A Stability, structure and bonding

Substitution of hydrogen, in an alkene, by fluorine leads to increased reactivity for a number of processes for example, with tetrafluoroethene, heats of addition of chlorine, hydrogenation and polymerisation are 58.5, 66.9 and 71.1kJmol greater, respectively, than for the analogous reactions with ethene [3, 29]. These observations could be attributed either to an increase in the carbon-fluorine bond strength upon changing the hybridisation of the carbon atoms bonded to fluorine [30] or to ir-bond destabilisation by fluorine [31]. 

A consideration of the cyclobutene ring-opening reactions [34, 35] (Table 7.4) reveals that the changes in hybridisation of carbon bonded to fluorine are the same for both compounds T7.4A and T7.4B, and so any changes in carbon-fluorine bond energies must also be the same. Consequently, as A// for T.7.4A is more endothermic than for T7.4B, 

These conclusions are supported by the measurement of the ir-bond dissociation energy [36] of CF2=CF2, which is 29kJmol less than that for ethene. However, the situation is less clear for partly fluorinated systems such as CF2=CH2, in which the ir-bond is 12.5kJmol more stable than in ethene [37]. 

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