Fragmentations are reactions in which a molecule breaks into pieces by the cleavage of a C-C single bond. Bond cleavage occurs due to the ‘pushing’ and ‘pulling’ of electrons, similar to that of the pinacol rearrangement. In order for this ‘push/pull’ scenario to occur, the molecule must contain an electron source (typically a lone pair of electrons) and an electron sink (a leaving group of some description).
The atom numbering in the diagrams goes from the electron source to sink. Two examples of bond polarization are shown below using different electron sinks. Click the links below to view the individual reactions:
Type of electron sink