Phenyl cation, high energy

Intersystem crossing (ISC) should be efficient for fragmentation to occur from the triplet in order to generate the phenyl cation with the same multiplicity if this condition is not fulfilled, sensitization with high-energy triplet donors can be used instead [13]. 

FIGURE 9. Reactions of chlorobenzene radical cations with ammonia (a) mass spectrum of the ions produced within the quadrupole collision cell (the intensity of the m/z 112 peak is reduced by a factor of 25), and (b) high energy (8 keV, nitrogen collision gas) of the so-produced m/z 94 ions. Spectrum (c) corresponds to the reactions of the m/z 11 phenyl cation, formed in the ion source, with ammonia (the intensity of the m/z 77 peak is reduced by a factor of 40)... 

The SN1 pathway requires formation of a carbocation. We saw earlier (Section 13-9) that alkenyl halides are unreactive toward displacement reactions. The Ss-2 pathway is poor for the same reason we stated above for benzene compounds. In addition, alkenyl cations are high-energy species because the placement of a positive charge on an -hybridized carbon is very unfavorable. The same issue arises in the case of benzene The phenyl cation is a high-energy species and forms only with difficulty because it contains an s/r-hybridized. positively charged carbon atom. 

Unlike secondary and tertiary alkyloxonium ions derived from alcohols, phenyloxonium derivatives do not dissociate to form phenyl cations, because such ions have too high an energy content (see Section 22-10). The phenyl-oxygen bond in phenols is very difficult to break. However, after protonation of alkoxybenzenes, the bond between the alkyl group and oxygen is readily cleaved in the presence of nucleophiles such as Br or U (e.g., from HBr or HI) to give phenol and the corresponding haloalkane. 

It is of interest to mention some detailed mass spectrometric results that were obtained by performing distinct experiments (a) ion-molecule reactions in mixtures of haloben-zenes and ammonia under chemical ionization conditions (b) ion-molecule reactions of mass-selected ionized halobenzenes toward ammonia in a quadrupole collision cell of a hybrid tandem mass spectrometer (c) ion-molecule reactions of phenyl diazonium cations with ammonia in the same quadrupole collision cell and, finally, (d) electrospray ionization of anilines in a hybrid quadrupole-time of flight mass spectrometer (QTof). Characterization of the product ions relies on collisional activation experiments in the low or high kinetic energy regime98. 

---