Stereochemical Specification

R-7.1.1 cis and trans Isomers

Atoms or groups are called cis or trans to one another when they project respectively on the same or on opposite sides of a reference plane identifiable as common among stereoisomers. The compounds in which such relations occur are termed cis/trans-isomers. For doubly bonded atoms, the reference plane contains these atoms and is perpendicular to the plane containing the doubly bonded atoms and the atoms directly attached to them. For cyclic compounds, the reference plane is that in which the ring skeleton lies or to which it approximates. When alternative numberings of a ring or ring system are possible, that numbering is chosen which gives a cis attachment at the first point of difference.

Examples to R-7.1.1

When one substituent and one hydrogen atom are attached at each of more than two positions of a monocycle, the steric relations of the substituents are expressed by adding r (for reference substituent) followed by a hyphen and the locant of another substituent, thus expressing its relationship to the reference substituent.

Examples to R-7.1.1

When two different substituents are attached at the same position of a monocycle, then the lowest-numbered substituent named as a suffix is selected as reference group. If none of the substituents is named as a suffix, then that substituent of the pair of substituents having the lowest number, and which is preferred by the sequence rule (see R-7.2.1), is chosen as the reference group. The relationship of the sequence-rule-preferred substituent at geminally substituted positions, relative to the reference group, is cited as c- or t-, as appropriate.

Examples to R-7.1.1


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