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Imperator Männlich
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IUPAC Basic Terminology of Stereochemistry:

cis, trans

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 -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 For alkenes the terms cis and trans may be ambiguous and have therefore largely been replaced by the E, Z convention for the nomenclature of organic compounds.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 (See IUPAC, Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F and H, Pergamon Press, 1979, p. 478, Rule E-2.3.3, E-2.3.4; IUPAC, A Guide to IUPAC Nomenclature of Organic Chemistry, Blackwell Scientific Publications, 1993, pp. 149-151, Rule R-7.1.1).

E, Z

The approved stereodescriptors of stereoisomeric alkenes R1R2C=CR3R4 (R1 R2, R3 R4; neither R1 nor R2 need be different from R3 or R4), cumulenes R1R2C[=C=C]n=CR3R4 and related systems e.g. R1R2C=NOH, HON=C{[CH2]n}2C=NOH. The group of highest CIP priority attached to one of the terminal doubly bonded atoms of the alkene, oxime, etc. or cumulene (i.e. R1 or R2) is compared with the group of highest precedence attached to the other (i.e. R3 or R4). The stereoisomer is designated as Z (zusammen = together) if the groups lie on the same side of a reference plane passing through the double bond and perpendicular to the plane containing the bonds linking the groups to the double-bonded atoms; the other stereoisomer is designated as E (entgegen = opposite). The descriptors may be applied to structures with a fractional bond order between one and two; and to double bonds involving elements other than carbon. They are not used to describe ring substitution relationships. See also cis-trans isomers.
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