The rotation of plane-polarized light is known as optical activity

The rotation of plane-polarized light is known as optical activity 

The observed angle through which the light is rotated is given the symbol a. By dividing this value by the path length l (in dm) and the concentration c (in g dm-3) we get a value, [a], which is specific 

Most [ot] values are quoted as [a] (where the D indicates the wavelength of 589 nm, the D line of a sodium lamp) or [a]o° the 20 indicating 20 °C. These define the remaining variables. 

Here is an example. A simple acid, known as mandelic acid, can be obtained from almonds in an enantiomerically pure state. 

28 mg was dissolved in 1 cm of ethanol and the solution placed in a 10 cm long polarimeter cell. An optical rotation a of -4.35° was measured (that is, 4.35° to the left) at 20 °C with light of wavelength 589 nm. 

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Helical chains such as Se o (Fig. 3.20a) may be right- or left-handed and are chiral. 6-Coordinate complexes such as [Cr(acac)3] ([acac], see Table 7.7) in which there are three bidentate chelating ligands also possess non-super-posable mirror images (Fig. 3.20b). Chiral molecules can rotate the plane of plane-polarized light. This property is known as optical activity and the two mirror images are known as optical isomers or enantiomers. We return to this in Chapter 19. 

Methamphetamine contains a chiral carbon, a carbon atom with four different groups attached, and therefore exists as two optical isomers, which are non-superimposable mirror images of each other. The isomers have identical chemical reactions, solubilities and melting points. They differ in their opposite rotations of plane-polarized light and, because they have different 3D structures, they ht protein receptors differently, and therefore have different effects upon the body. L-Methamphetamine is simply a decongestant, and has no stimulant activity. But its optical isomer, D-methamphetamine, is the stimulant commonly known as speed . 

A chiral molecule is one which exists in two forms, known as enantiomers. Each of the enantiomers is optically active, which means that they can rotate the plane of plane-polarized light. The enantiomer that rotates the plane to the right (clockwise) has been called the d (or dextro) form and the one that rotates it to the left (anticlockwise) the I (or laevo) form. Nowadays, it is more usual to refer to the d and I forms as the ( + ) and (—) forms, respectively. 

One form (L-form) exhibits the ability to rotate the plane of polarization of plane polarized light to the left (levorotatoiy), whereas the other form (D-form) rotates the plane to the right (dextrorotatory) Although the direction of optical rotation exhibited by these optically active forms is different, the magnitude of their respective rotations is the same. If equal amounts of dextro and evo forms are admixed, the optical effect of each isomer is neutralized by the other, and an optically inactive product known as a racemic modification or racemate is secured. 

This particular example represents one class of stereoisomers known as enantiomers, which may be defined as two molecules that are mirror images but are nonetheless nonsuperimposable. Such molecules are said to possess opposite configuration. If these isomers are separated (resolved), the separate enantiomers have been found to rotate the plane of plane-polarized light. This phenomenon of optical activity has been known for well over a century. A 50-50 mixture of two enantiomers is optically inactive or racemic, since the rotation of light by one enantiomer is precisely compensated by the rotation of tight in the opposite direction by the other enantiomer. 

The single most important physical property that differentiates enantiomers is their ability to rotate the plane of plane polarized light. This property is called optical activity and is displayed only by chiral molecules. Thus, stereoisomers which are also chiral are known as optical isomers. Chiral molecules that rotate polarized light in a clockwise fashion are termed dextrorotatory (d) while those that rotate the beam counterclockwise are levorotatory (/). Enantiomers have optical rotations of die same magnitude but of different signs (d or /). 

Optical Activity.—We come now, in the case of the isomeric alcohols, to a new and inost interesting example of isomerism. The five carbon alcohol 2-methyl butanol-i, differs from the other seven structurally isomeric amyl alcohols not only in structure, but also in other striking ways. Three different amyl alcohols are known all of which have the constitution of 2-methyl butanol-i. Two of these three are known as optically active all the other amyl alcohols being inactive. Certain substances either in the crystalline form, as in the case of quartz in solution, as in the case of sugar or in the liquid form, as in the case of the alcohol we are considering possess this physical property of optical activity. This property is shown by the fact that the compound has the power to turn or rotate the plane of vibration of a ray of light that has been polarized. 

In general, compounds which contain asymmetric carbon atoms rotate the plane of polarization of plane-polarized light. For this reason they are said to be optically active. When the molecular symmetry is such that the optical activity of one portion of the molecule is cancelled by that of the second portion of the molecule, the compounds are said to be internally compensated and are called meso compounds. The tartaric acid with the formula (X) is such a compound and has been known as the meso-tartaric acid. The tartaric acids identified as (VIII) and (IX) have been known as d-tartaric acid and Z-tartaric acid because of the sign of their optical rotations (dextro and levo, respectively). (The nomenclature of these acids is discussed later in this chapter.) The compounds (VIII) and (IX) are non-superimposable mirror images, called enantiomorphs. The existence of such pairs of asymmetric isomers is the fundamental basis of optical activity. The asymmetry may be in either the molecular structure or the crystal structure. Asymmetric carbon atoms are not always present in optically active molecules. 

Diasteroisomers, also known as geometric isomers, have different relative orientations of their metal-ligand bonds. Enantiomers are stereoisomers whose molecules are nonsuperposable mirror images of each other. Enantiomers have identical chemical and physical properties except for their ability to rotate the plane of polarized light by equal amounts but in opposite directions. A solution of equal parts of an optically active isomer and its enantiomer is known as a racemic solution and has a net rotation of zero. 

Optical activity is one of the best known, but least understood, phenomena of organic chemistry. It is observed as the ability of certain substances to interact with linearly polarized light by rotating the plane of polarization. Linear polarization means that the electromagnetic radiation vectors oscillate in fixed orthogonal planes that intersect along the propagation vector. 

Dextro, Levo and Inactive Compoimds.— All optically active substances rotate the plane of polarized light either in one direction to the right or in the contrary direction to the left. On this account they are known as right-handed or dextro-rotatory and left-handed or levo-rotatory. The phenomenon of polarization being purely a physical one will not be discussed here. An explanation of it may be found in text books of physics or in chemical books which consider in detail such subjects as the sugars. All that need be added is that optically active compounds are readily examined by means of an instrument known as a polariscope and the direction of rotation (right or left) and the exact amount of rotation in degrees may be accurately determined. 

Optical activity of a substance leads at least to three spectroscopic phenomena which are, however, related Optical Rotatory Dispersion (ORD), i.e. the rotation of the plane of linearly polarized light by traversing the substance. Circular Dichroism, expressed as Ae = e - e (if the relative molar mass of the molecules is known, or - in case of biopolymers - the mean residual relative molar mass), and Molar Ellipticity, [e] = 3300 x Ae. A necessary condition for optical activity of a substance is the Chirality of its molecules, i.e. the property of being not superposable onto its mirror image. This allows still the presence of (proper) axes of rotation Cp, but not improper ones (Sp, including... 

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