect to alkyl carbocations. Let s compare the isopropyl cation with the iso-propenyl cation. In the latter, the central C has two cr bonds, one it bond, and one empty orbital, so it is sp-hybridized (linear). Both ions are stabilized by the C(sp3)-H cr bonds of the CH3 group on the right. In the isopropyl cation there is an additional interaction with C(sp3)-H cr bonds on the left, whereas in the isopropenyl cation there is an additional interaction with C(sp2)-H cr bonds on the left. Because C(sp2) orbitals are lower in energy than C(sp3) orbitals, the... 

Interesting data have been obtained from the solvolysis of alkyl-substituted bicyclo[3,l,0]hexanes The high solvolytic activity of cis-epimers in comparison with trans-epimers, the stereochemistry of this reaction and the equilibration of deuterium in the products testify to the intermediate formation of trishomocyclo-propenyl cations upon acetolysis of cis-raonomethyl- and dimethylbicyclo[3,l,0]-hexyl-3-tosylates. 

The literature describes several intermediate formations of the trishomocyclo-propenyl cation from cyclopropane rings with a distance of more than one C atom from the ionization centre Sargent has solvolysed allyUc dinitrobenz ate... 

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