Bromine, dissociation energy

Bromination of methane is exothermic but less exothermic than chlorination The value calculated from bond dissociation energies is AH° = -30 kJ Al though bromination of methane is energetically fa vorable economic considerations cause most of the methyl bromide prepared commercially to be made from methanol by reaction with hydrogen bromide... 

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 same ideas may be applied to the other processes of Fig. 1. The work required to dissociate a diatomic molecule into two electricallt/ neutral atoms may he quite small the dissociation energy of the bromine molecule Br2 in a vacuum, for example, is only 1.915 electron-volts. On the other hand, the work to dissociate a molecule into two atomic ions in a vacuum cannot be as small as this, since work must be done to set up the full electrostatic field of the positive ion, and the full electrostatic field of the negative ion and this must amount to at least a few electron-volts.1 In addition, the non-electrostatic forces may make a small or large contribution. 

Why does bromination with NBS occur exclusively at an allylic position rather than elsewhere in the molecule The answer, once again, is found by looking at bond dissociation energies to see the relative stabilities of various kinds of radicals. 

Alkylbenzenes such as toluene (methylbenzene) react with NBS to give products in which bromine substitution has occurred at the position next to the aromatic ring (the benzyiic position). Explain, based on the bond dissociation energies in Table 5.3 on page 156. 

The physical properties of the interhalogens are intermediate between those of their parent halogens. Trends in the chemistry of the interhalogen fluorides can be related to the decrease in bond dissociation energy as the central halogen atom becomes heavier. The fluorides of the heavier halogens are all very reactive bromine trifluoride gas is so reactive that even asbestos burns in it. 

There is a clear antiperiplanar preference for the reaction (Scheme 4.2) due to the stabilization of the radical by coupling of the unpaired electron with bromine (ESR) in the first case. The weaker bond dissociation energy leads to a more favorable standard potential and a weaker intrinsic barrier. When the two conformers are present and can convert one into the other, the reduction follows a CE mechanism (Section 2.2.2), which goes through the more reducible of the two.1 2... 

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. 

Write initiation, propagation, and termination steps for this radical-chain reaction. Estimate a AH0 for the overall reaction using the bond-dissociation energies of Table 4-6. Would you expect bromotrichloromethane to be a selective or nonselective brominating agent Explain. 

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. 

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 ... 

Bromine— The dissociation energy of bromine has been well established spectroscopically by a band convergence limit 217. We follow the N.B.S. in putting A///(Br g)25 c = 26-71 kcal529. 

A bromine radical can abstract either a 1° or a 2° hydrogen from propane, generating either a 1 ° radical or a 2° radical. Calculating AH° using bond dissociation energies reveals that both reactions are endothermic, but it takes less energy to form the more stable 2° radical. 

This important dissociation energy D(R-H) was obtained from the thermochemical relationship D(R-H) = Z)(H-I) + Ef- Ef. This method was exploited by Benson and co-workers to great effect to obtain large numbers of BDE values. Parallel but less extensive work was done on bromination kinetic studies (by Kistiakowsky, Whittle and others). 

---