(i) Cis-trans Nomenclature:
The presence of a carbon-carbon double bond restricts the freedom of rotation about a double bond. The designation cis (Latin word: same side), is used to denote the presence of like atoms or groups on the same side and trans (Latin word, across) is used when they are on opposite sides. Isomerism seen in non-cyclic, open-chain compound due to the presence of a double bond, is called π diastereoisomerism while when it occurs in a cyclic skeleton lacking a double bond, it is termed as σ diastereoisomerism.
(ii) E/Z System of Nomenclature:
The simple convention of denoting the geometrical isomers by cis/trans is not possible when there are more than two different substituents on a double bond. Hence a new system of nomenclature known as the E/Z notation method is to be adopted.
Except for very simple alkenes, the nomenclature for alkene officially uses E/Z notation. In cyclic alkanes, cis and trans terminology is retained. The E/Z notation is not used in cyclic alkanes.
The group of highest priority on double-bonded carbon atoms is first chosen according to Cahn-Ingold-Prelog (CIP) priority sequence rule.
For example, in the following structure,
Here, the priority of functional groups attached to both double-bonded carbon atoms is Br > Cl and I > H. It means the highest priority functional groups are on the same side of the double bond (i.e., cis-isomer). Hence, it is named Z-form (from the German word, Zussamen meaning together). If the highest priority functional groups are on the opposite sides of the double bond (i.e., trans-isomer), it is named as E-form (from the German word, Entagagen meaning opposite). Thus, E stands for opposite side and Z for the same side. Generally, cis-isomer is said to be Z-form, and trans-isomer is said to be E-form with few exceptions.
CIP Rules for determining priorities:
(a) The atom which has the highest atomic number is given the higher priority, in the case of simple structure, and
(b) In the most complicated molecule where a group of atoms constitutes the functional groups attached to the carbons of the double bond, the priority depends on the atomic weights of other atoms present in the group. For example, the priority in the given molecules is CH3CH2 – > CH3 and CHO > CH2OH. In the case of CHO, oxygen is counted twice because of the carbon-oxygen double bond. Hence, the given structure has Z-form. If an atom is triply bonded to another atom, treat it as if it were singly bonded to three of those atoms.
(c) For molecules with multiple double bonds, it is necessary to indicate the alkene location for each E or Z symbol. For example, the prefix (2E, 4E, 6Z, 8E) used in the IUPAC name of alitretinoin indicates that the alkenes starting at positions 2,4,8 are E while the alkene at position 6 is Z.
(iii) Syn/Anti system of Nomenclature:
The cis-trans isomerism in some classes (such as oximes, diazoates, and azo) containing one or more carbon to nitrogen or nitrogen to nitrogen double bonds is designated by syn/anti-isomerism. Syn/anti nomenclature is based upon two substituents in an acyclic molecule. For example, in stereoisomeric oxime, the configuration of oximes is usually denoted by prefixes “syn” and “anti” instead of cis and trans.
The oximes may be of two types.
(a) Aldoximes: These are derived when aldehydes are treated with hydroxylamine. (either R or R’ is hydrogen), and
(b) Ketoximes: These are derived when ketones are treated with hydroxylamine (both R or R’ are alkyl/aryl groups).
The geometrical isomerism in oximes occurs due to the restricted rotation of the C = N bond. In syn-aldoximes, both the hydrogen and the hydroxyl group are on the same side of the C = N, and in anti-form, they are on the opposite side. However, in the case of ketoxime, the syn and anti descriptors indicate the spatial relationship between the group (whose name appears first in the name of the compound) and the hydroxyl group. For example, the ketoxime of butanone may be named as either syn methyl ethyl ketoxime (methyl and OH are syn) or anti-ethyl methyl ketoxime (ethyl and OH are anti). As per E-Z notation, the syn acetaldoxime is named E-acetaldoxime whereas the anti-form is named Z-acetaldoxime.
Similarly, Syn/Anti nomenclature is also used for octahedral complex fused rings. The syn-isomer has adjacent fused rings whereas the anti-isomer has opposite fused rings.
Make sure you also check our other amazing Article on : Racemic Modification