C-F bond strength

Despite the body of evidence in favor of the Mayo mechanism, the formation of diphenylcyclobutanes (90, 91) must still be accounted for. It is possible that they arise via the 1,4-diradical 94 and it is also conceivable that this diradical is an intermediate in the formation of the Diels-Alder adduct 95 (Scheme 3.64) and could provide a second (minor) source of initiation. Direct initiation by diradicals is suggested in the thermal polymerization of 2,3,4,5,6-pentafluorostyrene where transfer of a fluorine atom from Diels-Alder dimer to monomer seems highly unlikely (high C-F bond strength) and for derivatives which cannot form a Diels-Alder adduct. 

Because of the exceptional C-F bond strength, the successful preparation of a-halocyclopropyl c-complexes is realized by substitution of 1-bromo-l-fluoro-trans-2,3-dimethylcyclopropane 179 with Fp [90], Silica gel column chromatography of the thus obtained cr-complex 180 results in ring opening to the alcohol 181 as a single stereoisomer. The allene complex 182 is produced by treatment with BF3OEt2, indicating that 181 is derived from 182 and water. The 7i-allyl complex 183 is formed by photolysis via a disrotatory process. 

C—C and C—F bond strengths, 23 181-182 and plasma generation of trifluoro-methyl radicals, 23 180-192 Hexafluorogermanate ion charge distribution, 22 73 enthalpy, 22 73 halide ion affinity, 22 74 lattice energy, 22 73 radius, 22 73, 74 structure, 22 73... 

A consequence of the high C —F bond strength is that fluorine is an especially poor leaving group. Alkyl fluorides are 102-106 less reactive toward nucleophilic displacement than other alkyl halides.8 9 gera-Difluorides and trifluoromethyl groups are essentially unreactive toward nucleophilic displacement of fluorine. 

Despite the strong C-F bond strength, fluoride is a good leaving group in these substitution reactions. Reactions such as these usually require hi ly fluorinated substrates for reasonable yields. No new metal fluorine bond is formed. 

The carbon-fluorine bond is very stable. Further, where two fluorine atoms are attached to a single carbon atom there is a reduction in the C—F bond distance from 1.42 A to 1.35 A. As a result bond strengths may be as high as 504 kJ/mole. Since the only other bond present is the stable C—C bond, PTFE has a very high heat stability, even when heated above its crystalline melting point of 32TC. 

The C-F bonds in 1 fluoroalkenes and fluorobenzenes also are very strong (Table 17), but alkene k bond strengths vary with the level of fluonnabon (Table 18) Both CHF=CF2 and CF2=Cp2 have significantly weaker 7t bonds than CH2= CH2, CH7=CHF, and CH2=Cp2, consistent with other data indicatuig that tn- and tetrafluonnation thermodynamically destabilize double bonds [75] The low n bond energy in Cp2=CF2 underlies its propensity to undergo thermal biradical [2-1-2] cycloaddibons [103] (see p 767 )... 

A very stable bond involving fluorine is the carbon-fluorine bond. The strength of this C—F bond is comparable to the C—H bond, and has led to the existence of a series of compounds known as the fluorocarbons. These are analogous to the hydrocarbons and can be imagined as being derived from them by substituting F atoms for H atoms. For example,... 

The bond dissociation energy of fluoromethane is 115 kcal mol , which is much higher than the other halides (C-Cl, C-Br and C-1, respectively 84, 72 and 58 kcal mol ) [6], Due to its strength, the carbon-fluorine (C-F) bond is one of the most challenging bonds to activate [7], A variety of C-F bond activation reactions have been carried out with different organometallic complexes [8], Among them, nickel [9] and ruthenium complexes have proven to proceed selectively under mild conditions [10],... 

Figure 3.63 illustrates the gauche effect for vicinal lone pairs and polar C—F bonds with the examples of (a) hydrazine and (b) 1,2-difluoroethane, respectively. As seen in Fig. 3.63(a), the

lone pairs are anti to one another (thus squandering their powerful donor strength on vicinal moieties with no acceptor capacity) is disfavored by 3.2 kcal mol-1 relative to the preferred = 93.9° conformer in which each nN hyperconjugates effectively with... 

Hydrogen abstraction by fluorine is thermodynamically advantageous, since C—H bond strengths are about 410kJ/mol compared to 560kJ/mol for H—F and 450-500 kJ/mol for C—F bonding. 

The stability towards oxidative processes is generally attributed to the strength of the C-F bond, which is more important than that of the C-H bond. However, biological oxidations do not only involve the homolysis of a C-X or C-H bond, and then, they cannot be directly associated with the bond strength. It seems more judicious to invoke the difficult formation of the 0-F bond to explain the... 

Although the fluoride anion is not a good leaving group (because of the great strength of the C-F bond), ketones, imines and jS-fluoroesters easily afford this S-elimination reaction (Fig. 22) [77], The S-elimination process remains efficient for CF2 and CF3 compounds, while the C-F bond is stronger. Indeed, fluorine atoms render more acidic the a proton, which makes easier the formation of the anion. 

Modifications of the chemical reactivity generated by the presence of fluorine atoms in a molecule are connected to three main factors the strength of the C— F bond, the electron-withdrawing character of the fluorinated substituents, and the possible loss of a fluoride ion or of HF in the processes of )S-elimination. On these bases and taking into account the ability of fluoro-substituents to sterically or electronically mimic other... 

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