Optical isomerism defined

Naturally Derived Materials. The following are descriptions of some of the most important naturally derived materials in use. Importance in this context is defined in terms of the total value of the materials, which range from expensive, low volume materials that have great aesthetic value to relatively inexpensive and widely used products. Eor some of the naturals, it is indicated whether they can be distilled to provide individual chemicals for use as such or as intermediates. Materials produced in this way from a given natural source are usually not interchangeable with those from other naturals or synthetics. In some cases this may be due to optical isomerism, which can have a significant effect on odor, but usually it is due to trace impurities. 

Optical isomerism is concerned with chirality, and some important terms relating to chiral complexes are defined in Box 19.2. The simplest case of optical isomerism among fi -block complexes involves a metal ion surrounded by three didentate ligands, for example [Cr(acac)3] or [Co(en)3] (Figures 3.16b and 19.12). These are examples of tris-chelate complexes. Pairs of enantiomers such as A-and A-[Cr(acac)3] or A- and A-[Co(en)3]Cl3 differ only in their action on polarized hght. However, for ionic complexes such as [Co(en)3], there is the opportunity to form salts with a chiral counter-ion A. These salts now contain two different types of chirality the A- or A-chirality at the metal centre and the (-I-) or (—) chirality of the anion. Four combinations are possible of which the pair (A-(- -) and A-(—) is enantiomeric as is the pair A-(—) and A-(- -). However, with a given anion chirality, the pair of salts A-(—) and A-(—) are diastereomers (see Box 19.2) and may differ in the packing of the ions in the solid state, and separation by fractional crystallization is often possible. 

The history of optical isomerism goes back to the year 1815, when the phenomenon was discovered by the French physicist Jean-Baptist Biot [1]. Optical activity was defined as the ability of a substance to rotate the plane of polarisation of light. Some years later, his student, Louis Pasteur [2] proposed that this optical activity of certain organic compounds was a consequence of their molecular asymmetry, that produces non-super-imposable mirror-image structures. A molecule which is not super-imposable on its mirror image is said to be chiral Conversely, a molecule which is superimposable on its mirror image is described as achiral... 

The occurrence of enantiomers (optical isomerism) is concerned with chirality, and some important terms relating to chiral complexes are defined in Box 19.3. Enantiomers of a coordination compound most often occur when chelating ligands are involved. Figure 19.13a shows [Cr(acac)3], an octahedral tris-chelate complex, and Fig. 19.13b shows cw-[Co(en)2Cl2]", an octahedral bis-chelate complex. In this case, only the cw-isomer possesses enantiomers the tra 5-isomer is achiral. Enantiomers are distinguished by using the labels A and A (see Box 19.3). 

Define the terms geometric isomerism and optical isomerism and give an example of each. 

Definitions. Define and illustrate the following terms (a) isomerism, (b) homologous series, (c) conformation, (d) configuration, (e) geometrical isomerism, (f) optical isomerism, (g) aromaticity, (h) functional group, (i) unsaturation, (j)... 

Define each term related to optical isomerism enantiomers, chiral, dextrorotatory, levorotatory, racemic mixture. 

Before the relationship between chirality and optical activity was known, enantiomers were called optical isomers because they seemed identical except for their opposite optical activity. The term was loosely applied to more than one type of isomerism among optically active compounds, however, and this ambiguous term has been replaced by the well-defined term enantiomers. 

These authors have found that the decrease in specific rotation at low pH is preceded by a moderate increase, measured at 313 m/i, which is coincident with the isomerizing process as followed electrophoretically. The selective action of thiocyanate and perchlorate has enabled them to resolve this process into two well-defined optical transitions, one of which coincides with electrophoretic change and the other with the appearance of a solvent-induced difference spectrum for tyrosyl residues. One might expect the increase in specific rotation to reflect an increase in helical content, but dispersion data at various pH s in the isomerization indicate that the opposite occurs, for the helical content by bo drops from 48 to 40%. 

In order to define the stereochemical characteristics of the cyclopentene forming reaction, the thermal isomerization of optically pure (-f)( 15,25)-rruns,rrans-2-methyl-l-propenylcyclopropane (205) has been investigated (equation 138). 

Geometric isomerism was first defined by Wislicenus in 1887 as isomerism occurring in compounds where rotation is restricted by double bonds or ring systems. Geometric isomers do not rotate the plane of polarized light (unless they also contain a chiral center), and hence are not optically active. 

The data collected so far (23-28) has shown that the norisoprenoids 8-11 are present in natural substrates in well-defined mixtures, thus enabling inter alia authenticity control of natural flavoring material. (Note The only exception are raspberries, which showed considerable variations in their composition of isomeric theaspiranes depending on their origin). The data also shows that obviously different enzymes are operative in plants, catalyzing the formation of different optical antipodes of Ci3-volatiles. TWs is evident from the fact that, e.g., vanilla beans were found to contain optically pure (2S)-isomers of vitispiranes lOb/d, whereas white-beam (Sorbus aria) leaves contain mainly the (2/f)-configured isomers lOa/c (25). 

The structural features of the optically inactive scopoline from the hydrolysis of scopolamine are not as clearly defined as those of tropine. The hydrolysis of scopolamine and the isolation of scopoline was first reported in 1881. Early analyses indicated a formula isomeric with that of tropine (CsHisON) (20), which was later revised to C8H15O2N, and was finally corrected to C8H13O2N (25). 

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