Enantiomers Optical isomers

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. 

Optical isomerism is the result of a dissymmetry in molecular suhstitution. The basic aspects of optical isomerism are discussed in various textbooks of organic chemistry. Optical isomers (enantiomers) may have different physiological activities from each other provided that their interaction with a receptor or some other effector structure involves the asymmetric carbon atom of the enantiomeric molecule and that the three different substituents on this carbon atom interact with the receptor. The Easson-Stedman hypothesis assumes that a three-point interaction ensures stereospecificity, since only one of the enantiomers will fit the other one is capable of a two-point attachment only, as shown in figure 1.13 for the reaction with a hypothetical planar receptor. However, it is reasonable to assume that receptor stereospecificity can also undergo a change when the receptor conformation is altered by a receptor-drug interaction. 

The first step in method development is selecting an adequate HPLC mode for the particular sample. This choice depends on the character of the sample compounds, which can be either neutral (hydrophilic or lipophilic) or ionic, low-molecular (up to 2000 Da) or macromolecular (biopolymers or synthetic polymers). Many neutral compounds can be separated either by reversed-phase or by normal-phase chromatography, but a reversed-phase system without ionic additives to the aqueous-organic mobile phase is usually the best first choice. Strongly lipophilic samples often can be separated either by non-aqueous reversed-pha.se chromatography or by normal-phase chromatography. Positional isomers are usually better separated by normal-phase than by reversed-phase chromatography and the separation of optical isomers (enantiomers) requires either special chiral columns or addition of a chiral selector to the mobile phase. 

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. 

Pasteur devised three methods to resolve paratartaric acid the first was manual, the second was chemical, and the third could be considered biological or physiological. Because paratartaric acid (also called racemic acid) was the first inactive compound to be resolved into optical isomers (enantiomers, an equimolar mixture of two enantiomers is now called a racemate. 

GEOMETRICAL ISOMERS, OPTICAL ISOMERS, ENANTIOMERS, and DIASTEREOMERS. 

See optical isomer enantiomer asymmetry polymer, stereospecific Ziegler catalyst geometric isomer. 

Pasteur, Louis (1822-1895). (1) First to recognize infective bacteria as disease-causing agents (2) developed concept of immunochemistry (3) initiated heat sterilization of wine and milk (pasteurization) (4) observed optical isomers (enantiomers) in tartaric acid. 

Figure 15.9 Two chiral molecules. A, 3-Methylhexane is chiral because C-3 is bonded to four different groups. These two models are optical isomers (enantiomers). B, The central C in the amino acid alanine is also bonded to four different groups.

The terminogloy absolute configuration particularly refers to the arrangement of atoms in space of a chiral compound. It has been observed that there is a stark and distinct difference in specific biologic activity of the optical isomers (enantiomers) having the (R) and (S) configuration. A typical example of Levorphanol and Dextrorphan has already been discussed under Section 6.2 in this chapter. 

Thus, the two cis structures are optical isomers (enantiomers). We say that cts-[Co(en)2Cl2] is a chiral complex. 

A consequence of inversion symmetry is that die wave functions have to be eigenfunctions of the inversion operator with eigenvalues D = 1 (i.e., the wave function is symmetric), or fl = — 1 (i.e., file wave function is antisymmetric). Any asymmetric wave function corresponding to a stationary state is therefore excluded (illegal). However, two optical isomers (enantiomers), corresponding to an object and its mirror image, do exist (Fig. 2.5). " ... 

An interesting corollary to the Amoore theory is the postulate that if the receptor sites are chiral, then optical isomers (enantiomers) of a given substance might have different odors. This circumstance proves true in several cases. It is true for (-1-)- and (—)-carvone we investigate the idea in Experiment 16 in this textbook. 

Optical isomers Enantiomers. Isomers that have a mirror-image relationship. 

Octet rule the tendency of atoms in molecules to have eight electrons in their valence shells (two for hydrogen atoms). (9.4) Optical isomers (enantiomers) isomers that are nonsuperim-posable mirror images of one another. (23.5)... 

The two optical isomers (enantiomers) of menthol have identical chemical properties. They differ in those properties that depend upon the chiral carbon, like smell. The (-) isomer can be described as fresh, sweet, minty, cooling, refreshing . The (+) isomer is similar, but less minty, more herby, with musty, bitter, phenolic and herbaceous notes , and is less refreshing. The (-) isomer also has about four times the cooling power of the (+) isomer. 

Optical isomers Enantiomers compounds with chiral atoms that rotate plane polarized light in opposite directions (+/— or d/1). 

Optical isomers are nonsuperimposable mirror images. We call a pair of optical isomers enantiomers. The rotation of polarized light is measured with a polarimeter. 

The term enantio- pertains to optical activity, and is frequently designated as enantiomeric excess (e.e.), a ratio showing the predominance of a given optical isomer (enantiomer) preferentially formed. 

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