Plane of polarized hght

If two structures are nonsuperimposable mirror images, what can you say about the direction that each wih rotate the plane of polarized hght Would a mixture of the two be optically active Explain. 

Compounds in which rotation is restricted may exhibit cis-trans isomerism. These compounds do not rotate the plane of polarized hght (unless they also happen to be chiral), and the properties of the isomers are not identical. The two most important types are isomerism resulting from double bonds and that resulting from rings. 

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. 

Molecules that are not chiral cause no difference in velocity of the two circularly-polarized beams hence there is no rotahon of the plane of polarized hght described by their vector sum. Achiral molecules, therefore, are not ophcahy active. 

Optical isomers. Optical isomers exist for octahedral complexes that do not possess a center of inversion or a mirror plane of symmetry. The complex and its mirror image are not superimposable. One isomer will rotate the plane of polarized light to the left, the other will rotate polarized hght to the right. The complexes are said to be chiral and optically active. Some examples are [Co(ox)3] cis [Rh(en)2Cl2] and cis, cis, cis [PtCl2Br2(NH3)2]. 

Chirality is common in organic compounds. It is often not observed, however, because when a chiral substance is synthesized in a typical reaction, the two enantiomers are formed in precisely the same quantity. The resulting mixture is called a racemic mixture, and it does not rotate the plane of polarized light because the two forms rotate the Hght to equal extents in opposite directions, aoo (Section 23.4)... 

This tilts the plane of polarization. The analyzer prism must be rotated to the left to maximize the transmission of Hght. If rotation is counterclockwise, the angle of rotation is defined as (—) and the enantiomer that caused the effect is called levorota-tory (Z). Conversely, clockwise rotation is defined as (-F), and the enantiomer is dextrorotatory (d). Tilting the plane of polarization is called optical activity. Note that if a solution of equal amounts of a d and an / enantiomeric pair is placed in the beam of the polarimeter, no rotation is observed. Such a solution is racemic it is an equimolar mixture of enantiomers. 

A good crystal should be of a size that does not exceed the diameter of the X-ray beam, typically between 0.05 and 0.3 mm, in order to be evenly illuminated by it. The selection of a suitable crystal is usually done under a microscope, often aided by polarized light. Most of the crystals of molecular compounds (apart from those belonging to very highly symmetric space groups) rotate the plane of polarization of hght. Therefore, if... 

Minimal transmission occurs when the plane of polarization of the Ught is perpendicular to that of the analyzer through which it is viewed. This effect can be demonstrated using two pairs of polarized sunglasses as shown in Figure 10.5. If the plane of polarization is rotated to the right, the isomer is said to be dextrorotatory and is labeled d if it is rotated to the left, the isomer is called levorotatory and labeled /. Enantiomers always rotate the hght by the same amount, but in opposite directions. Thus, in an eqnimolar mixture of both enantiomers, called a racemic mixture, the net rotation is zero. 

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. 

As indicated, P is the difference between the two observed intensities divided by their sum. Fluorescence polarization is measured by placing a mechanically or electrically driven polarizer between the sample cuvet and the detector. A diagram of a fluorescence polarization measurement system is shown in Figure 3-16. In the normal instrumentation mode, the sample is excited with polarized hght to obtain maximum sensitivity. The polarization analyzer is positioned first to measure the intensity of the emitted fluorescence hght in the vertical plane ly), and then the polarization analyzer is rotated 90° to measure the emitted fluorescence hght intensity in the horizontal plane (Ij,). P is then calculated manually or automatically by use of equation (13). 

K. Ichimura, Y. Hayashi, and N. Ishizuki, Photocontrol of in-plane alignment of a nematic liquid crystal by a photochromic spiropyran monolayer absorbing linearly polarized hght, Chem. Lett. 1992,1063-1066. 

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. 

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