Descriptors which show the relationship between two
ligands attached to separate atoms that are connected by a double bond or are contained in
a ring. The two
ligands are said to be located
cis to each other if they lie on the same side of a plane. If they are on opposite sides,
their relative position is described as
trans. The appropriate reference plane of a double bond is perpendicular to that of the
relevant σ-bonds and passes through the double bond. For a ring (the ring being in
a
conformation, real or assumed, without re-entrant angles at the two substituted atoms) it is the
mean plane of the ring(s). For
alkenes the terms
cis and
trans may be ambiguous and have therefore largely been replaced by the
E,Z convention for the nomenclature of organic compounds. If there are more than two
entities attached to the ring the use of
cis and
trans requires the definition of a reference substituent.
Source:
PAC, 1996, 68, 2193
(Basic terminology of stereochemistry (IUPAC Recommendations 1996))
on page 2203