Chlorine, bond dissociation energy

Carbon-Hydrogen and Carbon-Chlorine Bond Dissociation Energies of Selected Compounds... 

Of this group only benzyl chloride is not an aryl halide its halogen is not attached to the aromatic ring but to an. v/r -hybridized carbon. Benzyl chloride has the weakest carbon-halogen bond, its measured carbon-chlorine bond dissociation energy being only 293 kJ/mol (70 kcal/mol). Homolytic cleavage of this bond produces a resonance-stabilized benzyl radical. 

Compare chlorination of methane with lodina tion The relevant bond dissociation energies are given in the equation... 

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-... 

What are the reasons for the observed reactivity order of alkane hydrogens toward radical chlorination A look at the bond dissociation energies given previously in Table 5.3 on page 156 hints at the answer. The data in Table 5.3 indicate that a tertiary C—H bond (390 kj/mol 93 kcal/mol) is weaker than a secondary C-H bond (401 kj/mol 96 kcal/mol), which is in turn weaker than a primary C H bond (420 kj/mol 100 kcal/mol). Since less energy is needed to break a tertiary C-H bond than to break a primary or secondary C-H bond, the resultant tertiary radical is more stable than a primary or secondary radical. 

The hemolytic bond dissociation energies, AH°, of hydrogen and chlorine ... 

The heat of reaction is simply the bond dissociation energy for a chlorine molecule, and it is highly endothermic. 

The presence of chlorine atoms in the ketone molecule leads to a further complication, since the C-Cl bond dissociation energy may be of the order of 80 kcal. mol.-1, so that a photodecomposition of type C becomes energetically feasible. 

In our second example we look at the reduction of chlorinated ethenes at a nickel electrode and at the surfaces of two zero-valent metals [Fe(0), Zn(0)]. To gain insight into the rate-limiting process(es) in these cases, we consider how the relative overall reduction rates (relative to PCE) of PCE, TCE, and the three DCE isomers (see Fig. 14.15 for structures) vary as a function of two common descriptors used in QSARs, the one-electron reduction potential (EJ Fig- 14.17a) and the bond dissociation energy (DR X Fig. 14.176). In all these systems, the reduction rates were found to be significantly slower than diffusion of the compounds to the respective surfaces. Therefore, the large differences in the relative reactivities of the compounds between the systems reflect differences in the actual reaction at the metal surface. 

Once produced, a chlorine atom can remove a hydrogen atom from a methane molecule and form a methyl radical and a hydrogen chloride molecule. The bond-dissociation energies of CH4 (104 kcal) and HC1 (103.1 kcal) suggest that this reaction is endothermic by about 1 kcal ... 

Use of bond-dissociation energies gives a calculated AH0 of—26 kcal for this reaction, which is certainly large enough, by our rule of thumb, to predict that Kqq will be greater than 1. Attack of a methyl radical on molecular chlorine is expected to require a somewhat more oriented collision than for a chlorine atom reacting with methane (the chlorine molecule probably should be endwise, not sidewise, to the radical) but the interatomic repulsion probably should not b much different. 

Similarly, if we look at the H-C bond-dissociation energies of the hydrocarbons shown in Table 4-6, we would infer that Cl- would remove a hydrogen most rapidly from the carbon forming the weakest C-H bond and, again, this is very much in accord with experience. For example, the chlorination of methylbenzene (toluene) in sunlight leads to the substitution of a methyl hydro-... 

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