The term is traditionally applied to any reaction that involves a change of @[email protected] (sometimes including hydrogen), and violates the so-called 'principle of minimum structural change'. According to this oversimplified principle, @[email protected] do not isomerize in the course of a @[email protected], e.g. @[email protected], or the change of a @[email protected] of a chemical species into a different @[email protected] is not expected to involve the making or breaking of more than the minimum number of bonds required to effect that @[email protected] For example, any new substituents are expected to enter the precise positions previously occupied by displaced groups. The simplest type of @[email protected] is an @[email protected] reaction in which the product is @[email protected] with the reactant (one type of '@[email protected] @[email protected]'). An example is the first step of the Claisen @[email protected]: The definition of molecular @[email protected] includes changes in which there is a @[email protected] of an atom or bond (unexpected on the basis of the principle of minimum structural change), as in the reaction: where the @[email protected] can formally be represented as the '1,2-shift' of @[email protected] between adjacent carbon atoms in the @[email protected]: Such migrations occur also in radicals, e.g.: The definition also includes reactions in which an @[email protected] takes up a different position from the @[email protected], with accompanying @[email protected] An example of the latter type is the 'allylic @[email protected]': A distinction is made between '@[email protected] rearrangements' (or 'true molecular rearrangements') and '@[email protected] rearrangements' (or 'apparent rearrangements'). In the former case the atoms and groups that are common to a reactant and a product never separate into independent fragments during the @[email protected] (i.e. the change is @[email protected]), whereas in an '@[email protected] @[email protected]' a migrating group is completely free from the parent molecule and is re-attached to a different position in a subsequent step, as in the Orton reaction: