Carbocation center

One possible explanation is that adamantyl cation, an intermediate in the reaction, is particularly unstable because it cannot accomodate a planar carbocation center (see Chapter 1, Problem 9). Examine the geometry of adamantyl cation. Does it incorporate a planar carbocation center Compare electrostatic potential maps of adamantyl cation and 2-methyl-2-propyl cation. Which cation better delocalizes the positive charge Assuming that the more delocalized cation is also the more stable cation, would you expect adamantyl tosylate to react slower or faster than tcrf-butyl tosylate Calculate the energy of the reaction. 

Compare electrostatic potential maps for ethyl, 2-propyl, 2-methyl-2-propyl and 2-butyl cations. Does the extent to which positive charge is localized at the carbocation center parallel proton affinity Explain. 

Hyperconjugation, as it i termed, implies that the electron pair associated with out-of-plane CH bond is donated into the empty p Orbital at the carbocation center. 

What is the preferred geometry about the radical center in free radicals Carbocation centers are characterized by a vacant orbital and are known to be planar, while carbanion centers incorporate a nonbonded electron pair and are typically pyramidal (see Chapter 1, Problem 9). 

On the other hand, if the single substituent can stabilize an adjacent carbocation center following protonation of the alkene, then reduction may occur. 

The cyclopentyl cation (39) undergoes a rapid degenerate rearrangement which can be frozen out at cryogenic temperatures as shown by solid state CPMAS 13C NMR spectra.57 MP2/6-31G(d,p) calculations show that cyclopentyl cation has a twisted conformation 4058 in which the axial hydrogens are bend toward the carbocation center. This is due to the pronounced geometrical distortion caused by the hyper-conjugative interaction of the /i-cr-C-H-bond with the formally vacant 2pz-orbital at the C+ carbon of this secondary carbocation. 

The results suggest that chinoid type structures are the predominant resonance contributors for 88. The IGLO/DZ//3-21G calculated 13C NMR chemical shifts of benzylic monocations 88 correlate reasonably well with the experimentally obtained data. The 13C NMR chemical shifts of the carbocation centers (CH2 carbon) are calculated 10.6-12.5 ppm too deshielded. Similar results were obtained for benzylic dications 89. NMR chemical shifts of arenium ions derived from various classes of polycyclic aromatic hydrocarbons have been calculated using GIAO-DFT methods.103... 

This is clearly illustrated in the example given below for the reaction of the methoxymethyl cation with pivaldehyde. There are four possible reaction channels for the unimolecular dissociation of the initially formed adduct, which is presumed to be of the form of a covalent species resulting from the interaction of the carbocation center of the methoxymethyl cation with the carbonyl oxygen of the aldehyde, 2. For smaller carbonyl compounds this adduct has been demon-... 

Due to the 18-CH3 —> 17-CH3 shift, a carbocation centered at C13 is formed, and further 14a-H elimination originates the A13-double bond. 16p-Epimers can be formed due to an acid-catalyzed retro-aldol equilibrium involving the 16-hydroxy-20-keto function of the rearranged steroid, under the reaction conditions employed, which is responsible for the epimerization at C16, as previously discussed by Herzog et al. [125, 126] and reviewed by Wendler (Scheme 34) [111]. 

The expressions (Eqs. 5-34 and 5-42) for Rp in cationic polymerization point out one very significant difference between cationic and radical polymerizations. Radical polymerizations show a -order dependence of Rp on while cationic polymerizations show a first-order depenence of Rp on R,. The difference is a consequence of their different modes of termination. Termination is second-order in the propagating species in radical polymerization but only first-order in cationic polymerization. The one exception to this generalization is certain cationic polymerizations initiated by ionizing radiation (Secs. 5-2a-6, 3-4d). Initiation consists of the formation of radical-cations from monomer followed by dimerization to dicarbo-cations (Eq. 5-11). An alternate proposal is reaction of the radical-cation with monomer to form a monocarbocation species (Eq. 5-12). In either case, the carbocation centers propagate by successive additions of monomer with radical propagation not favored at low temperatures in superpure and dry sytems. 

Y is either a solvated electron (displaced electron formed during the radiolytic reaction) or the product of the electron having reacted with some compound in the reaction system [Allen et al., 1974 Hayashi et al., 1967 Kubota et al., 1978 Williams et al., 1967]. If Y is an electron, the propagating carbocation centers are converted to radical centers that subsequently undergo reaction with some species in the reaction system to form molecular species. The termination rate is given by... 

This reaction is referred to as end capping or end blocking. The result is that reactive carbanion or carbocation centers do not form and depolymerization does not occur at the ceiling temperature of the polymer. The polymer chains are end blocked from depolymerization. The effective ceiling temperature is increased considerably above the ceiling temperature. Acetic anhydride is the usual capping reagent. 

Spiroorthocarbonates and spiroorthoesters are stable toward bases and nucleophiles but easily undergo cationic ROP. Cationic ROP of a spiroorthocarbonate (LXX) proceeds by initial formation of carbocation LXXI in which the carbocation center is stabilized by... 

The presence of fluorine strongly destabihzes a carbocation centered on the jS carbon because only the inductive effect takes place. " The effect on solvolysis or protonation reaction of double bonds can be very important. The destabilization of carbenium and alkoxycarbenium ions plays an importantrole in the design of enzyme inhibitors (cf Chapter 7) and in the hydrolytic metabolism of active molecules (cf. Chapter 3). 

This is a close analogy to what is commonly known by organic chemists as hyperconjugation, where an electron-deficient carbocation center grabs electrons from a neighboring CH bond. 

In this case, only in the planar arrangement may electrons be transferred from the filled k orbital on benzene to the empty orbital associated with the carbocation center. This leads to delocalization of the positive charge, which in turn contributes to the high stability of the planar cation. 

A new method for the synthesis of 2-substituted, as well as 2,4- and 2,5-disubstituted, cyclopentanones in 53-93% yield has been reported.81 For example, the Lewis acid catalyzed transformation of l-propanoyl-l-(4-tolylsulfanyl)cyclobutane gave 2-ethyl-2-(4-tolylsulf-anyl)cyclopentanone (1) in 93 % yield. The formation of the cyclopentanone is best explained by a mechanism which involves initial coordination of aluminum trichloride to the carbonyl oxygen, followed by ring expansion to form the sulfur-stabilized carbocation. Finally, migration of the ethyl group to the carbocation center regenerates concomitantly the carbonyl function.81... 

HIAs is only O.lkcalmol-1. This reflects almost complete cancellation of contributions from the extra CH2 group in the butyl structure between the cation and hydrocarbon. It indicates that HIAs provide a good approximation to differences in stability between a carbocation center and the corresponding group contribution from a hydrocarbon, independently of structural variations at carbon atoms not attached to the carbocation center. Moreover, a comparison between two secondary carbocations leads to almost complete cancellation of the contributions from the parent hydrocarbons and from alkyl groups of the carbocations too far removed from the charge center to influence stability. One is very close therefore to a comparison of stabilities comparable to that between isomeric cations. It should be noted that such intrinsic stabilities are not expressed in heats of formation of carbocations because they include uncanceled contributions from more remote portions of the structure. 

Also shown in Table 1 are differences in pAiR. These are multiplied by 1.364 to give free energies for easier comparison with HIAs. They correspond to intrinsic differences between tertiary, secondary, and primary carbocation centers (CH+, CH2+, and CH3+) and the corresponding values for the carbon bound to an OH functional group (C-OH, CH-OH, and CH2-OH). In principle, carbocation stabilities may be expressed relative to any functional group, but clearly the convenience and prevalence of measurements of pAiR give a special place to the OH group. 

Of course, the intrinsic stabilities, and the evidence for localization of the influence of carbocation centers, apply only to aliphatic ions. Phenyl or vinyl substitutents lead to extended delocalization of a positive charge. More generally, however, cancellations of group contributions between reactants and... 

Stabilization conferred by aromatic hyperconjugation resolves a puzzle concerning the relative stabilities of arenonium ions. As judged by rates of solvolysis reactions, normally a phenyl group is more effective than vinyl in stabilizing a carbocation center.166 This difference is moderated for cycloalkyl substrates, so that benzoannelation has little effect, for example, on the rate of hydrolysis of 3-chlorocyclohexene (Cagney H, Kudavalli JS, More O Ferrall... 

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