Cahn nomenclature

Cadiot-Chodkiewicz coupling 99 Cahn nomenclature 117 calamitic compounds 134 f... 

R.S. Cahn, O.C. Dermer, Introduction to Chemical Nomenclature, 5th ed., Butter-worths, London, 1979. 

The prefix (RS) is used to denote a racemic modification. For example, (RS)-Sec butyl chloride. The symbols R and S are applied to compounds whose absolute stereochemistry has been determined. However, while applying the nomenclature to projection formulae of compounds containing several asymmetric centres Cahn, Ingold. Pielog procedures are supplemented by the following conversion rule. 

FIGURE 1.1 Chemical and stereochemical nature of amino acids. Substituents in (a) and (b) are on opposite sides of the plane N-Ca-C, the bold bond being above the plane. Interchange of any two substituents in (a) changes the configuration. For the Cahn-Ingold-Prelog system of nomenclature, the order of preference NH2 > COOH > R2 relative to H is anticlockwise in (a) = (S) and clockwise in (c) = (R). 

Nano-structures comments on an example of extreme microstructure In a chapter entitled Materials in Extreme States , Cahn (2001) dedicated several comments to the extreme microstructures and summed up principles and technology of nano-structured materials. Historical remarks were cited starting from the early recognition that working at the nano-scale is truly different from traditional material science. The chemical behaviour and electronic structure change when dimensions are comparable to the length scale of electronic wave functions. Quantum effects do become important at this scale, as predicted by Lifshitz and Kosevich (1953). As for their nomenclature, notice that a piece of semiconductor which is very small in one, two- or three-dimensions, that is a confined structure, is called a quantum well, a quantum wire or a quantum dot, respectively. 

Scheme 2.1 Kinetic resolution of glycidyl butyrate catalyzed by porcine pancreatic lipase, PPL The preferential conversion of (S)-glycidyl butyrate into (R)-glycidol and butyric acid results from the stereochemical nomenclature rules under the Cahn, Ingold, Prelog convention, the configuration around the chiral centre is not affected.

In response to this nomenclature dilemma, the Cahn-Ingold-Prelog (IUPAC, International Union of Pure and Applied Chemistry) system of nomenclature was developed and is now the standard mediod to specify the relative configuration of chiral centers in molecules. Each chiral center will have two possible mirror-image configurations, which are designated as eidter R or S. 

Notice that in the previous examples, the atom of the higher atomic weight is given the higher priority (Br = 79.1 versus Cl = 15.5, and I = 126.0 versus N = 14.0). These assignments are based on the priority rules of Cahn-Ingold-Prelog nomenclature. 

The d, l nomenclature is chosen in this chapter in order to unmistakably distinguish homochiral stereoisomers. According to the specifications of the Cahn-Ingold-Prelog nomenclature L-l would equal (S)-l, while L-2 equals (J )-2. 

Cahn RS, Dermer OC (1979) Introduction to chemical nomenclature, Butterworth,... 

Deprotonation of a-alkylated acetic acid esters (e.g., the propionic acid ester of Figure 10.13) with LDA at —78°C selectively yields the -enolatcs.Thc quotation marks indicate that this application of the term is based on an extension of the E/Z-nomenclature here, the Cahn-Ingold-Prelog priority of the O I, i1 substituent is considered to be higher than the priority of the OR group. The deprotonation of the ester shown in Figure 10.13 occurs via the strain-free transition state A. The alternative transition state B is destabilized by a 1,3-diaxial interaction. 

In Figure 21, attention is directed to a molecule for which identical modules are bound to non-overlapping halves of a ring. Cahn and Dermer [45], name this compound trans-p-(l-4-r 5-8-r -cyclo-octatetraene)- bis-(tricarbonyliron). Godwin [46], on the other hand, recognizes that there is the possibility for connections to only some of the atoms and thus elects to include all relevant locant numbers namely p-(l,2,3,4-r - 5,6,7,8-T -cycloocta-l,3,5,7-tetraene)- bis-(tricarbonyliron). In the nomenclature being developed in this... 

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