Radicals - Introduction
Radicals of Bicyclic Compounds
Radical reactions with bicyclic compounds have a good possibility of getting information about the three-dimensional structure of radicals if the radical is generated at one of the bridgehead atoms.
Due to the high rigidity of the bicycle, these radicals cannot have a planar structure with a sp2 hybridized radical carbon atom. Such radicals are just formed particularly slowly. This provides proof of the higher stability of planar carbon-centered radicals.
The reaction rate of the radical decarboxylation of bicyclo[2.2.2]octane-1-carbaldehyde into bicyclo[2.2.2]octane is, for example, 500 to 1000 times lower than the reaction rate of a comparable reaction with an intermediately formed t-butyl radical. However, as far as the analogous ionic solvolysis is concerned, the reaction rate of the bicyclic reactant may be up to times lower.
The three-dimensional, interactive molecular model (Chime) below illustrates that the bridgehead radical carbon and its substituents can, obviously, not consume a planar structure due to the high rigidity of the bicycle. If this were the case, the ring strain would then be too strong.