Iodine bond dissociation energies

With the assumptions that (2) represents the slow step and that kj = Ogg calculated Arrhenius parameters for the concurrent bimolecular (1) and unimolec-ular (2) processes. The Arrhenius activation energy for step (2) was calculated to be 43 kcal.mole for each of the alkyl iodides, and, assuming Ogg s mechanism, this value should equal the carbon-iodine bond dissociation energy in each of these molecules. Modern studies show that the D(R—I) values are in the range 50-55 kcal.mole" and for this and other reasons Ogg s mechanism is now considered unsatisfactory. 

From this mechanism, the carbon-iodine bond dissociation energies were estimated by Sullivan as 55 and 52 kcal.mole for methyl iodide and ethyl iodide respectively, these values being in good agreement with those obtained by other tech-niques . Semenov has proposed a similar mechanism but without steps (c) and (e). 

The endothermic radical lO has also been studied in the gas phase the interatomic distance is 186.7 pm and the bond dissociation energy 175 20kJmol . It thus appears that, although the higher oxides of iodine are much more stable than any oxide of Cl or Br, nevertheless, lO is much less stable than CIO (p. 849) or BrO (p. 851). Its enthalpy of formation and other thermodynamic properties are A//f(298K) 175.1 kJmol", AGf(298 K) 149.8 kJmol-, 5°(298 K) 245.5 J K- mor . 

Although we will deal with organic radicals in solution, it is worth mentioning that the reactivity of atoms and small organic radicals with silanes in the gas phase has been studied extensively. For example, the bond dissociation energies of a variety of Si-H bonds are based on the reaction of iodine or bromine with the corresponding silanes.1... 

Since the bond dissociation energy of hydrogen iodide is less than that of hydrogen bromide [DH-i, 70.5 kcal. DH-Br, 86.5 kcal. (3)], Reactions 6, 7, and 8 should occur more readily with hydrogen iodide. This would account for the fact that the promoting effect of iodine compounds on ignition is less than that of bromine compounds. 

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-iodine bonds 108-116, 83 5, 70, and 56 kcal/mol, respec-... 

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... 

When molecular bromine or molecular iodine is used instead of molecular chlorine in this reaction, the chain reaction does not proceed effectively. The bond dissociation energies of Br-Br and I-I are 46 and 36 kcal/mol in the starting materials, and those of CH3-Br, CH3-I, H-Br, and H-I in the products are 70, 56, 88, and 71 kcal/mol, respectively. Thus, the difference in the bond dissociation energies between the starting materials and the products in these reactions tends to be small. Especially, iodination does not proceed at all. Therefore, the considerable difference in bond dissociation energies between the starting materials and the products is the driving force of radical reactions. 

In the case of HI the weak carbon-iodine bond (bond dissociation energy = 52 keal/mol [217 kJ/mol]) causes the addition step to be slow ... 

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 ... 

This same approach was used by Doncaster and Walsh to study the iodine reaction with MesGcH and to derive a bond dissociation energy of 340 10 kJ mol for McsGe— These results suggest that methyl substitution has no appreciable effect on the Ge—H bond energy within experimental error. On the other hand, iodine substitution as in GeH3l was shown to decrease the Ge—H bond energy by 14 kJmoP. ... 

Notes. Key parameters for consideration of the photolysis of iodine-containing gases in the air (data taken from the International Union of Pure and Applied Chemistry (lUPAC) Subcommittee for Gas Kinetic Data Evaluation online database http //www.iupac-kinetic.ch.cam.ac.uk/). a Photo-dissociation threshold wavelength, b Bond dissociation energy (298 K). c Wavelength of maximum absorption, d Peak molecular absorption cross-section (298 K). 

If one looks at the average bond dissociation energies for X2, C-X, H-X, and C-H bonds (Table 2.2), an average heat of reaction for the halogenation of alkanes can be calculated. The results in kcal/mol are as follows F = -101, Cl = -22, Br = -4, and I = 16. The variation in these numbers comes from a continual decrease in H-X and C-X bond strengths in the series F, Cl, Br, and I. These heats of reaction reflect a dramatic change in reactivity. Free radical fluorination is so exothermic that it occurs spontaneously and very explosively. Chlorination and bromination can be controlled and are useful reactions. Free radical iodination rarely occurs. 

The polar and steric effects on the bond dissociation energy have been investigated, and several reports can help chemists to select and/or design the proper stmcture for the iodinated transfer agent. ... 

Electron affinity and hydration energy decrease with increasing atomic number of the halogen and in spite of the slight fall in bond dissociation enthalpy from chlorine to iodine the enthalpy changes in the reactions... 

The proportions of ground-state and (2P ) excited iodine atoms produced in a photolysis using monochromatic radiation can be approximately calculated from Fig. 1. To conserve momentum, essentially all the energy from the primary process in excess of that used in bond dissociation (HI -> H+I) =... 

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