Tropenyl chloride

Solution-Phase Ionization of Tropenyl Chloride and Triphenyl-methyl Chloride. Insofar as thermal degradation is concerned, the critical phase in PVC processing occurs in the melt, which may reach temperatures in the vicinity of 200°C (73) over a period of some minutes. The dielectric constant of PVC (3-4 at 25°C, 11-12 at 140°C) (74) is comparable with that of small chlorocarbons (75). Of the organic chlorides whose ionization has been studied in such solvents, tropenyl chloride and triphenylmethyl chloride seem to have received the most attention. 

Tropenyl Chloride. Tropenyl chloride is thought to exist completely in the form of an ion pair in methylene chloride (69) (dielectric constant of 8.9 at 25°C) (75). Charge-transfer spectra of some tropenyl halides (69) are shown in Figure 9. In a solvent such as acetonitrile (dielectric constant of 37.5 at 20°C) (75), these absorptions are absent, and the tropenyl halides are assumed to be dissociated into solvent-separated ion pairs which are in equilibrium with free ions (as determined by conductance measurements) (69). Thermodynamic data (Appendix 2) allow the following gas-phase energy change (between tropenylium cation and cycloheptatriene) to be estimated ... 

Now this process is analogous to Reactions 5 (Table VII and Figure 5) and 6 (Table VIII and Figure 7). If we assume that the C-Cl hetero-lytic bond dissociation energies in tropenyl chloride and the polyenyl chlorides parallel the C-H bond strengths toward hydride abstraction, then it is reasonable to use the above results to extrapolate the behavior of the polyenyl chlorides into the solution phase. On this basis it seems clear that the establishment of equilibrium in Reaction 2a in solution is eminently reasonable—bearing in mind that Reaction 1 occurs in the melt and that we require only a kinetic amount of the ionized chloride... 

Ion-Pair Mechanism for the Thermal Dehydrochlorination of Poly (vinyl chloride). Clearly there is nothing in our theoretical analysis of the electronic structures and energetics of the polyenyl cations which would preclude their participation in Reaction 2 as either transition states or intermediates. However, it remains to establish the values of n at which the pathway of Reaction 2 will be competitive with other reaction processes. On the basis of our comparison (see above) with the ionization of tropenyl chloride, it is apparent that kinetic amounts of contact and perhaps even solvent-separated ion pairs will be available at n values (Equation 2) of about 1 or 2. However, in view of our previous arguments, it seems unlikely that free ions will ever achieve a high concentration in PVC. Given the fact that proton abstraction by chloride ion (Equation 2b) is necessary to complete the elimination... 

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