The term is traditionally applied to any reaction that involves a
change of
connectivity (sometimes including hydrogen), and violates
the so-called '
principle of minimum structural change'. According to
this oversimplified principle,
chemical species do
not isomerize in the course of a
transformation, e.g.
substitution, or the
change of a
functional group of a chemical species into a different
functional group is not expected to involve the making or breaking of
more than the minimum number of bonds required to effect that
transformation. For example, any new substituents are expected to
enter the precise positions previously occupied by displaced
groups. The simplest type of
rearrangement is an
intramolecular reaction in which the product is
isomeric with the reactant (one type of '
intramolecularisomerization'). An example is the first step of the Claisen
rearrangement:
The definition of molecular
rearrangement includes
changes in which there is a
migration of an atom
or bond (unexpected on the basis of the principle of minimum
structural change), as in the reaction:
where the
rearrangement stage
can formally be represented as the '
1,2-shift' of
hydride between
adjacent carbon atoms in the
carbocation:
Such migrations occur also in radicals, e.g.:
The definition also
includes reactions in which an
entering group
takes up a different position from the
leaving
group, with accompanying
bond migration. An example of the
latter type is the '
allylic rearrangement':
A distinction is made
between '
intramolecular rearrangements' (or '
true molecular
rearrangements') and '
intermolecular
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
rearrangement
stage (i.e. the change is
intramolecular), whereas in an
'
intermolecularrearrangement' 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:
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1142
