Optical activity, nomenclature

The discoveries of optical activity and enantiomeric structures (see the box, page 97) made it important to develop suitable nomenclature for chiral molecules. Two systems are in common use today the so-called d,l system and the (R,S) system. 

Dipentene is the racemic form of the optically active d-limonene and 1-limonene, terpenes which are found to a very large extent in essential oils. Since an equal mixture of d-iimonene and f-limonene is dipentene, it is obvious that whenever optically active limonene is found with a rotation below the maximum, it must contain dipentene. Mixtures of equal quantities of a compound of the optically active limonenes are identical with the corresponding compound prepared from dipentene. It is therefore obvious that the nomenclature is unfortunate and dipentene should be termed i-limonene. 

The (I ,S)-nomenclature still reminds the user of the right and left handed helical pattern arising from Fresnel s 29> interpretation of optical activity. These patterns are characterized by the combination of a translational and a rotational direction. The Ta skeletal symmetry of tetracoordinate systems submits itself to the pictorial models not applicable to other configurational types. The CIP rules may as well be used to define a configurational nomenclature on the basis of the Fischer projection. If one specified that in such a projection of an (R)-... 

Since the first reaction undoubtedly proceeds with inversion of configuration at sulfur, and since additional experiments demonstrated that the formation of chlorosulfurane 177 from sulfoxide 180 takes place with retention at sulfur, (5>chirality was assigned to (+>177. As for the designation of absolute configuration, Martin and Balthazor (195) proposed a system of nomenclature for optically active pentacoordinate species. 

It is now possible to incorporate the configuration of the compound into its nomenclature to give more detail. (—)-Serine becomes (—)-(5 )-serine, whilst (+)-malic acid becomes (+)-(/f)-malic acid. Because there is no relationship between (+)/(—) and configuration it is necessary to quote both optical activity and configuration to convey maximum information. The... 

The nomenclature for describing optically active inorganic complexes and ligand conformations requires some discussion. Formerly, active... 

R) and (S) Nomenclature of Asymmetric Carbon Atoms 181 5-4 Optical Activity 185... 

The alternative names for these two isomers in terms of R and S nomenclature refers to their relative positions within the molecule, but the explanation of this system is beyond the scope of this text and it does not add further relevance or explanations to the principles outlined. (See optical activity in the Glossary.)... 

The transfer of the nomenclature of isotropic optical activity is surely acceptable where it evidently yields a descriptive picture of the experimental observations. However, precautions are necessary as to apparently self-evident implications, this is all the more important since the anisotropic nature of the sample is by no means obvious when observing parallel to the optical and thus helical axis. An unbiased and complete record of how a cholesteric sample acts on the measuring radiation can be obtained by el-lipsometry. This method (compare Sec. 6.4) yields a comprehensive description of the state of polarization including the degree of polarization (Rbseler, 1990). An adequate simulation can be based on the Berreman formalism (1972) rendering possible a study of particularities observed, such as pronounced depolarization related to the selective reflection band (Reins et al., 1994). 

As each of these menthenes will yield isomeric mentha di-enes the number of isomers possible in this group is still larger. That is, one men-thane yields six menthenes and these a larger number, viz., fourteen, mentha-di-enes. Furthermore, stereo-isomerism with accompanying optical activity due to the presence of asymmetric carbons, increases the number of possible isomers. It will not be necessary to dwell further upon the isomerism of the terpenes it being necessary simply to explain the fact of the existence of structural isomers and of stereo-isomers with optical activity. The system of nomenclature of the isomers will not be considered. Reference to larger books will be necessary to make this plain. 

Over the years, several nomenclature systems have been developed to characterize the relationship between enantiomers. The system based on optical activity and the classification of enantiomers as dextrorotatory [d or (+)] or levorotatory [1 or (—)] already has been described. However, this system of nomenclature is of limited applicability because the sign of rotation, (+) or (—), does not predict the absolute configuration or the relative spatial arrangement of atoms in the enantiomers. In an attempt to designate the precise configurations about carbon centers of asymmetry, the Cahn-Ingold-Prelog RjS system have been developed and adopted as the most commonly used nomenclature system for isomers. 

Remember that the R/S, +/-, and d/l nomenclatures all arise from different observations and the fact that a molecule has, say, the R configuration gives no clue as to whether it will have + or - optical activity or be labelled D or L. Never try and label a molecule as d/l, or +/-, simply by working it out from the structure. Likewise, never try and predict whether a molecule will have a + or - specific rotation by looking at the structure. 

The system of nomenclature for alkaloids of the tetrahydro series is somewhat confusing. Naturally occurring members of this series often occur in one of their optically active forms understandably, at the time of isolation, they were given trivial names of their own. The trivial names have survived despite the fact that the relationship of these alkaloids as tetrahydro derivatives of known quaternary alkaloids of the dibenzo Iquinolizine series (cf. berberine), which themselves have trivial names, has in most cases been clearly established. Although it would seem desirable to retain the names of the quaternary alkaloids of the series as parent names and to designate alkaloids of the tetrahydro series accordingly, this has its disadvantages in that the related quaternary bases of some alkaloids of the tetrahydro series are not known to occur in nature. 

Comment. This rule is simply an attempt to adapt the best organic nomenclature practices to coordination compounds. Difficulty arises because those who have worked with optically active inorganic compounds almost invariably have used d- and l- to designate the sign of rotation of the coordinated group and large capitals D and L for the sign of rotation of the coordination entity. 

Besides the classification of the elements and compounds, there are related chemical systems that illustrate the intricacies and not quite hierarchical structure of scientific classification schemes. Molecular systems that crystallize are further classified by crystal structure. Organic molecules have an elaborate system of classification called "nomenclature" based on both their chemical composition and their symmetries. The classifications are correlated with properties that do not define the classification. Thus, the classification of the elements is associated with a numerical property—atomic weight—and molecular symmetries are associated with optical activity. What are the formal structures that make multiple classifications correct, and how does the existence of one classification scheme constrain others What is the purpose, methodological or otherwise, of corresponding properties Chemistry provides our most familiar examples of settled scientific classifications, and when we want to know what makes for such certainty, chemistry is where we should look. 

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