Molecules optically active

Discovery of Optically Active Molecules and Determination of Absolute Configuration... 

Figure 9.5 Schematic representation of a polarimeter. Plane-polarized light passes through a solution of optically active molecules, which rotate the plane of polarization.

The amount of rotation observed in a polarimetry experiment depends on the number of optically active molecules encountered by the light beam. This number, in turn, depends on sample concentration and sample pathlength. If the concentration of sample is doubled, the observed rotation doubles. If the concentration is kept constant but the length of the sample tube is doubled, the observed rotation is doubled. It also happens that the amount of rotation depends on the wavelength of the light used. 

Optically active molecules show circular dichroism. Their extinction coefficients f l and are different and change as a function of wavelength. Using a suitable spectroelectrochemical cell, Af = fl -which is usually small compared to conventional extinction coefficients, can be measured. Combined with the special properties of a thin layer cell kinetic data can be extracted from CD-data [01 Liu]. (Data obtained with this method are labelled CD.)... 

A substance displays optical activity Molecules of the substance are asymmetrical. Interactions between the molecules and electromagnetic radiation is such that the plane of polarisation of the radiation is always rotated in the same direction... 

The methods which have been used to determine the configurational stability of organotin compounds and those which have successfully been applied to synthesize such optically active molecules are reviewed, including the chromatographic resolution of racemic mixtures. 

The asymmetric synthesis achieved when the base is an optically active one is proof that the base is present in a transition state with the carbonyl and not just an agent for removal of protons from hydrogen cyanide. It has further been shown that asymmetric synthesis is still achieved even if the only optically active molecules present are quaternary ammonium compounds, i.e., positive ions without any protons to donate. This probably means that the important thing is to have some positive ion near the carbonyl oxygen, an actual covalent... 

An electric dipole operator, of importance in electronic (visible and uv) and in vibrational spectroscopy (infrared) has the same symmetry properties as Ta. Magnetic dipoles, of importance in rotational (microwave), nmr (radio frequency) and epr (microwave) spectroscopies, have an operator with symmetry properties of Ra. Raman (visible) spectra relate to polarizability and the operator has the same symmetry properties as terms such as x2, xy, etc. In the study of optically active species, that cause helical movement of charge density, the important symmetry property of a helix to note, is that it corresponds to simultaneous translation and rotation. Optically active molecules must therefore have a symmetry such that Ta and Ra (a = x, y, z) transform as the same i.r. It only occurs for molecules with an alternating or improper rotation axis, Sn. 

Molecules which exhibit optical activity are molecules which have a handedness in their structure. They are chiral . Chemists often have reasons to obtain chemical pure aliquots of particular molecules. Since the chirality of molecules can influence biological effect in pharmaceuticals, the chiral purity of a drug substance can pose a challenge both in terms of obtaining the molecules and in assaying the chiral purity by instrumental methods. While diastereomers can have different physical properties including solubility, enantiomers have the same physical properties and the same chemical composition. How then to separate optically active molecules ... 

The first example of the deliberate separation of optically active molecules is appropriate as an example of physical separation in the clearest sense of the term. The molecules are referred to as optically active because polarized light interacts differently with right- and left-handed molecules. In the case of simple diastereomers the RR and SS forms are enantiomers while the RS and SR forms are not. The separation of the latter and former was first done under a microscope using crossed polarizers and the crystals which were seen were separated from those that caused little or no rotation of plane-polarized light by hand using tweezers. A truly physical separation of chemical species using a physical property of chemical origin ... 

The experimentally observable phenomenon of optical activity is usually considered in the context of variation of molecular chirality arising from a particular stereochemical configuration at a particular atom such that the molecule has no improper rotation S axis. Molecules with opposite chirality configurations are enantiomers and show oppositely signed optical activity. Molecules differing only in conformation are called conformers or rotational isomers. In most cases, the difference in energy between rotational isomeric states is very small, such that at ambient temperature all are populated and no optical activity results. However, if one particular conformer is stabilized, for example, by restriction of rotation about a bond, the molecule can become chiral, and thus optically active. 

Therefore, as we have seen, we can not prepare an optically active compound from an inactive one by the ordinary laboratory methods. Such a synthesis however, can be accomplished by attaching an optically active molecule or groups to the original compound and then removing it after the new asymmetric atom has been produced. 

The synthesis of an asymmetric compound carried under the influence of an optically active molecule or group is termed asymmetric synthesis. 

The reasons for the increasing acceptance of enzymes as reagents rest on the advantages gained from utilizing them in organic synthesis Isolated or wholecell enzymes are efficient catalysts under mild conditions. Since enzymes are chiral materials, optically active molecules may be produced from prochiral or racemic substrates by catalytic asymmetric induction or kinetic resolution. Moreover, these biocatalysts may perform transformations, which are difficult to emulate by transition-metal catalysts, and they are environmentally more acceptable than metal complexes. 

The osmotic pressure, at constant temperature, of a dilute solution is proportional to the concentrations of the dissolved substances i.e., U = RTXc, where II is the osmotic pressure, R is the molar gas constant, T is the absolute temperature and Ci is the molar concentration of species i. 2. In stereochemistry, all optically active molecules contain one or more multivalent atoms united... 

CD and ORD have also been employed to detect conformational properties of acyl heterocycles in optically active molecules especially in biological systems. These techniques have been employed, for example, in the conformational study of proline derivatives (70MI2 70M13 73BCJ3894), and as complementary approaches to NMR measurements. 

Halving the concentration or the tube length would halve the number of optically active molecules and the new rotation would be +30° if the substance was dextrorotatory or -150° if levorotatory. 

Let us look at the example of glyceraldehyde, an optically active molecule. Glyceraldehyde can exist as enantiomers, i.e. (- -) and (—) forms, hut the sign does not describe the exact configuration. 

The needs to observe the CD of samples having ordered structures such as mitochondria and viruses will increase in the future. Much attention should be paid to CD observations of samples in which optically active molecules are aligned to each other and are concentrated in the ordered systems. 

The Mirror Plane, cr Most flowers, cut gems, pairs of gloves and shoes, and simple molecules have a plane of symmetry. A single hand, a quartz crystal, an optically active molecule, and certain cats at certain times4 do not possess such a plane. The symmetry element is a mirror plane, and the symmetry operation is the reflection of the molecule in the mirror plane. Some examples of molecules with and without mirror planes are shown in Fig. 3.1... 

The directors (long molecular axes) of the constituent molecules in nematic phases are parallel to one another on average. This is the only order present in nematic liquid crystals, which are the most fluid type of liquid-crystalline phase. Molecules that form cholesteric phases must be optically active or contain an optically active dopant. As the phase name implies, the constituent molecules are frequently steroids and most commonly are cholesteric esters or halides. A conceptual model of the cholesteric phase includes layers of molecules in nematic-like positions, each layer being twisted slightly with respect to the ones above and below it. When the phase consists only of optically active molecules, the angle of twist between layers is typically less than one degree. Several subclasses of discotic phases exist. In all, the molecular planes of the constituent molecules are parallel. However, the discs can pack in nematic-like arrangements (ND) or in columns that are internally ordered (D ) or disordered (Dd) and may be stacked vertically,... 

Many examples of optically active molecules contain an asymmetric carbon atom, that is, one with four different groups attached, as in I-111. A wide variety of other atoms may also be asymmetric (1V-VI). An asymmetric center is by no means... 

A good test for the absence of artifacts that can arise at higher absorbances is to run a blank with a non-optically active molecule, such as 3-methylindole or a DL-amino acid, at the same absorbance as the test samples. The scan should be indistinguishable from the solvent blank. 

Irradiation at room temperature of BN isotropic solutions containing largecon centrations of optically active molecules,... 

A wide range of fluorinated compounds are applied as pharmaceuticals and agrochemicals. Several stereoselective methods are used for synthesis of optically active molecules bearing a C-F bond at the stereogenic carbon atom [72, 73]. These are mainly based on diastereoselective fluorination of chiral molecules or enantioselective alkylation of fluoroorganic compounds. Asymmetric introduction of a fluorine... 

Addition of nucleophiles to electrophilic glycine templates has served as an excellent means of synthesis of a-amino acid derivatives [2c, 4—6]. In particular, imines derived from a-ethyl glyoxylate are excellent electrophiles for stereoselective construction of optically active molecules [32], This research and retrosyn-thetic analysis led us to believe that amine-catalyzed asymmetric Mannich-type additions of unmodified ketones to glyoxylate derived imines would be an attractive route for synthesis of y-keto-ce-amino acid derivatives [33], Initially, L-proline-catalyzed direct asymmetric Mannich reaction with acetone and N-PMP-protected a-ethyl glyoxylate was examined in different solvents. The Mannich-type reaction was effective in all solvents tested and the corresponding amino acid derivative was isolated in excellent yield and enantioselectivity (ee >95 %). Direct asymmetric Mannich-type additions with other ketones afford Mannich adducts in good yield and excellent regio-, diastereo- and enantioselectivity (Eq. 8). 

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