Fluorine bond dissociation energy

It is reasonable to postulate that the main driving force for the reaction of (MeC5H4)3U(t-Bu) with hexafluorobenzene is thermodynamic. A weak uranium-carbon bond and a carbon-fluorine bond have to be broken. This is offset by the formation of a strong uranium-fluorine bond and either a carbon-carbon or a carbon-hydrogen bond. As illustrated in equation 8, the uranium-fluorine bond energy can be estimated to be on the order of 150 kcal/mol based on known thermochemical data for uranium fluorides (21), The carbon-fluorine bond dissociation energy for hexafluorobenzene is reported to be 154 kcal/mol (17), The... 

The bond dissociation energy of the hydrogen-fluorine bond in HF is so great that the above equilibrium lies to the left and hydrogen fluoride is a weak acid in dilute aqueous solution. In more concentrated solution, however, a second equilibrium reaction becomes important with the fluoride ion forming the complex ion HFJ. The relevant equilibria are ... 

The last example represents a fairly rare elimination of hydrogen fluoride in preference to hydrogen chloride, a reaction that deserves a more detailed discussion A comparison of bond dissociation energies of carbon-halogen bonds shows that the carbon-fluorine bond is much stronger than the carbon-chlorine, carbon-bromine, and carbon-iodme bonds 108-116, 83 5, 70, and 56 kcal/mol, respec-... 

FIGURE 2.16 The bond dissociation energies, in kilojoules per mole of nitrogen, oxygen, and fluorine molecules. Note how the bonds weaken in the change from a triple bond in N, to a single bond in F,... 

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],... 

The standard state of fluorine is the difluorine molecule, F2, which has an electronic configuration identical with that of the peroxide ion. The two species are isoelectronic. The bond order is 1, and the bond dissociation energy of 155 kj mol-1 and bond length of 144 pm are very similar to the values for 022-. 

Table 1. Bond-Dissociation Energy for Various Nonfluori-ruited and Fluorinated Alkanes3-4 6...

The limited stabilization of a radical by partial fluorination is reflected in the corresponding bond-dissociation energies (sec Table 10) 87 however, increasing degrees of fluorination. such as the introduction of a trifluoromethyl group into a molecule, destabilize the radical (vide supra) 02 104 Even a single fluorine in the /J-position is destabilizing. 

Carbon-hydrogen bonds, such as those in hydrocarbons like methane and ethane, have dissociation energies close to 400 kj-mol whereas single bonds between carbon and fluorine have dissociation energies close to 500 kj-mol-1. The greater strength of a carbon-fluorine bond helps to explain why fluorocarbon polymers are very resistant to chemical attack. They are used to construct valves for corrosive gases and to line the interiors of chemical reactors. 

Molecular fluorine, because of its very low bond dissociation energy, usually reacts uncontrollably with organic compounds.43 Merritt, however, has observed electrophilic addition of F2 to cis- and trans-i -phenylpropenes at low temperatures. The mode of addition is predominantly syn. A fluoronium ion, in which the fluorine is positively charged, would be very unstable and apparently does not form.44 Attempts to form a three-membered ring fluoronium ion in superacid medium have also failed.45... 

Br —4, and 1+16 kcal mole-1)119 reflect the decreasing bond dissociation energies for H—X and C—X bonds in the series F, Cl, Br, I, and the relatively constant Z)(X—X) (Table 9.4). The highly exothermic fluorination requires no external initiators and occurs violently and uncontrollably on mixing fluorine with a hydrocarbon either in the gas or liquid phase. Chlorination must be initiated, but proceeds readily, whereas bromination frequently requires elevated temperatures. Iodination is rarely successful, and indeed is more likely to occur in the reverse direction as reduction of alkyl iodides by HI.120... 

An important aspect of fluorine thermochemistry is the effect of increasing a-fluorine substitution on the C—F bond dissociation energy (BDE). This geminal stabilization is illustrated by the C—F BDE in the fluorinated methanes—the CHF series with associated C—F and C—H BDEs given below (for radical heats of formation used see Table 10, for halon atom heats of formation see Table 8) ... 

We can summarize the above discussion as follows increasing the number of fluorines on a carbon atom increases the C—F and C—Cl bond dissociation energy, but it does not have a systematic effect on the C—H BDE. Increasing the number of other halogen atoms on the carbon atom does not systematically increase BDEs and seems to show in some cases a slight systematic decrease in BDE. 

The bond dissociation energies of chlorine, bromine, and iodine have been known for a long time. In Linus Pauling s classical book The Nature of the Chemical Bond (1948), the bond dissociation energies of halogens are listed as 57.8, 46.1, and 36.2 kcal/mol (242, 193, and 152 kJ) for chlorine, bromine, and iodine, respectively. What is the bond dissociation energy of fluorine ... 

Because the bond dissociation energy of fluorine was not known at that time, its value was estimated by extrapolation of the values for the other three halogens (63.5 kcal/mol [/], or 63-70 kcal/mol [2], It was believed that the fluorine atoms in the molecule of fluorine are held tighter than the other halogen atoms. 

Therefore, there was much surprise when the measurements of the bond dissociation energy for fluorine gave a much lower value of 37.5 kcal/mol (157 kJ) [3],... 

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