Isomers and enantiomers

This chapter has reported the only extensive and coordinated investigation of the effects of chirality on the properties of monolayer films spread at the air-water interface. Twenty compounds of varied headgroup and chain length have been examined carrying one and two chiral centers. In every case, all of the optical isomers—enantiomers and diastereomers—were made and their properties measured both as pure compounds and as mixed monolayers in order to compare phase changes in the films with mixed melting points of the crystals. 

Stereoisomers are chemical componnds having the same elemental composition bnt differing in strnctnre. We have seen three snbtypes of stereoisomers constitutional isomers, enantiomers, and diastereomers. 

There are two major types of stereoisomer conformational isomers and configurational isomers. Configurational isomers include optical isomers, geometrical isomers, enantiomers and diastereomers. 

X-ray crystal structures were used for the production of computer projected images of inclusion complexes of structural isomers, enantiomers and dlastereomers with a- or B-cyclodextrin. These projections allow for a visual evaluation of the interaction that occurs between various molecules and cyclodextrin, and an understanding of the mechanism for chromatographic resolution of these agents with bonded phase chromatography. 

Ion association or ion-pairing reactions are most commonly studied for clathrochelate complexes exhibiting unique inertness. These reactions attract particular interest due to their marked effect on the kinetics and direction of the redox and photochemical reactions and on the characteristics of electrochemical processes. In certain cases, ion association reactions govern the catalytic activity of compounds. The ion-pairing ability of clathrochelates is utilized to resolve racemates into optical isomers (enantiomers) and to separate optically active anions using clathrochelates as chiral eluents. 

GEOMETRICAL ISOMERS, OPTICAL ISOMERS, ENANTIOMERS, and DIASTEREOMERS. 

Diastereoisomers When a molecule contains several asymmetrical atoms of carbon (see chiral), two types of isomers, enantiomers and diastereoisomers, exist. The former are mirror images. The latter contain some carbon atoms which have an identical structure, others which are enantiomeric. For example, if we have a molecule with two asymmetrical carbons, the first carbon atom can have... 

The applicant should provide justification for using the racemate. Where the interconversion of the enantiomers in vivo is more rapid than the distribution and elimination rates, then use of the racemate is justified. In cases where there is no such interconversion or it is slow, then differential pharmacological effects and fate of the enantiomers may be apparent. Use of the racemate may also be justified if any toxicity is associated with the pharmacological action and the therapeutic index is the same for both isomers. For preclinical assessment, pharmacodynamic, pharmacokinetic (using enantiospecific analytical methods) and appropriate toxicological studies of the individual enantiomers and the racemate will be needed. Clinical studies on human pharmacodynamics and tolerance, human pharmacokinetics and pharma-cotherapeutics will be required for the racemate and for the enantiomers as appropriate. 

Note carefully the difference between enantiomers and diastereomers. Enantiomers have opposite configurations at all chirality centers, whereas diastereomers have opposite configurations at some (one or more) chirality centers but the same configuration at others. A full description of the four stereoisomers of threonine is given in Table 9.2. Of the four, only the 2S,3R isomer, [o]D= -28.3, occurs naturally in plants and animals and is an essential human nutrient. This result is typical most biological molecules are chiral, and usually only one stereoisomer is found in nature. 

Stereoisomers (Section 4.2) Isomers that have their atoms connected in the same order but have different three-dimensional arrangements. The term stereoisomer includes both enantiomers and diastereomers. 

Sequence rules, 180-183, 297-298 E,Z alkene isomers and, 180-183 enantiomers and. 297-300 Serine, biosynthesis of, 1177... 

These cases are completely different from the cis-trans isomerism of compounds with one double bond (p. 157). In the latter cases, the four groups are all in one plane, the isomers are not enantiomers, and neither isomer is chiral, while in allenes, the groups are in two perpendicular planes and the isomers are a pair of optically active enantiomers. 

When a molecule has two stereogenic centers, each has its own configuration and can be classified (R) or (5) by the Cahn-Ingold-Prelog method. There are a total of four isomers, since the first center may be (R) or (5) and so may the second. Since a molecule can have only one mirror image, only one of the other three can be the enantiomer of A. This is B [the mirror image of an (R) center is always an (5) center). The compounds C and D are a second pair of enantiomers and the rela-... 

Although four is the maximum possible number of isomers when the compound has two chiral centers (chiral compounds without a chiral carbon, or with one chiral carbon and another type of chiral center, also follow the rules described here), some compounds have fewer. When the three groups on one chiral atom are the same as those on the other, one of the isomers (called a meso form) has a plane of symmetry, and hence is optically inactive, even though it has two chiral carbons. Tartaric acid is a typical case. There are only three isomers of tartaric acid a pair of enantiomers and an inactive meso form. For compounds that have two chiral atoms, meso forms are found only where the four groups on one of the chiral atoms are the same as those on the other chiral atom. 

When a molecule contains a double bond and an asymmetric carbon, there are four isomers, a cis pair of enantiomers and a trans pair ... 

The term fine chemicals is widely used (abused ) as a descriptor for an enormous array of chemicals produced at small scale and is frequently assumed to infer a significant added value of the product derived from the degree of complexity (number of functional groups, geometric isomers, and enantiomers) and precision in their manufacture. Whether the term fine chemicals refers to the finesse of the chemistry or to the small scale of manufacture is far from clear. However, in order to assist our discussion the following division can be adopted [2] ... 

Stereo isomers have the same constitution, but a different spatial arrangement of their atoms they differ in their configuration. Two cases have to be distinguished geometric isomers (diastereomers) and enantiomers. 

Technical 1,2, 5, 6, 9,10-HBCD is produced industrially by addition of bromine to cis-trans-trans-1,5,9-cyclododecatriene. This process leads theoretically to a mixture of 16 stereoisomers (six pairs of enantiomers and four mesoforms) and the product usually is a mixture of the three diastereoisomers a-, p- and y-isomer [14]. Normally, the y-isomer is the most dominant in the commercial mixtures (ranging between 75 and 89%), followed by a- and then p-isomer (10-13% and 1-12%, respectively) [15]. The dissimilarities in the structure of a-, p- and y-isomer might raise differences in polarity, dipole moment and in solubility in water. For example, the solubility of a-, p- and y-HBCD in water was 48.8,14.7, and 2.1 pg/L, respectively. Therefore, these different properties may explain the differences observed in their environmental behavior [16]. Covaci et al. [17] and Morris et al. [18] found that in sediments, the distribution of HBCD isomers was the same of... 

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