Plane-polarized hght

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 presence of asymmetric carbon atoms also confers optical activity on the compound. When a beam of plane-polarized hght is passed through a solution of an optical isomer, it will be rotated either to the right, dextrorotatory (+) or to the left, levorotatory (—). The direction of rotation is independent of the stereochemistry of the sugar, so it may be designated d(—), d(+), l(—), or l(+). For example, the naturally occurring form of fructose is the d(—) isomer. 

Dextrorotatory stereoisomer that reflects plane-polarized hght to the right. 

Polarized light (Section 7.4) Light in which the electric field vectors vibrate in a single plane. Polarized hght is used in measuring optical activity. 

Although Pasteur s work opened the door to understanding the relationship between structure and optical activity, it was not until 1874 that the Dutch chemist van t Hoff and the French chemist LeBel independently came up with a basis for the observed optical activity tetrahedral carbon atoms bonded to four different atoms or groups of atoms. Thus, two enantiomers, which are identical to one another in all other chemical and physical properties, will rotate plane-polarized hght to the same degree, but in opposite directions. 

A further fluorescence effect, fluorescence polarization (FP), has recently been employed in immunoassays. FP occurs when a fluorophore is excited by plane-polarized Hght and emits fluorescence that is also polarized. Complete FP occurs only when the fluorophore is stationary. The degree of observed FP is therefore dependent on how fast a molecule tumbles in solution, a property related to its molecular mass. Changes in molecular mass, for example, caused by antibody-analyte complex formation, can therefore be detected by changes in FP as long as the temperature, viscosity, and other conditions remain constant. This is the principle of the fluorescence polarization immunoassay, in which the analyte disrupts a tracer complex introduced into the sample, reducing the degree of FP as a function of analyte concentration. 

The device that is used for measuring the effect of optically active compounds on plane-polarized hght is a polarimeter. 

As we have already established, enantiomers are different compounds, and we must expect, therefore, that they differ in some property or properties. One property that differs between enantiomers is their effect on the plane of polarized light. Each member of a pair of enantiomers rotates the plane of polarized hght, and for this reason, enantiomers are said to be optically active. To understand how optical activity is detected in the laboratory, we must first understand plane-polarized hght and a polarimeter, the instrument used to detect optical activity. 

Another class of stereoisomers are optical isomers, or enantiomers, which are non-superimposable mirror images. ( Snperimposable means that if one structure is laid over the other, the positions of aU the atoms will match.) Like geometric isomers, optical isomers come in pairs. However, the optical isomers of a compound have identical physical and chemical properties, such as melting point, boihng point, dipole moment, and chanical reactivity toward molecules that are not enantiomers themselves. Enantiomers differ from each other in their interactions with plane-polarized hght, as we will see. 

A useful way of depicting the structure of chiral molecules employs crossed lines (Fischer projections) to represent chiral carbon atoms. The prefixes d- and l-are used to distinguish between enantiomers, objective 4 (Section 17.3), Exercise 17.12. Signs indicating the rotation of plane-polarized hght to the right (-t) or to the left (-) may also be used to desiguate enantiomers. 

Figure 4 8. A representation of the rotation of the plane of plane-polarized hght by a chiral material.

Figure 4.61. A schematic description of a polarimeter. In the absence of a sample, the polarizer and analyzer are aligned so that a maximum amount of hght passes. (If they are crossed or at 90°, no light passes.) Introduction of a chiral sample between them causes the plane of the plane-polarized hght to be tilted, and the extent (the number of degrees) and the sense (clockwise or counterclockwise) the analyzer must be turned to again cause maximum transmission are noted. Generally, the observed rotation is reported as specific rotation.

A polarimeter is an instrument that allows plane-polarized hght to travel through a sample tube containing an organic compound. After the light exits the sample tube, an analyzer slit is rotated to determine the direction of the plane of the polarized light exiting the sample tube. There are two possible results. 

If a chiral substance rotates plane-polarized hght to the right—that is, in a positive (+) or clockwise direction—the substance is dextrorotatory (Latin dextra, right). If a chiral substance rotates plane-polarized light to the left—in a negative (—) or counterclockwise direction— the substance is levorotatory (Latin... 

What is the optical rotation of a mixture of enantiomers, and how is it related to the percentage of each enantiomer in the mixture When plane-polarized hght interacts with a single enantiomer of a chiral molecule, the plane is rotated in one direction. If the plane-polarized hght interacts with the other enantiomer, the plane is rotated in an equal and opposite direction. If a solution contains equal amounts of two enantiomers, the clockwise and counterclockwise rotations resulting from all molecules... 

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