Polycyclic Arenium Ions

The electronic absorption spectra of the ions from protonation of naphthalene and its derivatives are listed in Table 36. The molecular-orbital calculations according to the SCF method have led to the conclusion that the spectra correspond to the ions resulting from the addition of the proton at the a-position of the naphthalene ring. Protonation at the p-position should have resulted in a long-vrave absorption 

Similar calculations has been carried out for the addition of the proton at the meso-, a, and p-positions of anthracene t . 2,4i6,429,434) Comparison of tte results with the experimental spectrum of an anthracene solution in HF has confirmed the mentioned conclusion of V. Gold and F. Tye that anthracene is meetly protonated at the meso position. This is also proved by PMR. Among the anthra-cenium ions listed in Table 36, only protonated 2-methylanthracene has no PMR data available. The similarity of its electronic spectrum to that of the 9-H-anthra-cenium ion, however, testifies that too, the proton is attached at the meso position (for a-H- and P-H-ions long-wave absorption bands have been calculated near 550 and 720 nm, respectively 

The electronic absorption spectra of acid phenanthrene solutions are more complicated. According to molecular-orbital calculations, the most basic positions must be 1(8) and 9(10) and close to them positions 4(5). The protonation of i en-anthrene, therefore, could be expected to yield a mixture of isomeric ions. 

Molecular-orbital calculations predict the 9-H-phenanthrenium ion to have in the long-wavelength r on a stronger absorption band at about 530 nm, the 4-H-isomer near 400 nm (and a weaker one at about 600 nm) and the 1-H-isomer near 490 nm. In fact the spectrum of the 9,9,10-trimethylphenanthrenium ion (Table 37) shows only the first of these bands. Similar is the spectrum of a solution of 9-methylphenanthrene in the HF—BFj system, so this hydrocarbon must be mostly protonated at the 10-position. 

35 The reactivity of the three positions in anthracene can be estimated from the isotopic hydrogen exchange rate k meso) k(a) k(P) = 7250 5 1 . 

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The electronic spectroscopy combined with molecular-orbital calculations and isotopic hydrogen exchange has also been applied in the protonation of other polycyclic hydrocarbonsipyrene1,2- and 3,4-benzpyrenesperylene 171,416) 1,2-benzanthracene and its monomethylated derivatives i i,<23,433,434) tetracene 33.434) electronic spectra of polycyclic arenium ions ArH are compared with their negatively charged analogues ArH ... 

Effective oxidizers of aromatic hydrocarbons in equilibrium with arenium ions are the diprotonated quinones particularly diphenoquinone and its 3,3, 5,5 -tetra-bromo-derivative In this case the unsubstituted polycyclic arenium ions... 

In fused ring systems, the positions are not equivalent and there is usually a preferred orientation even in the unsubstituted hydrocarbon. The preferred positions may often by predicted as for benzene rings. Thus it is possible to draw more canonical forms for the arenium ion when naphthalene is attacked at the a position than when it is attacked at the p position, and the a position is the preferred site of attack,though, as previously mentioned (p. 682), the isomer formed by substitution at the p position is thermodynamically more stable and is the product if the reaction is reversible and equilibrium is reached. Because of the more extensive delocalization of charges in the corresponding arenium ions, naphthalene is more reactive than benzene and substitution is faster at both positions. Similarly, anthracene, phenanthrene, and other fused polycyclic aromatic hydrocarbons are also substituted faster than benzene. 

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... 

Laali et al. have characterized carbocations generated from substituted polycyclic aromatic compounds. The related cation 157 is a true aryl-methyl-type ion, whereas cations 158 have arenium ion character because the strongly electron-withdrawing a-CF3 group enhances charge delocalization into the pyrenyl and phenyl groups. 

Polycyclic aromatic compounds also undergo electrophilic aromatic substitution reactions. Because the aromatic resonance energy that is lost in forming the arenium ion is lower, these compounds tend to be more reactive than benzene. For example, the brotni-nation of naphthalene, like that of other reactive aromatic compounds, does not require a Lewis acid catalyst ... 

A series of 1- and 2-substituted acenaphthylenium ions has been generated by removing OH in strong acids from respective acenaphthenols The PMR spectra of the biphenylene and ancephthylene ions fail to reveal the spin-spin coupling of the CH3 protons with those of neighbouring carbons For the PMR spectra of the arenium ions formed by the protonation of 7-methyl-and 7,12-dimethylbenz[a]anthracene, pyrene, benz[a]pyrene, naphthacene and other polycyclic systems see... 

Lewis-type acids can serve as electron acceptors, as already pointed out, as well. When polycyclic aromatic hydrocarbons interact with BF3 in 1,2-dichlorethane the respective radical cations and particles which are suppc ally bipolar arenium ions of type (JSS) are recognized in the electronic absorption spectra These particles are formed rapidly, the radical cations slowly. The accumulation of the latter is accelerated by illumination. 

Recent publications calculate the basicity of aromatic compounds and the electronic structure of the respective arenium ions by quantum chemical methods in different approximations — by semi-empirical methods MO LCAO (methylbenzenes " ), CNDO, CNDO/2 and CNDO/2FK (benzene " , toluene and other monoalkylbenzenes " , anisole , a series of monosubstituted benzoles , poly-methylbenzenes , monomethylnaphthalenes and polycyclic aromatic hydrocarbons ) INDO (benzene , cresols ) MINDO-2 and MINDO-3 (benzene , toluene ) by nonempirical (ab initio) methods using the basis... 

A report describes a novel method for the deuteration of aromatic hydrocarbons, without the need for forcing conditions or oxidizing mineral acids (Scheme 19)." The chemistry utilizes catalytic amounts of arenium ion salts, such as the mesitylene derivative (82a), and deuterated solvents. The solvent acts as the deuterium source, while cyclohexadienyl cation is the deuteron carrier. Thus, an equilibrium is established between (82a) and the deuterated isotopomers (82b) and the deuterium exchange then occurs at the hydrocarbon. Deuteration was demonstrated for several polycyclic aromatic hydrocarbons, including triphenylene (83) (98% yield, 94% D incorporation). Corranulene was deuterated in 78% yield and 96% D incorporation using this chemistry. 

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