Hydrogen fluoride bond energy

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-... 

The following table lists the predicted bond energy of hydrogen fluoride computed with various methods using the 6-311-H-G(3df,3pd) basis set. We chose this basis set because it is near the basis set limit for this problem errors that remain can be assumed to arise from the method itself and not from the basis set. 

Hydrogen abstraction reactions potential surfaces for, 25-26,26,41 resonance structures for, 24 Hydrogen atom, 2 Hydrogen bonds, 169,184 Hydrogen fluoride, 19-20, 20,22-23 Hydrogen molecules, 15-18 energy of, 11,16,17 Hamiltonian for, 4,15-16 induced dipoles, 75,125 lithium ion effect on, 12... 

Another important question deals with the intramolecular and unimolecular dynamics of the X-—RY and XR -Y- complexes. The interaction between the ion and molecule in these complexes is weak, similar to the intermolecular interactions for van der Waals molecules with hydrogen-bonding interactions like the hydrogen fluoride and water dimers.16 There are only small changes in the structure and vibrational frequencies of the RY and RX molecules when they form the ion-dipole complexes. In the complex, the vibrational frequencies of the intramolecular modes of the molecule are much higher than are the vibrational frequencies of the intermolecular modes, which are formed when the ion and molecule associate. This is illustrated in Table 1, where the vibrational frequencies for CH3C1 and the Cr-CHjCl complex are compared. Because of the disparity between the frequencies for the intermolecular and intramolecular modes, intramolecular vibrational energy redistribution (IVR) between these two types of modes may be slow in the ion-dipole complex.16... 

In fluorine thermochemistry, two key heat values frequently occur. They are the dissociation energy of difluorine, required for evaluation of fluorine bond energies and the heat of formation of hydrogen fluoride, a product in hydrolysis, hydrogenation, fluorine combustion, or neutralization reactions. These values have been difficult to measure and have changed considerably over the years. A recommended set of values has been reported in recent CODATA bulletins (60) which are collected in Table I together with older values and corrections to update them. 

Eormula HE MW 20.006. A very stable polar covalent diatomic molecule H—E bond energy 136.1 kcaPmol at lower temperatures molecules are associated by hydrogen bonding H—bond length 0.92A partial ionic character 40% dipole moment 6.10 D hydrofluoric acid is an aqueous solution of hydrogen fluoride gas. 

The discussion of hydrogen fluoride in this paragraph is a little different from that in the first two editions of this book. In the first two editions the calculated energy curve for the extreme ionic structure was shown as falling below that for the normal covalent structure, and the conclusion was reached that the bond between the hydrogen atom and the fluorine atom in the molecule... 

The mechanistic borderline between E2 and ElcB mechanisms has been studied under various conditions.1,2 The mechanism of the elimination reaction of 2-(2-fluoroethyl)-1-methylpyridinium has been explored explored by Car-Parrinello molecular dynamics in aqueous solution.3 The results indicated that the reaction mechanism effectively evolves through the potential energy region of the carbanion the carbon-fluoride bond breaks only after the carbon-hydrogen bond. 

Then 33 is exposed in a one-pot reaction to tetrabutylammonium fluoride (TBAF), forming 34. Driving force of this reaction is the higher bond energy of an Si-F bond (467 KJ/mol) compared with that of the Si-C bond (251 KJ/mol). The following oxidation with aqueous hydrogen peroxide in the presence of potassium bicarbonate gives exclusively the diol 35. 

---