Reactions Involving Biradical Intermediates ER

To break the C—C bond in ethane requires a great deal of energy (88 kcal/mole), so the reaction 

A biradical is a species in whose ground state there are two unpaired electrons. This situation can occur either because the radical centers are separated by one or more saturated carbon atoms and so cannot interact with one another, or in special cases, such as the triplet ground state of O2 (p- 36), where the unpaired electrons occupy orbitals of different symmetry and so again are unable to interact. The term should not be used for excited states, in particular triplet excited states, since their behavior is different from that of true biradicals (see Chapter 6, p. 397). 

Stability of the latter depends on resonance interactions that are present neither in the reactants nor in the products, the rate of the overall reaction is unrelated to the heat of reaction. Processes of this kind are therefore of anti-BEP type and are termed ERj. 

The biradicals in ERa reactions are usually formed either by bond cleavage or by the combination of two unsaturated molecules. Thus two molecules of ethylene can combine to form a butadiyl biradical, 

This process is less endothermic than simple C—C bond fission, the calculated heat of reaction being + 62 kcal/mole. 

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