Fluorine bond enthalpy

Write the balanced chemical equation for the complete fluorination of methane to tetrafluoromethane. Using bond enthalpies, estimate the enthalpy of this reaction. The corresponding reaction using chlorine is much less exothermic. To what can this difference be attributed ... 

The authors of primary Reference 80 present their own and selected literature values for the R—NO bond enthalpies for the hydrocarbyl cases of Me, Et, t-Bu, allyl and benzyl, as well as mixed fluorinated, chlorinated methyl radicals. We now wish to compare nitroso species with the corresponding amino and nitro compounds. Choosing what we consider the most reliable and relevant nitroso compound data, and accompanying them with the corresponding radical data, we derive enthalpies of formation of gaseous nitrosomethane, 2-methyl-2-nitrosopropane and o -nitrosotoluene81 to be 65 2, —29 4 and 174 7 kJmol-1. (By comparison, the earlier values recommended8 for nitrosomethane and 2-methyl-2-nitrosopropane were 70 and —42 kJmol-1 respectively.)... 

Since the size of halogen atom Increases as we go down the group in the periodic table, fluorine atom is the smallest and iodine atom, the largest. Consequently the carbon-halogen bond length also Increases from C—to C— I. Some typical bond lengths, bond enthalpies and dipole moments are given in Table 10.2. 

The calculation of the bond enthalpy term for boron-fluorine bonds in BF3 is ... 

When hexafluorocyclobutene is treated with aluminum chloride, all fluorine atoms are replaced by chlorine. On treatment with aluminum bromide, all are replaced by bromine. The products are hexachlorocyclo-butene and hexabromocyclobutene, respectively. The reason for this quite unexpected reaction may be the difference in the strength of carbon-fluorine bond and aluminum-fluorine bond. As the aluminum-fluorine bond (595 kJ/mol, 142 kcal) is stronger than the carbon-fluorine bond (443 kJ/mol, 106 kcal), the lower reaction enthalpy may be the driving force for the halogen exchange [40]. 

Studies of the relative binding energies of the sixth ligand to a series of CF3- and CH3-substituted Co(III) coordination complexes RCoL4, where L4 is a tetradentate base, e.g., bis(acetylacetone)ethylenediimine(bae), found that the bond enthalpies for a typical monodentate ligand like pyridine at the sixth coordination site (trans to R) are higher for the CF3 derivatives than the CH3 derivatives, e.g., by 6 kcal/mol for bae. Similarly, fluorination of the L4 entity increases the Co—py bond enthalpy, but to a smaller extent, e.g., 2 kcal/mol for bae 62). 

The comparatively low bond enthalpy of F2 (155 kj/mol) accounts in part for the extreme reactivity of elemental fluorine. Because of its high reactivity, F2 is difficult to work with. Certain metals, such as copper and nickel, can be used to contain F2 because their surfaces form a protective coating of metal fluoride. Chlorine and the heavier halogens are also reactive, although less so than fluorine. 

When molecnlar fluorine dissociates, two fluorine atoms are produced. Since the enthalpy of formation of atomic fluorine is in units of kJ/mol, this number is half the bond enthalpy of the fluorine molecule. 

In contrast to the FVP synthesis of indenocorannulene (98) from phenyl-corannulene, the fluoro-substituted analogue 29 (X=F) turns out to be very efficient [126] (Scheme 28). Catalytic cyclization is initiated by reaction of a silyl cation with an aryl fluoride to generate a phenyl cation and, subsequently, Friedel-Crafts reaction to an intramolecular aryl coupling, flowed by deprotonation, to give 98, The enabling feature of this reaction is the exchange of carbon-fluorine for sihcon-fluoiine bond enthalpies. 

The larger the atoms joined by a particular bond, the longer the bond length. Large atoms have more electrons than smaller atoms and this results in an increase in repulsion between the electron shells of each atom. In addition, the nucleus of each atom is more effectively shielded (Chapter 12). Both of these effects lead to a weakening of the bond. For example, in the halc ens the bond strength weakens in the order chlorine, bromine and iodine. Fluorine, however, has a surprisingly low bond enthalpy, which is accounted for by lone pair—lone pair repulsion. 

Q1 a i Define the term average bond enthalpy. [3] ii Explain why the fluorine molecule, F2, is not suitable as an example to illustrate the term average bond enthalpy. [1]... 

On the basis of metal fluorocarbon combinations, pyrolants show superior exothermicity compared to many of the aforementioned fluorine-free systems [22]. This advantage is due to the high enthalpy of formation of the metal-fluorine bond not outperformed by any other combination of the respective metal. Thus, the exothermic step... 

The very low bond dissociation enthalpy of fluorine is an important factor contributing to the greater reactivity of fluorine. (This low energy may be due to repulsion between non-bonding electrons on the two adjacent fluorine atoms.) The higher hydration and lattice enthalpies of the fluoride ion are due to the smaller size of this ion. 

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