Carbon-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. 

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-... 

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-... 

Branching should incorporate weaker tertiary bonds in PVC, either carbon-hydrogen or carbon-chlorine. Secondary and tertiary C-H bonds have bond dissociation energies of 94 and 90 Kcal/mole respectively. A tertiary carbon-chlorine bond may require even more energy for homolytic cleavage than a secondary one, i.e. 75 versus 73 Kcal/mole (20). Cleavage of the branch... 

Examples of the temperature dependence for different classes of molecules are given as global plots of In KTm versus 1,000/T. The curves that are drawn used the equations for the complete model. Excited-state Ea have been measured with the ECD. The clearest indication of an excited state is structure in the data, as illustrated for carbon disulfide and C6F6. The temperature dependence of the ions formed in NIMS of the chloroethylenes indicate multiple states. NIMS also supports AEa, as in the case of SF6 and nitrobenzene. The quantity D Ea can be obtained from ECD data for DEC(2) dissociative thermal electron attachment. If one is measured, then the other can be determined. In the case of the chlorinated benzenes this quantity gives the C—Cl bond dissociation energy. The highest activation energy of 2.0 eV has been observed for the dissociation of the anion of o-fluoronitrobenzene. 

PROBLEM 23.1 Consider all the isomers of C7H7CI containing a benzene ring and write the structure of the one that has the weakest carbon-chlorine bond as measured by its bond dissociation energy. 

PROBLEM 11.26 Analyze the thermochemistry of the two propagation steps for the photochlorination of ethane. The bond dissociation energies of the chlorine-chlorine and carbon-chlorine bonds of ethyl chloride and the carbon-hydrogen bond of ethane are 59, 84.8, and 101.1 kcal/mol, respectively. 

Ionic or polar reactions of alkyl halides rarely are observed in the vapor phase because the energy required to dissociate a carbon-halogen bond heterolyti-cally is almost prohibitively high. For example, while the heat of dissociation of chloromethane to a methyl radical and a chlorine atom is 84 kcal mole-1 (Table 4-6), dissociation to a methyl cation and a chloride ion requires about 227 kcal mole-1 ... 

The effect on dissociation energies of bonding chlorine atoms to carbon, as compared wdth hydrogen atoms can be seen in the following set of dissociation energies ... 

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