Carbocations and carbanions

The equilibrium constant for the hydration of 31 has been determined as pATR = —15.9,470 Hence the equilibrium constant for the heterolytic dissociation of 9-fluorenol in water is Kdiii = KR/KW = 1019 9, which amounts to a ground-state free energy difference of ArG° = 114 kJ mol  

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NMR spectroscopy is ideal for detecting charged fluorinated intermediates and has been applied to the study of increasingly stable carbocation and carbanion species. Olah [164, 165] has generated stable fluorocarbocations m SbFj/SOjClF at low temperatures The relatively long-lived perfluoro-rerr-butyl anion has been prepared as both the cesium and tris(dimethylamino)sulfonium (TAS) salts by several groups [166, 167, 168], Chemical shifts of fluonnated carbocations and carbanions are listed m Table 23. 

A free radical (often simply called a radical) may be defined as a species that contains one or more unpaired electrons. Note that this definition includes certain stable inorganic molecules such as NO and NO2, as well as many individual atoms, such as Na and Cl. As with carbocations and carbanions, simple alkyl radicals are very reactive. Their lifetimes are extremely short in solution, but they can be kept for relatively long periods frozen within the crystal lattices of other molecules. Many spectral measurements have been made on radicals trapped in this manner. Even under these conditions, the methyl radical decomposes with a half-life of 10-15 min in a methanol lattice at 77 K. Since the lifetime of a radical depends not only on its inherent stabihty, but also on the conditions under which it is generated, the terms persistent and stable are usually used for the different senses. A stable radical is inherently stable a persistent radical has a relatively long lifetime under the conditions at which it is generated, though it may not be very stable. 

The synthetic procedures for isolation of the salt appear to be rather simple. First, one prepares a solution in which the carbocation and carbanion coexist free from any combination reactions. Then, the hydrocarbon cation-anion salt is isolated after separation of the concomitant inorganic salt and evaporation of the solvent. For the purification of the crude salt recrystallization or reprecipitation with proper solvents is used. 

Sufficient stability of the hydrocarbon ions, as the salt or in the solution, is an obvious prerequisite for these procedures, and, in practice, selecting or designing the stable ions and choosing a proper solvent are tasks of primary importance. As an ordinary stability index for the ions, thermodynamic scales referred to the water molecule, i.e. p CR+ and pKa values, are chosen for the carbocation and carbanion, respectively. 

In the course of the salt synthesis, it was found that a hydrocarbon [3-2], which was formed by an unfavourable cation-anion combination reaction, dissociates into the original carbocation and carbanion in a polar aprotic solvent (Okamoto et ai, 1985) (1). This was the first example of ionic dissociation of the carbon-carbon a bond in genuine hydrocarbons, although a few cases of heterolytic dissociation of carbon-carbon tr bonds had been reported by Arnett (Arnett et al., 1983 Troughton et al., 1984 Arnett and Molter, 1985) for compounds bearing cyano and nitro groups, e.g. [4-6] and [5-6] as in (2). 

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. 

As thermodynamic stability indexes for the hydrocarbon ions, pA R+ and pA a values [(4) and (5)] have been widely applied for the carbocation and carbanion, respectively, in solution. Here K + stands for the equilibrium constant for the reaction (6) of a carbocation and a water molecule stands for the equilibrium constant for the reaction (7) of a hydrocarbon with a water molecule to give the conjugate carbanion. The equilibrium constants are given by (8) and (9) for dilute aqueous solutions. Obviously, the reference system for the pKn+ scale is the corresponding alcohol, and... 

Unlike such unstable intermediates, the first, rare example of reversible dissociation of a carbon-carbon a bond into a stable carbocation and carbanion was reported for a nitro-dicyano compound (20) prepared from trimethyl- and triphenyl-cyclopropenylium tetrafluoroborate ([4" ]BF4 and [5 JBFJ) with the potassium salt of p-substituted-phenylmalononitrile anions (Arnett et al., 1983 Troughton et al., 1984 Arnett and Molter, 1985). Other ionically dissociative malononitrile derivatives have been prepared from such carbocations as the tropylium [S ] (Arnett and Troughton, 1983) and the tris(p-methoxyphenyl)methylium [93 j (Arnett and Troughton, 1983) ions. 

In view of the observations of the ionic dissociation of nitro-cyano compounds, it is hardly surprising that even a hydrocarbon could dissociate ionically into a stable carbocation and carbanion, provided that the medium is polar enough to prevent the recombination reaction and to ensure equilibration. 

These three types, radicals, carbocations and carbanions, by no means exhaust the possibilities of transient intermediates in which carbon is the active centre others include the electron-deficient species carbenes, R2C (p. 266), nitrenes, RN (p. 122) and also arynes (p. 174). 

The familiarity with qualitative valence bond descriptions of substituent effects in combination with the known substituent effects in carbocations and carbanions led Viehe and his group to the postulate of a captodative effect for free radicals (Stella et ai, 1978 Viehe et al., 1979). They did not seem to be aware of the earlier work which was of a more physical organic character. The fact that carbocations [8] are stabilized by + M substituents, and carbanions [9] by - M substituents, raised the idea that free radicals, as... 

Chain copolymerization is important from several considerations. Much of our knowledge of the reactivities of monomers, free radicals, carbocations, and carbanions in chain polymerization comes from copolymerization studies. The behavior of monomers in copolymerization reactions is especially useful for studying the effect of chemical structure on reactivity. Copolymerization is also very important from the technological viewpoint. It greatly increases the ability of the polymer scientist to tailor-make a polymer product with specifically desired properties. Polymerization of a single monomer is relatively limited as to the number of different products that are possible. The term homopolymerization is often used to distinguish the polymerization of a single monomer from the copolymerization process. 

Closed-shell ions are among the most important intermediates in solution chemistry, and no treatise on reactive intermediates (including the present one) would be complete without extensive sections on carbocations and carbanions, if not also on heteroanalogues of the above species. Nevertheless, closed-shell ions are conspicuously absent from matrix isolation studies, apart from a few cases where such species were coincidentally formed in discharges, or where charged species were deliberately isolated by mass spectrometry (cf. Section 6.4). The reason for... 

Up to this point the CM model and the More O Ferrall PES diagram lead to essentially identical conclusions. The analysis by More O Ferrall (1970) of the elimination mechanism spectrum may be summed up by Fig. 27. The two axes represent C—H and C—X bond-breaking co-ordinates. The third axis, perpendicular to the plane of the paper, is the energy co-ordinate. Two of the diagonal corners represent reactants and products while the other pair of diagonal corners represent the carbocation and carbanion intermediates. All possible mechanistic pathways are simultaneously indicated on the energy surface. 

Radicals themselves are also subject to oxidation and reduction. Radicals interact with oxidizing and reducing transition metal complexes with formation, respectively, of carbocation and carbanion products.111, 112... 

J. Carbonium Ions.—Carbonium ions are very important in organic chemistry160 and many carbocations and carbanions have aroused the interest of theoreticians. A detailed report of work in this area has been given by Pople161 and Hehre,162 and space does not allow for further discussion here. However, we do refer to selected papers of interest which have appeared more recently. 

Change in bond connectivity common with radical, carbocation and carbanion intermediates. 

J. P. Richard, T. L. Amyes, M. M. Toteva, Formation and Stability of Carbocations and Carbanions in Water and Intrinsic Barriers to Their Reactions, Acc. Chem. Ret. 2001, 34, 981-988. 

All of the mechanisms that have been presented so far have involved the reaction of electrophiles with nucleophiles. Carbocations and carbanions have been encountered as intermediates. In this chapter the chemistry of a new reactive intermediate, called a radical (or free radical), is presented. A radical is a species with an odd number of electrons. After a discussion of the structure of radicals, including their stability and geometry, various methods of generating them are described. Next, the general reactions that they undergo are presented. Finally, specific reactions involving radical intermediates are discussed. 

The dissociation constant KD, which is often derived from spectral properties in carbanion chemistry, therefore includes a covalent term that corresponds to Kx in carbocationic chemistry. As one would not expect equal reactivity of benzhydryl chloride and benzhydryl cations, one also should not expect equal reactivity for benzhydryl lithium and benzhydryl anions. As one realizes that the terms contact ion-pair and dissociation have a different meaning in carbocation and carbanion chemistry, the apparent discrepancies quoted above, will disappear. 

The book is divided into three parts. Part I deals with typical complex organic reactions such as (i) reactions involving carbocations and carbanions, (i/) Pericyclic and electrocyclic reactions and (ii/) Sigmatropic and Chelotropic reactions. This part also includes material useful for characterization of products from structural point of view such as Geometrical isomerism, Stereochemistry and Conformation. Part II is concerned with spectroscopic methods of structure determination such as U.V.,... 

Other nice examples of well-studied solvent-dependent ionization equilibria of ionogens are azidocycloheptatriene tropylium azide [282, 283] and (triphenylcyclo-propen-l-yl) (4-nitrophenyl)malononitrile ("2a) triphenylcyclopropenium dicyano(4-nitrophenyl)methide (3a), the latter being one of the first examples of direct heterolysis of a weak carbon-carbon bond to a carbocation and carbanion in solution [284],... 

In this section we wiU deal with the effects of a polyfluoroalkyl group as a whole attached to a saturated, and therefore not formally charged, carbon atom. The effect of fluorine and fluorinated groups directly bonded to reaction centres such as intermediate carbocation and carbanion sites will be treated in separate sections. 

The high reactivity of carbocations and carbanions is associated primarily with the strong coulombic interactions between opposite charges. A point charge... 

Typical carbocations and carbanions, even with significant charge delocalization, are still usually quite reactive. This property leads to two very important consequences from the point of view of organic synthesis. 

Such a dethroning of carbocations and carbanions can be carried another step further. Certain purely covalent compounds possessing weakly polarized bonds can serve as ionic reagents if their electronic system is highly polarizable. In such molecules the approach of a charged particle, or even a dipole, induces a significant displacement of electrons. The electron displacement is to such an extent that the reaction intermediates may become almost fully ionized, as if an ionic reagent had been actually used. A typical example is represented by the previously mentioned reactions of electrophilic substitution in aromatic series, where a neutral molecule of an aromatic hydrocarbon, ArH, behaves as an efficient equivalent to a carbanion, Ar . 

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