Benzyl ion

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. 

As already indicated in the preceding schemes, the resonance-stabilized benzyl ion, [CvHv]", initially formed by benzylic bond cleavage reversibly isomerizes to the tropylium and tolyl ion isomers. The isomerization of [CvHv]" ions has been the subject of numerous studies, [50] revealing the tropylium ion as the thermodynamically most stable isomer. [51,52]... 

In alkyl-substituted aromatic compounds, cleavage is very probable at the bond /3 to the ring, giving the resonance-stabilized benzyl ion or, more likely, the tropylium ion ... 

Any substituted benzyl- ions formed in the course of the reduction will yield eventually a polystyrene, and indeed, a small amount of polymer was found in the reduction products of styrene (17). However, the reduction of compounds which give radicals of higher electron affinity leads to a substantial amount of carbanions. i.e. with those compounds the electron-transfer to a radical competes efficiently with a hydrogen transfer from NH2, e.g. 1,1-diphenyl ethylene gives Pl C-CH3 ion under conditions which yield ethyl benzene from styrene (17). 

The interesting explanation offered by Wexler for this discrepancy is based on a striking parallel with the behaviour of the isomeric deuteriated toluenes under electron impact. The mass spectrometric work by Meyerson and Rylander (1957), Rylander et al. (1957) and by Meyerson et al. (1968), provided convincing evidence that the isomeric tolyl and benzyl ions produced by electron impact rearrange, before fragmentation, to a common seven-membered ring structure, the tropylium ion, which... 

Aryl-2-chloro-2,3-dimethylbutanes form both benzylic and equilibrating ions upon ionization. The benzylic ions are destabilized due to severe steric crowding in the planar conformation necessary for optimal charge delocalization. 

The mass spectra and pyrolysis of alkyl resorcinols are also subject to nonquantitative correlations. 3 > Lille and Kundel reported that the mass spectra of 2, 4 and 5 alkyl (C4 or larger) resorcinols were all dominated by jS-cleavage (benzylic) ions. The pyrolysis of the 2 or 4 alkylresorcinols at 700 °C was consistent with their mass spectra however, pyrolysis of 5-alkylresorcinols gave in addition to the anticipated / -cleavage products, 5,7-dihydroxyindene or an alkenylresorcinol. The yield of the unique olefin was of the same order as the peld of the 5-methylresorcinol. The basis for this reactivity discrepancy may be the scrambling of positions in the resorcinol on electron impact. 

In this example, ionization is favored by several factors. First, the benzyl ion formed is resonance-stabilized and bears a methoxy group in the para position that can further stabilize the cation by resonance. In addition, the leaving group is triflate, an exceptionally good leaving group. Finally, the ionization takes place in water, a polar solvent that can stabilize the two charged species formed. 

Fig.lc shows the distribution of the minor products as a function of pressure. Light hydrocarbons (Cs-Cg) are olefinic (OL) at atmospheric pressure, but at 10 bar are mainly paraffins (PAR) and at 40 bar the olefins are totally absent. At elevated pressure the carbonium ions (C4+) formed by sBB dealkylation, can interact with the reactant, which is present in a high concentration, and promote an external hydrogen transfer leading to the formation of benzylic ions and paraffins. Two afkylbenzenium ions can thus react together to form, through a biphenylalkane intermediate, the disproportionation products (di-sec-butylbenzene) and benzene. 

The disproportionation reaction deserves particular attention. It can take place with a Sn 1 and/or Sn2 mechanism. The SnI mechanism involves the dealkylation of a molecule of the reactant, with formation of a R+ carbonium ion, which can alkylate a second molecule of reactant, forming a dialkylate. The Sn2 mechanism involves the intervention of two molecules of reactant, interacting with formation of a biphenylalkane intermediate. The latter then reacts further to benzene and dialkylate. Different hypotheses have been formulated to explain the formation of the biphenylalkane intermediate. The reactant can be activated by very strong protonic sites [10] and/or by carbonium ions [11]. Both activators would promote an extern hydrogen transfer, leading to the formation of a benzylic ion. 

Thus, the reaction with aromatic aldehydes is second order in aldehyde and first order in hydroxide ion,31 and no deuterium becomes attached to carbon in the alcohol fragment when the reaction is carried out in deuterium oxide solution.32 It is interesting that when the reaction is carried out with benzaldehyde in the cold and in the absence of excess alkali, benzyl benzoate has been isolated.33 Although the point has not yet been settled, it seems probable that the ester is formed by a secondary reaction between the benzylate ion which is formed initially (XXIII) and two molecules of benzaldehyde 2M0... 

If, however, the tertiary carbocation is not benzylic, rearrangement to a secondary, benzylic ion can be observed ... 

Figure 19 shows the LAMMA 1000 positive spectra of Irganox 1010 (Clba Geigy a high molecular weight (1176), multifunctional anti-oxidant and thermal stabilizer. The LAMMA matches the EI spectra well, with the LAMMA giving additional information the t-butyl, phenyl and benzyl ions suggest an aromatic compound. 

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... 

Rates that are independent of aromatic substrate concentration have been found for reaction of benzyl chloride catalyzed by TiC or SbF in nitromethane. This can be interpreted as resulting from rate-determining formation of the electrophile, presumably a benzyl ion pair. The reaction of benzyl chloride and toluene shows a second-order dependence on the titanium chloride concentration under conditions where there is a large excess of hydrocarbon. This is attributed to reaction through a 1 2 benzyl chloride-TiC complex, with the second TiC molecule assisting in the ionization reaction ... 

If we have the mass spectrum of a molecule containing an aromatic ring, then we should expect to find a peak from a very stable ion, the benzyl ion, PhCH2, provided the structure permitted this ion to form. An unsubstituted benzyl peak is found at m/z 91 substitution will obviously change the mass of the ion. Formation of a benzyl ion is shown in the next spectrum. Figure 2.4, which shows the mass spectrum of propylbenzene, PhCH2CH2CH3. 

The spectrum shows a large molecular ion peak at m/z 120, and a single large peak at m/z 91, resulting from the loss of 29 Daltons, consistent with the loss of C2H5. The peak at m/z 91 is the benzyl ion, and the molecule shows a single fragmentation ... 

If an aromatic molecule cannot form a benzyl ion, then the phenyl ion CeHs, m/z 77, may be formed, but formation of the benzyl ion is favoured. 

Substituted aromatics such as shown in Equation 16.24 fragment preferentially by loss of R to give the stable benzyl ion, which is known to rearrange to the symmetrical tropylium ion A)... 

The above regularities are mainly retained in the halogenation of tetrafluorobenzo-dihydrobarrelene 306 as well, but in this case the processes with the participation of the bond (k ) and that of the reagent (k,) compete the latter route results in trans-dibromides with the same skeleton " . In ion 307 the skeletal 1,2-shift of the 1-7 bond formating the benzylic ion 308 competes with the capture by a nucleophile ion 308 yields non-stereospecifically formed products 309 and 310,... 

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