Optical Acfivfiy Charles D. Mickey Texas ABM University at Galveston Galveston. TX 77553
A remarkable result of the early analyses of organic compounds was the discovery that two compounds, totally different in their properties, might nonetheless he composed of the same elements in precisely the same proportions. This phenomenon was first noticed, in the case of tartaric and racemic acids, by the Swedish chemist Jons Berzelius (1827). In 1828, the German chemist Friedrich Wohler discovered that ammonium cvanate and urea have the same molecular ; ~ ~ ~different properties. Thus, he formula, C H ~ N Z Oentirely added to the small hut growing list of suhstances that were obviously different while still having identical molecular formulas. In 1832, Berzelius proposed that such suhstances he called isomeric, from the Greek isos, equal, and meros, part. Moreover, he suggested that the difference hetween compounds having identical molecular formulas must lie in a different arrangement of their parts, a prediction later confirmed Hlstorlcal Background of Stereolsornerisrn Certain ~uzzlinafeatures about tartaric acid led Louis to one of his most tkmuur.discoveries. Chemists Pasteur (1k0) had found that solutions of tartaric acid rotated lane-~olnrized light to the right. A seemingly identical substanck, racemic acid (from the Latin racemus, hunch of grapes), produced no effect on polarized light. Subsequently, Pasteur found that two kinds of crystals can he obtained from racemic acid.' By separately dissolving the two types of crystals in water, Pasteur found that each solution was optically active. Each solution had the same magnitude of optical activity, hut they rotated the plane-polarized light in opposite directions. How could this hanoen? The explanation came from the Dutch physical che&t, J. H. van%Hoff, and his former colleague, the French chemist J. A. LeBel(1874). Simultaneously, but independently, they proposed that molecules such as tartaric acid must exist in two isomeric forms, a right-handed and a left-handed f o m , depending on the relative orientation of the atoms making up the molecule. This discovery by Pasteur, and its suhsequent explanation by van't Hoff and LeBel, founded the science of stereochemistry, which studies the effects of a substance's structure upon its physical and chemical properties. Many other suhstances have since been found to exist, like tartaric acid, in two or more forms which seem chemically identical hut differ in the way in which their atoms are arranged in space. Turpentine, sugars, camphor, and amino acids are good examples. Properties of Light
The phenomenon of optical activity is best understood hy Brst reflecting on the properties of light. A beam of ordinary white light is an electromagnetic radiation involving the propagation of both electric and magnetic forces oscillating 3s waves in all nossihle planes ner~endicularto the beam's A cioss-section of the cylindrical lirection of propagation beam of lieht mav he visualized as a wheel with an infinite number oi;poke;(planes) radiating from the central axis as shown in Figure 1. Kach planeof vibration is the resultant of two component waves vibrating in mutually perpendicular planes. This model represents a beam of unpolarized light. The French physicist Etienne Malus (1808) discovered that
Flgure 1. Cross-sectionof a beam of white light vibrating in all pwsibie planes (O0-360').
unpolarized light can he polarized. A convenient way to induce polarization is to allow a beam of ordinary light to pass through a device called a Nicol prism, or a sheet of special plastic known as Polaroid." These devices interact with the &cillating electrical field in such a way that the emerging light vihrates in a sinale plane onlv, as shown in Firmre 2. Hence the name plane-poiarGed light:
Figure 2. Light is polarized into vibrations in one plane when it passes through certain materials called polarizers.
The lnleraction of Light with Polarold@ If a light beam is viewed through a pair of identical Polaroid" disks while slowly rotating one of the disks, the light intensity begins to drop, reaching a minimum when the two disks are in a position such that their axes are perpendicular, as shown in Figure 3. The plane-polarized light from the first disk is absorbed by the second disk. Thus, a beam of polarized light will pass completely through a second sample of polarizing suhstance only if the axis of the second suhstance is parallel to that of the first. This fact serves as the basis for the construction of a polarimeter, the instrument used to measure optical activity. The Pdnclples of a Polarlrneter A ~olarimeteris used to measure the number of deerees of rotation of plane-polarized light as i t passes through"an optically active suhstance. Figure 4 illustrates the principal working parts of a simple polarimeter which includes e light source, two polarizing...misms, a samde cell placed between the two polarizers, an eyepiece, and a protractor for measuring the angular rotation of ~olarizedlight caused hv the samole. The s o h e of illurninatfon must provide monochromatic light
71).
1 He laboriously separated them, using a magnifying glum and tweezers.
442 1 Journal of Chemical Education
Figwe 3. The regions d overlap of me uMligned PolamidO disks appear opsque. although alone each disk is transparent.
Figure 4. The principal working parts of a polarimeter.
(a single wavelength), since the angle of rotation is affected bv the wavelength of polarized light. Normally, monochromatic light from a sodium vapor lamp is employed. As shown in Figure 4, if a solution containing an optically active compound is placed in the sample cell, the polarized light encounters asymmetric molecules, and its plane of oscillation is rotated. The deeree of rotation is measured bv the angle through which the analyzer must he rotated in order to be parallel to the rotated plane of light. Thus the angle a,referred to as the observed rotation, is equal to the number of degrees that the optically active substance has rotated the plane-polarized light. Customarily, the analyzer is rotated in the direction requiring the fewer degrees to restore the original intensity of the~plan~-polarized light. If the analyzer is rotated to the right (clockwise)torestore the original intensity of the light, the degrees are recorded as positive (+), and the optically active substance is classified as dextrorotatory. If the rotation is to the left (counterclockwise), the degrees are recorded as negative (-),and the substance is classified as levorotatory.
direction of oscillation. The effect per molecule is extremelv small, hut in the aggregatemay he measurable as a net rotation of the plane-polarized light. Molecules like carbon tetrachloride, acetylene and acetone which have aplane of symmetry (i.e., an imaginary plane that divides a molecule into two halves that are mirror images of each other) do not rotate plane-polarized light because their symmetries are such that every optical rotation in one direction is cancelled by an equal rotation in the opposite direction.
Asparagine-A Molecule with One Chiral Center The relationship uf optical acti\.ity to chemical 3tructure can tx illuitrated u,ith asrwauine, a white solid isulared in . 1806 from the juice of asparagus and in 1886 from vetch, a plant widelv used as fodder. Figure . 5 shows that one carbon in asparagine has four different suhstituents; viz., -H, COOH, -NH2, and NH2COCH2- attached to it. Such a carbon is defined as an asymmetric carhon. Because of the tetrahedral nature of the s p 3 orbitals of the asymmetric carbon atom, the four different substituent groups can occupy two different spatial arrangements around the asymmetric Specific Rotation carbon atom. Such molecules are stereoisomers related to each other as non superimposahle mirror images called enantioThe angle a is a function of the concentration of the optimers (from the Greek enantios, opposite, and meros, part). cally active compound in solution, the sample cell length, the Molecules like asparagine, that can exist in right-handed temperature of the solution, and, to a lesser extent, the solvent. and left-handed forms, are referred to as chiral molecules In order to permit a comparison of the optical activity of dif(from the Greek chier, hand), and the asymmetric carbon is ferent substances, it is necessary to standardize all these pareferred to as the chiral center. This phenomenon of stererameters. This standardization is effected by determining the specific rotation, [a],which is defined by the equation: oisomerism, called chirality (handedness), has a net effect on incident plane-polarized light since the electromagnetic in100a b l * -T teractions do not average to zero; such substances are characterized as being optically active (2). where a = observed rotation in degrees, 1 = cell length in Inasmuch as the intrandear distances and bond angles are decimeters, c = concentration in grams of sampleI100 ml of identical within this enantiomeric pair (Fig. 5), their polarities solution, X = wavelength of incident light, and t o = temperare therefore identical. Conseauentlv. ".as is the case with all ature in degrees Celsius. enantiomeric pairs, their melting points, boiling points, For examole. when cholesterol is reported as havine-. lo~ln~~' .solubilities, densities, and spectra are identical. = -31.61 (c'= ~,chloroform), this means that cholesterol has Two important differences, however, do exist. The aspara specific levorotation of 31.61° at a concentration of 2gI100 agine from asparagus has a hitter taste and is dextrorotatory, ml of chloroform solution a t 20°C when contained in a 1 dec[a]$= +5.41°, whereas the asparagine from vetch has a sweet imeter samole tube. the rotation beine measured with sodium taste and is levorotatorv. .. l .al...ko= -5.41°. More detailed disD light, which has wavelength of 5g93 A. cuisions of the influence of m(~lecularstructure on a molerule's The specific rotation of a compound is an important ohvsicai, chemical, and hidoeicnl are available in - -properties physical constant, comparable to its boiling point or melting the literature ( 3 , 4 ; 5). ooint. It serves as another ~ orooertv l . . that a chemist -h v.s i c acan use to characterize a compound. Moreover, by measuring Tartaric Acid-A Molecule with Two Chiral Centers actual rotation (a)of a solution of a compound of known For tartaric acid, the compound whose optical activity was specific rotation in a sample tube of fixed length, one can first studied carefully by Louis Pasteur, there are three calculate the concentration of the solution from the equation stereoisomeric forms, shown in Figure 6. Two of the stereoisofor specific rotation. mers are optically active enantiomers, while one is an optically The Orlgin of Optical Rotation In 1815, the French physicist, Jean Baptiste Biot discovered that certain crystalline substances such as quartz and some organic compounds, of which camphor, turpentine, and sugar are examples, when placed in the path of plane-polarized light, caused a rotation of the plane of light. The question naturaliy arises a s to why compounds such as sugar, camphor, etc., interact with polarized light in this manner while others do not. Simply stated, the electric forces in a beam of plane-polarized light impinging on a molecule interacts to some extent with the electrons within the molet"AN.,O*E., cule, causing their pularieat~t,n.This interaction is important Figure 5. Three dimensional structure of Asparagine and its mirror image since it causes the electric field of the radiation to change its
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Volume 57, Number 6, June 1980 1 443
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