Allyl cation relative stability

Although steric effects and substituent effects leading to carbonium ion stabilization are of greatest importance in governing the mechanism and relative rate of nucleophilic substitution processes, there are other substituent effects that are recognized and of importance. We have mentioned earlier in this chapter that arylmethyl and allylic cations are stabilized by electron delocalization. It is therefore easy to understand why substitution reactions of the ionization type proceed more rapidly in such systems than in simple alkyl systems. It has also been observed that nucleophilic substitutions of the direct displacement type also take place more readily, but the reason for this is not apparent. Allyl chloride is 33 times more reactive than ethyl chloride toward iodide ion in acetone, and benzyl chloride is 93... 

Experimental evidence concerning the relative stabilities of cation conformations is limited. The few examples known, however, strongly support our analysis. For example, the hydrolysis of a-methyl allyl chloride under Sn 1 conditions affords exclusively the trans cotyl alcohol, presumably via a transoid butenyl cation179) ... 

Perhaps more informative are the data in Table 22 for the relative rates of reaction of various cyclopropanones with furan. The results are consistent with the mechanism given in Scheme 32 where k >kz [furan], although the kinetic data do not distinguish between the closed cyclopropanone form and the zwitterionic intermediate. Increasing substitution would, of course, increase the stability of the allyl cation. 

A substantial number of substituted allylic cations were prepared using the reaction of SbF5 and fluoroolefin and characterized by NMR spectroscopy. Spectroscopic data along with data on relative stabilities of polyfluorinated allylic cations were summarized and thoroughly discussed in a comprehensive review [63]. Stability of polyfluorinated allylic cations bearing a substituent at central carbon decreases in the following order [63] ... 

The energetic effects of conjugation are largest when empty or half-empty p-orbitals interact with a 7r-system. Typical examples include allyl cations or allyl radicals, respectively. In these cases, the allylic stabilization was estimated to be 20 kcal/mol.26 In comparison, the effect on neutral, closed-shell molecules is relatively small. The conjugative effect on the rotation of 1,3-butadiene 2 is, for example, with 3 kcal/mol much smaller. 

The orientation of electrophilic addition to trifluoropropene was originally thought to be a reflection of the relative stabilities of the intermediate carbocations 4.15A and 4.15B (Figure 4.15), but it was subsequently found that trifluoropropene is dimerised, rather than protonated in highly acidic media [47, 48]. Deuterium labelling studies indicated that the reaction proceeds via initial fluoride ion abstraction to yield an intermediate allyl cation [49] (Figure 4.16). 

Removal of one electron should make no difference to the relative stabilities of polyene molecule ions or even electron polyene fragments as compared to their neutral counterparts, e.g. butadiene and the allyl radical should have the same relative stabihties as the butadiene molecule ion, and the allyl cation. Removal of one electron will, however, alter the stabihties, and thus the reactivities of cychc polyenes. The molecule ions of aromatic hydrocarbons will be substantially less aromatic then their neutral counterparts. Correspondingly the molecule ions of antiaromatic hydrocarbons will not be as antiaromatic as their neutral analogs, e.g. cyclobutadiene + should be relatively more stable than cyclobutadiene. The largest charge effects in hydrocarbons will be observed in nonaltemant ) monocychc hydrocarbons. The cyclopropenium ion 7 and the tropillium ion 2 are both strongly aromatic as compared to their neutral analogs. Consequently CsHs is a very common ion in the mass spectra of hydrocarbons while cyclopropene is not a common product of hydrocarbon pyrolysis or photo-... 

With larger bicyclo[n. 1. OJalkyl cyclopropyl derivatives such as 8 and 9, their solvolytic behavior follows from that of the simple alkyl-substituted cyclopropyl derivatives. With smaller bicyclo[n.l.0]alkyl cyclopropyl derivatives such as 10 and 11, however, where a trans-a y cation cannot be accommodated in the ring, the order of reactivity is reversed. In both the [6.1.0] and [3.1.0] examples mentioned above, the rates are given relative to cyclopropyl tosylate. The much higher reactivity of the endo-[3.1.Qi] system (11) over the endo-[6.1.0] system (9) reflects the stability of the almost strain-free cyclohexenyl allylic cation versus the cyclononenyl allylic cation which possesses both torsional and transannular strain. 

Carbocations that are adjacent to tt bonds, as in allylic and benzylic carbocations, are strongly stabilized by delocalization. The stabilization in the gas phase is about 60 kcal/mol for the allyl cation and 75 kcal/mol for benzyl ions, relative to the methyl... 

Because the allyl and benzyl cations have delocalized electrons, they are more stable than other primary carbocations. (Indeed, they have about the same stability as secondary alkyl carbocations.) We can add the benzyl and allyl cations to the group of carbocations whose relative stabilities were shown in Sections 4.2 and 6.4. 

Have a look at Figure 14-8 in the text At high temperature, an equilibrium mixture exists because there is enough energy for molecules to move from any location on the reaction coordinate to any other location on cl In other words, all three species—the two products and the intermediate allylic cation—are interchanging rapidly, and at any given time the relative quantities of each are governed by their relative thermodynamic stabilities. 

The number of electrons changes stability in a more complex way in three-center systems, i.e. the allyl and related species. In this case, delocalization of charge is much more important than delocalization of spin. For example, rotation around the C-C bond becomes much more difBcult in the allyl cation (-38 kcal/mol) compared to the allyl radical (-13 (calculated), 15.7 (experimental)kcal/mol). Allylic anions have a lower rotation barrier relative to the cation (-23 vs. -38kcal/mol). In the case of anions, additional stabilization to the twisted form (-8-14 kcal/mol) is provided by rehybridization, which partially offsets the lower efficiency of hyperconjugation in the twisted anion than in the twisted cation. The calculated barriers for the allyl system depend strongly on the methods employed, but the trend of cation > anion > radical remains. The same trend is observed for the rotation barriers in the benzyl radical and cation (Figure 3.10). ... 

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