Methyl chloride, bond dissociation energy

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 ... 

A fundamentally similar situation is obtained with the initiation of styrene by alkylaluminum halides or trialkylaluminum compounds. In the case of this monomer the demarcation between poor and good coinitiators is perhaps even more evident than with isobutylene. Fig. 2 shows the essential data obtained in a series of experiments in which various alkyl chlorides were added to styrene/Al(C2H5)2Cl charges in methyl chloride 24). Alkyl halides with high R—Cl heterolytic bond dissociation energies are very poor coinitiators compared with those with lower dissociation energies. Thus 3-chloro-l-butene, crotyl chloride, fert-butyl chloride, 1-chloroethylbenzene and diphenyl chloromethane are efficient because the carbocations which arise from these chlorides... 

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 ... 

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