Rearrangements are of two types: intramolecular processes in which the group which migrates does not become completely detached from the system in which rearrangement is occurring, and intermolecular processes, in which the migrating group is first detached and later re-attached at another site. Intramolecular rearrangements are conveniently subdivided into those which occur in electron-deficient systems and those which occur in electron-rich systems. One of the simplest systems within which carbon migrates, with its bonding-pair, to an electron-deficient carbon atom is the neopentyl cation. The migration step is similar to that in the pinacol reaction: the movement of R may be visualized as being brought about by the combination of the 'pull' from nitrogen and the 'push' from the oxyanion. In an unsymmetrical ketone, that group migrates which is the better able to supply electrons, as in the Wagner–Meerwein and related rearrangements. The Hofmann rearrangement is convenient providing that functional groups in the molecule do not react with bromine and alkali.