Benzyl-cation

Reactions involving benzylic cations benzylic radicals and aUcenylbenzenes will be dis cussed m Sections 11 12 through 11 17... 

The positive charge m benzyl cation is shared by the carbons ortho and para to the benzylic carbon... 

Most stable Lewis structure of benzyl cation... 

Although this cleavage is probably driven by the stability of benzyl cation evidence has been obtained suggesting that tropyhum cation formed by rearrangement of benzyl cation IS actually the species responsible for the peak... 

The intermediates in these base-catalyzed reactions are beheved to be of the nature of a benzyl cation because the reaction product from toluene and... 

Computational comparison of structures of the benzyl cation (A) and singlet phenylcarbene (B) indicates a much greater double-bond character for the exocyclic... 

The diarylmethyl cations listed in Table 5.1 are 6-7pATr+ units less stable than the corresponding triarylmethyl cations. This indicates that the additional aryl group has a cumulative, although not necessarily additive, effect on stability of the carbocation. Primary benzylic cations (monparylmethyl cations) are generally not sufficiently stable for determination of pATr+ values. A particularly stable benzylic ion, the 2,4,6-trimethyl-phenylmefliyl cation, has a pATr+ of — 17.4. 

The Yukawa-Tsuno r values have been measured for the solvolysis reactions fonning benzyl cations and several a-substituted derivatives, 6-3IG charges and bond orders have been calculated for the presumed cationic intermediates. Analyze the data for relationships between r and the structural parameters. (Hint. Plot r versus the bond orders and the charges at C-1, C-2, C-3, and C-4.)... 

Rates that are independent of aromatic substrate concentration have been found for reaction of benzyl chloride catalyzed by TiCl4 or SbFj in nitromethane. This can be interpreted as resulting from rate-determining formation of the electrophile, presumably a benzyl cation. The reaction of benzyl chloride and toluene shows a second-order dependence on titanium tetrachloride concentration under conditions where there is a large excess of hydrocarbon. ... 

Addition of carbon and fluorine can also be initiated by elettraphilic attack on a/luorinated otefin under strongly acidic conditions [250, 251, 252,253 254 255] Best known are fluoroalkylations ot tetrafluoroethylene by tertiary or highly halogenated allylic or benzylic cations in the presence of antimony pentafluoride (equation 53)... 

See Learning By Modeling for an electrostatic poten-tial map of benzyl cation. 

Draw all reasonable resonance contributors for both planar and perpendicular conformers of benzyl cation. Identify the site(s) of the positive charge in each. Which cation would you expect to be more stable Which is the more stable Compare energies of planar and perpendicular conformers of benzyl cation. 

Electrostatic potential map for planar benzyl cation shows most positively-charged regions (in blue), and less positively-charged regions (in red). 

We saw in Section 6.9 that the stability order of alkyl carbocations is 3° > 2° > 1° > —CH3. To this list we must also add the resonance-stabilized allvl and benzyl cations. Just as allylic radicals are unusually stable because the... 

FIGURE 2.8 Energy levels for the benzyl cation, free radical, and carbanion. Since a is the energy of a p orbital (p. 35) the nonbonding orbital has no bonding energy. 

A number of benzylic cations have been obtained in solution as SbFg salts, Diarylmethyl and triarylmethyl cations are still more stable. Triphenylchloro-methane ionizes in polar solvents that do not, like water, react with the ion. In SO2, the equilibrium... 

However, a number of examples have been found where addition of bromine is not stereospecifically anti. For example, the addition of Bf2 to cis- and trans-l-phenylpropenes in CCI4 was nonstereospecific." Furthermore, the stereospecificity of bromine addition to stilbene depends on the dielectric constant of the solvent. In solvents of low dielectric constant, the addition was 90-100% anti, but with an increase in dielectric constant, the reaction became less stereospecific, until, at a dielectric constant of 35, the addition was completely nonstereospecific.Likewise in the case of triple bonds, stereoselective anti addition was found in bromination of 3-hexyne, but both cis and trans products were obtained in bromination of phenylacetylene. These results indicate that a bromonium ion is not formed where the open cation can be stabilized in other ways (e.g., addition of Br+ to 1 -phenylpropene gives the ion PhC HCHBrCH3, which is a relatively stable benzylic cation) and that there is probably a spectrum of mechanisms between complete bromonium ion (2, no rotation) formation and completely open-cation (1, free rotation) formation, with partially bridged bromonium ions (3, restricted rotation) in between. We have previously seen cases (e.g., p. 415) where cations require more stabilization from outside sources as they become intrinsically less stable themselves. Further evidence for the open cation mechanism where aryl stabilization is present was reported in an isotope effect study of addition of Br2 to ArCH=CHCHAr (Ar = p-nitrophenyl, Ar = p-tolyl). The C isotope effect for one of the double bond carbons (the one closer to the NO2 group) was considerably larger than for the other one. ... 

---