Rotation of plane

Some physical properties of the three stereoisomers are listed in Table 9.3. The (+)- and (-j-tartaric acids have identical melting points, solubilities, and densities but differ in the sign of their rotation of plane-polarized light. The meso isomer, by contrast, is diastereomeric with the (+) and (-) forms. As such, it has no mirror-image relationship to (+)- and (-)-tartaric acids, is a different compound altogether, and has different physical properties. 

The rotation of plane polarized hght (either -I- or -) is not a man-made convention. It is a physical effect that is measured in the lab. It is impossible to predict whether a compound will be -l- or - without actually going into the lab and trying. If a stereocenter is R, this does not mean that the compound will be +. It could just as easily be. In fact, whether a compound is -l- or - will depend on temperature. So a compound can be + at one temperature and - at another temperature. But clearly, temperature has nothing to do with R and S. So, don t confuse R/S with +/-. They are totally independent and unrelated concepts. 

The direction of rotation of plane-polarized light is often incorporated into the names of optically active compounds ... 

Material like sapphire, which is a uniaxial crystal, does not show nonlinear fluorescence, and direct visualization of filaments is, consequently, not an option. However, the effect of rotation of plane polarization can also be probed in such materials by monitoring the spatial size of the white light disc, and of the spectrum of the super continuum that is produced. Figure 5.3 depicts some typical results obtained with 3 mm long sapphire crystals. The extent of the supercontinuum spectrum is seen to vary as a function of polarization angle. The supercontinuum spectrum has two components [39] symmetric broadening about the incident wavelength that is essentially ascribable to... 

This phenomenology is, of course, consistent with optically active domains in a conglomerate. It should be pointed out that this kind of rotation of plane polarized light in a 4-. im path cannot derive from molecular chirality but should be due to some kind of macroscopic helical chirality. 

Polarimetry is the technique of following the rotation of plane-polarized light. 

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 ... 

Quantitative measurements of simple and enzyme-catalyzed reaction rates were under way by the 1850s. In that year Wilhelmy derived first order equations for acid-catalyzed hydrolysis of sucrose which he could follow by the inversion of rotation of plane polarized light. Berthellot (1862) derived second-order equations for the rates of ester formation and, shortly after, Harcourt observed that rates of reaction doubled for each 10 °C rise in temperature. Guldberg and Waage (1864-67) demonstrated that the equilibrium of the reaction was affected by the concentration ) of the reacting substance(s). By 1877 Arrhenius had derived the definition of the equilbrium constant for a reaction from the rate constants of the forward and backward reactions. Ostwald in 1884 showed that sucrose and ester hydrolyses were affected by H+ concentration (pH). 

Enantiomers possess identieal physieal properties namely, melting point, boiling point, refraetive index, ete. They only differ with respeet to the rotation of plane polarised light. If one of the enantiomer is dextro rotatory, the other will be laevo rotatory. 

This method was the first accurate spectroscopic method for determining chemical reaction rates. In the mid-eighteenth century, kinetic measurements of changes in the rotation of plane polarized light upon acid-catalyzed hydrolysis of sucrose led to the concept of a dynamic equilibrium. 

Circular dichroism (CD) S Change in rotation of plane-polarized light upon binding to measure conformational change... 

Enantiomers can be distinguished by their rotation of plane-polarized light at a specific wavelength, or over a range of wavelengths (optical rotatory dispersion, ORD), as well as by the difference in absorption of right and left circularly polarized light (circular dichroism. Cotton effect, CD). 

The upper half of(the )nolecule is a non-superimposable mirror image of the lower half, making the top half an enantiomer of the lower half However, since the two halfs are in the same molecule the rotation of plane-polarized light by the upper half is cancelled by the lower half and the compound is optically inactive. 

Figure 6.1 Rotation of plane polarized light by enantiomers.

An asymmetric carbon atom has four different atoms or groups of atoms attached to it and may be a source of dissymmetry in the molecule. The asymmetric carbon atom may have two possible arrangements of the groups around it. The two structures may be nonsuperimposable mirror images, and can be expected to differ in the rotation of plane-polarized light to an equal extent but in the opposite direction. 

Diastereomcrs have different physical properties (i.e. rotation of plane-polarized light, melting points, boiling points, solubilities, etc...). Their chemical properties also differ. 

The labeled compounds are each checked with a polarimeter for the rotation of plane-polarized light. Only Compounds A. B. arui C are optically active. 

B is correct. Absolute configuration describes the R or S configuration around a chiral atom. Observed rotation describes the direction of rotation of plane-polari2ed light. The direction of rotation cannot be predicted by the absolute configuration abne. 

Pasteur showed that optical activity was related to molecular right- or left-handedness (chirality). Later, van t Hoff and LeBel proposed that the four valences of carbon are directed toward the corners of a tetrahedron. If the four attached groups are different, two arrangements are possible and are related as an object and its nonsuperimposable mirror image. Enantiomers differ only in chiral (or handed) properties, such as the direction of rotation of plane-polarized light. They have identical achiral properties, such as melting and boiling points. 

Figure 12.5 Rotation of Plane Polarized Light by a Chiral Sample...

All of the 20 standard amino acids, except for glycine, have four different groups arranged tetrahedrally around the Ca atom and thus can exist in either the d or l configuration. These two enantiomers are nonsuperim-posable mirror images that can only be distinguished on the basis of their different rotation of plane-polarized light. Only the l isomer is found in proteins. 

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