4 Properties of Chiral Molecules: Optical Activity Any molecule with a plane of symmetry or a center of symmetry is achiral, but their absence is not sufficient for a molecule to be chiral. a molecule lacking a center of symmetry or a plane of symmetry is likely to be chiral, but the superposability test should be applied to be certain 7. 4 PROPERTIES OF CHIRAL MOLECULES: OPTICAL ACTIVITY The experimental facts that led vant Hoff and Le bel to propose that molecules having The phenomenon of optical the same constitution could differ in the arrangement of their atoms in space concerned ctivity was discovered by the physical property of optical activity. Optical activity is the ability of a chiral sub- the French physicist Jean- stance to rotate the plane of plane-polarized light and is measured using an instrument Baptiste Biot in 1815 alled a polarimeter.( Figure 7.5) The light used to measure optical activity has two properties: it consists of a sin- le wavelength and it is plane-polarized. The wavelength used most often is 589 nm (called the D line ), which corresponds to the yellow light produced by a sodium lamp Except for giving off light of a single wavelength, a sodium lamp is like any other lamp in that its light is unpolarized, meaning that the plane of its electric field vector can have any orientation along the line of travel. A beam of unpolarized light is transformed to plane-polarized light by passing it through a polarizing filter, which removes all the waves except those that have their electric field vector in the same plane. This plane polarized light now passes through the sample tube containing the substance to be exam ined, either in the liquid phase or as a solution in a suitable solvent(usually water, ethanol, or chloroform). The sample is"optically active "if it rotates the plane of polar ized light. The direction and magnitude of rotation are measured using a second polar izing filter (the"analyzer")and cited as a, the observed rotation. To be optically active, the sample must contain a chiral substance and one enantiomer must be present in excess of the other: A substance that does not rotate the plane of polar ized light is said to be optically inactive. All achiral substances are optically inactive. What causes optical rotation? The plane of polarization of a light wave undergoes a minute rotation when it encounters a chiral molecule. enantiomeric forms of a chiral molecule cause a rotation of the plane of polarization in exactly equal amounts but in Sample tube with solution of optically Angle rotatIon ctive substance Analyzer light oscillates Plane-polarized 180° FIGURE 7.5 The mp emits light moving in all When the light passes through stance. The plan contains a solu econd polarizing filter(called the analyzer)is ed in degrees that is used to measure the angle of rotation (Adapted from M. Silberberg, Chemistry, 2d edition, McGraw-Hill Higher Education, New York, 1992,p.616) Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteAny molecule with a plane of symmetry or a center of symmetry is achiral, but their absence is not sufficient for a molecule to be chiral. A molecule lacking a center of symmetry or a plane of symmetry is likely to be chiral, but the superposability test should be applied to be certain. 7.4 PROPERTIES OF CHIRAL MOLECULES: OPTICAL ACTIVITY The experimental facts that led van’t Hoff and Le Bel to propose that molecules having the same constitution could differ in the arrangement of their atoms in space concerned the physical property of optical activity. Optical activity is the ability of a chiral sub￾stance to rotate the plane of plane-polarized light and is measured using an instrument called a polarimeter. (Figure 7.5). The light used to measure optical activity has two properties: it consists of a sin￾gle wavelength and it is plane-polarized. The wavelength used most often is 589 nm (called the D line), which corresponds to the yellow light produced by a sodium lamp. Except for giving off light of a single wavelength, a sodium lamp is like any other lamp in that its light is unpolarized, meaning that the plane of its electric field vector can have any orientation along the line of travel. A beam of unpolarized light is transformed to plane-polarized light by passing it through a polarizing filter, which removes all the waves except those that have their electric field vector in the same plane. This plane￾polarized light now passes through the sample tube containing the substance to be exam￾ined, either in the liquid phase or as a solution in a suitable solvent (usually water, ethanol, or chloroform). The sample is “optically active” if it rotates the plane of polar￾ized light. The direction and magnitude of rotation are measured using a second polar￾izing filter (the “analyzer”) and cited as , the observed rotation. To be optically active, the sample must contain a chiral substance and one enantiomer must be present in excess of the other. A substance that does not rotate the plane of polar￾ized light is said to be optically inactive. All achiral substances are optically inactive. What causes optical rotation? The plane of polarization of a light wave undergoes a minute rotation when it encounters a chiral molecule. Enantiomeric forms of a chiral molecule cause a rotation of the plane of polarization in exactly equal amounts but in 7.4 Properties of Chiral Molecules: Optical Activity 265 The phenomenon of optical activity was discovered by the French physicist Jean￾Baptiste Biot in 1815. 0° 180° 270° 90° Analyzer Rotated polarized light Plane-polarized light oscillates in only one plane Sample tube with solution of optically active substance α Polarizing filter Unpolarized light oscillates in all planes Light source Angle of rotation FIGURE 7.5 The sodium lamp emits light moving in all planes. When the light passes through the first polarizing filter, only one plane emerges. The plane-polarized beam enters the sam￾ple compartment, which contains a solution enriched in one of the enantiomers of a chiral sub￾stance. The plane rotates as it passes through the solution. A second polarizing filter (called the analyzer) is attached to a movable ring calibrated in degrees that is used to measure the angle of rotation . (Adapted from M. Silberberg, Chemistry, 2d edition, McGraw-Hill Higher Education, New York, 1992, p. 616.) Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website