Nomenclature Cahn-Ingold-Prelog priority system

Derivatives and Isomers of Alkenes Nomenclature The Cahn-Ingold-Prelog Priority System Relative Stability of Alkenes Heats of Formation Double Bonds in Rings Physical Properties of Alkenes Alkynes Structure and Bonding... 

FIGURE 1.9 cis trans (or Z/ ) stereochemistry. If a = a we use the cis/trans nomenclature, but if a a (which is the case for fatty adds because the terminus of the a group is the CH3 group and the terminus of the a group is carboxylate), we should more correctly use the Z/E system based on Cahn-Ingold-Prelog priority rules. In practice, it must be noted that most people still use the cis/treats system for the stereochemistry of fatty acids. 

In the stereochemical nomenclature of coordination compounds, the procedure for assigning priority numbers to the ligating atoms of a mononuclear coordination system is based upon the standard sequence rules developed for chiral carbon compounds (the Cahn, Ingold, Prelog or CIP rules6, see Section IR-9.3.3.2). 

The first proposed nomenclature suggested that isomers should be called cis when W and Y are on the same side of the double bond and trans when they are on the opposite side (Fig. 1.2), provided W X and Y Z. However, this nomenclature was limited to the particular case where W and Y are identical. The more recent nomenclature of Cahn-Ingold-Prelog, based on the German Zusammen (Z) and Entgegen (E) notation, was extended to systems where W and Y are different substituents (Fig. 1.2). There is no direct relation between the two nomenclatures since they depend on the nature of substituents and so the Z isomer is not necessarily cis. Moreover, the order of priority is determined by the atomic number of each atom connected to the C=C double bond [1]. Although the E/Z nomenclature may also be applied to compounds B/B and C/C, these are considered as conformational isomers, whereas compounds A/A are configurational isomers. 

In order to be able to define the absolute stereochemistry of a molecule, we therefore use another system of nomenclature which was proposed by Cahn, Ingold and Prelog in 1956. This system uses the prefixes R- and S-, derived from the Latin words rectus (right) and sinister (left) and the same system of priority setting that was discussed above in relation to E- and Z-isomers. We start by assigning the priorities of the four groups... 

To differentiate two enantiomers, we need a system of nomenclature that indicates the arrangement or configuration of the four groups about the chiral center. Such a system was developed by three chemists, R. S. Cahn, C. Ingold, and V. Prelog. The first step is to rank the four substituents in order of decreasing priority. The rules for assigning priorities will be described shortly. 

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