Benzenoid cations

Multi-Mode Jahn-Teller and Pseudo-Jahn-Teller Effects in Benzenoid Cations... 

The Jahn-Teller Effect in Benzenoid Cations Theory and Experiment," T. A. Miller and V. E. Bondybey, "Molecular Ions Spectroscopy, Structure, and Chemistry," T. A. Miller and V. E. Bondybey, eds., (North-Holland, 1983). 

Miller T, Bondybey VE (1983) The Jeihn-TeUer effect in benzenoid cations theory and experiment. In Molecultir ions spectroscopy, stiucture and chemistry. North-Holland, Amsterdam, p 201... 

Aminoazobenzene is a very weak base, and consequently it will not form salts with weak organic acids, such as acetic acid, although it will do so with the strong mineral acids, such as hydrochloric acid. Aminoazobenzene is a yellowish-brown compound, whilst the hydrochloride is steel blue. The colour of the latter is presumably due to the addition of the proton to the phenyl-N-atom, the cation thus having benzenoid and quinonoid forms ... 

Many benzenoid quaternary cationic surfactants possess germicidal, fungicidal, or algicidal activity. Solutions of such compounds, alone or in combination with nonionic surfactants, are used as detergent sanitizers in hospital maintenance. Classified as biocidal products, their labeling is regulated by the U.S. EPA. The 1993 U.S. shipments of cationic surfactants represented 16% of the total sales value of surfactant production. Some of this production is used for the preparation of more highly substituted derivatives (101). 

It is apparent from simple valence bond considerations as well as from calculations of rr-electron density, " that isoindoles should be most susceptible to electrophilic attack at carbon 1. This preference is most clearly evident when the intermediate cations (85-87) from electrophilic attack (by A+) at positions 1, 4, and 5 are considered. The benzenoid resonance of 85 is the decisive factor in favoring this intermediate over its competitors. 

As another example, the tropylium ion [3 ], which is stabilized by virtue of the 67t electrons spread over a heptagonal sp hybridized carbon framework [Hiickel s (4n 4- 2)v rule with = 1], is also unstable in the gas phase. Its formation from toluene or the benzyl cation has been a long-standing problem in organic mass spectrometry, and the reaction mechanism and energetics have recently been exhaustively discussed (Lif-shitz, 1994). It was, however, isolated as the bromide salt by Doering and Knox (1954, 1957), and was the first non-benzenoid aromatic carbocation. 

In 1970, the synthesis of the orange-red sandwich cation 1 from cobaltocene and PhBCl2 (1) marked a further starting point in the chemistry of boron metal compounds. The presence of a planar benzenoid C5H5B ligand moiety in 1 was deduced from XH and UB NMR data (1). This was made ironclad by two X-ray structure determinations which revealed typical centrosymmetric sandwich structures for the 19-e complexes Co(C5H5BOMe)2 (6) (21,22) and Co(C5H5BMe)2 (7) (22) as shown in Fig. 1. 

Addition of electrophiles to a coordinated benzenoid ligand is a characteristic reaction of 20-e complexes. [Co(C5H5BPh)2] in 59 is readily protonated at C-2, producing complex 79 (60) which alternatively can be obtained by hydride addition to cation 61 (69). Formal electrophilic addition is also observed in the reaction of [Co(C5H5BMe)2] in 58 with Phi which affords the two isomeric phenylation products 80 and 81 (60). 

While benzene was the first aromatic system studied, the formulation of HtickePs rule and the theory behind it created an impetus to prepare non-benzenoid species such as the tropylium cation and cyclopentadienyl anion that also obeyed Huckel s rule to see if these species were also aromatic. This required that the properties of aromatic compounds be defined. 

The simultaneous annihilation of the aromatic cation radical by its triad partners N02 and pyridine, as presented in Scheme 12, is most apparent in the nature of the aromatic product (i.e., stoichiometry) from mesitylene, among all the other aromatic (benzenoid) donors. Thus, the direct incorporation of N02 and pyridine into the aromatic nucleus is shown by the concomitant ring nitration and pyridination of mesitylene (Kim et al., 1993),... 

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