Hydrocarbon, salts

Ionic Dissociation of Carbon-Carbon a- Bonds in Hydrocarbons and the Formation of Authentic Hydrocarbon Salts... 

Structural factors necessary for carbocation and carbanion stability are discussed briefly. The mechanistic problems of dissociation of the carbon-carbon a bond are then discussed, and finally the physicochemical nature of hydrocarbon salts and the related novel hydrocarbons. 

Bordwell, 1988), contributed to the successful isolation of the first hydrocarbon salt [l 2 ], when it was combined with Agranat s cation, tris(3-guaiazulenyl)cyclopropenylium ion [1" ] (Agranat and Aharon-Shalom, 1976). 

In view of the ease of dissociation of the a bond into resonance-stabilized hydrocarbon ions, it is expected that an authentic hydrocarbon salt could be isolated from an extraordinarily stable cation and anion. The first synthesis of a salt composed solely of carbon and hydrogen was achieved on the combination of tris(3-guaiazulenyl)cyclopropenylium ion [1" ] and tris(7//-dibenzo[c,g]fluorenylidenemethyl)methanide ion [2 ] (Okamoto et al., 1985). 

The experimental procedure for the preparation of analytically pure hydrocarbon salts was as follows. First, the deep green solution of Kuhn s... 

The solution of Agranat s cation [1 ] instantly gave black precipitates of mixed KCIO4 and [l 2 ]. The crude hydrocarbon salt was isolated by recrystallization from DMSO as greenish black needles. 

The complete dissociation of the hydrocarbons [l 2 ], [28" 2 ] and [40" 2 ] in DMSO has been demonstrated by quantitative generation of both Kuhn s carbanion [2 ] and carbocations [1" ], [28" ] and [40" ] as determined by UV-vis spectra (Table 6 and Eig. 4). However, since carbocation [24 ] has no absorptions at a wavelength region longer than 220 nm in the UV spectrum, there remained an ambiguity that this cation might have decomposed in the DMSO solution. A clue to this problem could be obtained by determination of the electric conductivity of DMSO solutions of hydrocarbon salts (Table 7) (Okamoto et al., 1990). 

Table 5 Some physical properties of hydrocarbon salts prepared from Kuhn s anion [2... 

Table 7 Equivalent conductances" at infinite dilution for hydrocarbon salts, [l 2 ] and [24 2 ], and related ionic species in DMSO at 25°C. ...

Recently two additional examples, [26-2] and [28-2], of hydrocarbons existing solely in solution have been found in the chloroform solutions of hydrocarbon salts [26" 2 ] and [28 2 ] (Table 5) (Komatsu et al., 1989 Takeuchi et al., 1993). 

The hydrocarbon salt [28 2 ] dissolves in DMSO without any reaction, giving a deep green solution. In contrast, when this salt is dissolved in chloroform, it affords an orange-brown solution, spectrophotometrically suggesting the formation of a hydrocarbon [28-2]. The formation of the covalent hydrocarbon in chloroform is also supported by the fact that the anion [2 ] is regenerated upon dilution of the freshly prepared solution with 10 volumes of DMSO. 

As the cation becomes progressively more reluctant to be reduced than [53 ], covalent bond formation is observed instead of electron transfer. Further stabilization of the cation causes formation of an ionic bond, i.e. salt formation. Thus, the course of the reaction is controlled by the electron affinity of the carbocation. However, the change from single-electron transfer to salt formation is not straightforward. As has been discussed in previous sections, steric effects are another important factor in controlling the formation of hydrocarbon salts. The significant difference in the reduction potential at which a covalent bond is switched to an ionic one -around -0.8 V for tropylium ion series and —1.6 V in the case of l-aryl-2,3-dicyclopropylcyclopropenylium ion series - may be attributed to steric factors. 

Considering the long saga of hydrocarbon chemistry, it is surprising that two new classes of hydrocarbon - ionically dissociative hydrocarbons and hydrocarbon salts - have been discovered in the last decade. The syntheses of authentic samples as analytically pure solids have revealed the very existence of such novel hydrocarbons in an unquestionable way, but the investigation of their basic features is just in the inchoate stage. The search for such novel hydrocarbons depends primarily on the synthesis and examination of highly stabilized hydrocarbon cations and anions. As mentioned above, until now such elaboration has been concentrated on the carbocation side, and examination of the carbanion moiety has only just started. 

Computational approaches remain almost untouched, and the main reason probably is the inevitable structural complication which is required of molecules and ions to gain sufficient resonance stabilization. Recently, a molecular orbital calculation has been applied to a simplified hydrocarbon salt model by Fujimoto et al. (1991). 

Ionic dissociation of carbon-carbon a-bonds in hydrocarbons and the formation of authentic hydrocarbon salts, 30, 173 Ionization potentials, 4, 31 Ion-pairing effects in carbanion reactions, 15, 153 Ions, organic, charge density-NMR chemical shift correlations, 11,125 Isomerization, permutational, of pentavalent phosphorus compounds, 9, 25 Isotope effects, hydrogen, in aromatic substitution reactions, 2,163... 

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