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General Chemistry Principles, Patterns, and Applications, 2011

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Although enantiomers have identical densities, melting <strong>and</strong> boiling points, colors, <strong>and</strong> solubility in most<br />

solvents, they differ in their interaction with plane-polarized light, which consists of electromagnetic<br />

waves oscillating in a single plane. In contrast, normal (unpolarized) light consists of electromagnetic<br />

waves oscillating in all directions perpendicular to the axis of propagation. When normal light is passed<br />

through a substance called a polarizer, only light oscillating in one direction is transmitted. A polarizer<br />

selectively filters out light that oscillates in any but the desired plane (Figure 24.8 "Detecting the Optical<br />

Activity of Chiral Substances").<br />

Figure 24.8 Detecting the Optical Activity of Chiral Substances<br />

When<br />

polarized light is passed through a solution that contains an achiral compound, there is no net<br />

rotation of the plane of polarization of the light. In contrast, when polarized light is passed through<br />

a solution that contains one enantiomer of a chiral compound, as shown here, the light is rotated<br />

either clockwise [dextrorotatory, (+) enantiomer] or counterclockwise [levorotatory, (−)<br />

enantiomer] by an angle that depends on the molecular structure <strong>and</strong> concentration of the<br />

compound, the path length, <strong>and</strong> the wavelength of the light.<br />

When plane-polarized light is passed through a solution, electromagnetic radiation interacts with the<br />

solute <strong>and</strong> solvent molecules. If the solution contains an achiral compound, the plane-polarized light<br />

enters <strong>and</strong> leaves the solution unchanged because achiral molecules cause it to rotate in r<strong>and</strong>om<br />

directions. The solute is therefore said to be optically inactive. If the solution contains a single<br />

enantiomer of a chiral compound, however, the plane-polarized light is rotated in only one direction, <strong>and</strong><br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

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