Asymmetric carbon atom, description

Asymmetric carbon atom, description, 57 Asymmetric molecule, enantiomorphic forms, 57... 

The dissection of a molecular model into those components that are deemed to be essential for the understanding of the stereochemistry of the whole may be termed factorization (9). The first and most important step toward this goal was taken by van t Hoff and Le Bel when they introduced the concept of the asymmetric carbon atom (10a, 1 la) and discussed the achiral stereoisomerism of the olefins (10b,lib). We need such factorization not only for the enumeration and description of possible stereoisomers, important as these objectives are, but also, as we have seen, for the understanding of stereoselective reactions. More subtle differences also giving rise to differences in reactivity with chiral reagents, but referable to products of a different factorization, will be taken up in Sect. IX. 

According to the above rule, for instance, one has to select Xq = 228 nm for compound 20 (Table 2) and also = 228 nm for 24 (Table 2). The description of the ORD of the atomic asymmetry term is more complicated. Each of the three terms (representing the ORD of the three asymmetric carbon atoms (k) (k = 1, 2, 3) of I) may exhibit a different wavelength dependence according to equation 19. ... 

A more precise description of the absolute configuration of the asymmetric carbon atom is the R and S convention. The basic rules are simple. 

While the structural description of low molecular weight compounds with asymmetric carbon atoms is explicit, there are no similarly accurate rules for the description of polymers. Tertiary carbon atoms in polyolefin chains are not asymmetric in a general chemical sense. Even with one of the substituents bearing a double bond at its end and the other terminated by an ethyl group, they are very similar. Therefore, these carbon atoms are often called pseudo asymmetric. The differences between the three forms of polypropene with identical molecular weight distribution and branching percentage are considerable (Table 13). 

The arrangement of atoms in three-dimensional space changes both structure and reactivity and is described in several different ways in chemical substance nomenclature (see Stereochemistry Representation and Manipulation). The most common stereochemical descriptions arise from multiple bonds and from asymmetric carbon atoms. Often, as in CAS systematic nomenclature, these stereochemical descriptions are handled as separate character strings not directly part of the chemical substance name. 

A second problem that has repeatedly concerned us is the inability of the Sequence Rule to provide descriptors for some elements of stereoisomerism. When Cahn et al. (16) first encountered this problem with the all-cis and all-trans isomers of inositol, they attributed it to the fact that the symmetry has become so high that they have no asymmetric, nor even a pseudo-asymmetric atom. This interpretation, we believe, is incorrect. If the two ring ligands of any carbon atom of m-inositol were not heteromorphic, their exchange could not yield an isomer, as it clearly does. Each atom is a center of stereoisomerism with a pair of enantiomorphic ligands (Cg+g hi) and indistinguishable from the traditional pseudoasymmetric atom. The description of cu-inositol as all-5 could be accomplished by the same device that would allow one to specify the configurations of C(l) and C(4) of 4-methylcyclohexanol. 

Colmnn 13 of the tables indicate briefly the conformation of the polymer chain in the crystal (in a hehcal notation) either as reported in the reference cited or as inferred from the value of the fiber axis. The designation n p/q specifies the munber (n) of skeletal atoms in the asymmetric imit of the chain and the number of such asymmetric units (p) per q turns of the helix in the crystallographic repeat. Thus, poly(ethylene), as listed, has two carbon atoms in the backbone with one such imit per turn in the repeat - it is designated as a 2 1/1 helix. Alternatively, poly(ethylene) considered a poly(methylene) would be designated as a 1 2/1 helix - an entirely equivalent description of the eonformation. Note that n may differ from the number of ehain atoms in the constitutional base unit. On the one hand, isotactic poly(propene) has two skeletal atoms in the asymmetric unit and three units per turn (2 3/1). On the other hand, syndiotactic poly-(propene) with the same eonstitutional base unit as isotactie... 

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