Optical rotation

Optical rotation or optical activity is a fundamental property of certain substances that have the ability to rotate the plane of polarization of plane-polarized light. This phenomenon is observed in liquids, solutions, and gases of optically active substances, which are typically chiral molecules or enantiomers. The direction and magnitude of the rotation depend on several factors, including the wavelength of light (usually specified as the sodium D line, λ = 589 nm), the temperature, the length of the path through the material, and the concentration of the substance.

Definition

Optical rotation is measured using a polarimeter, an instrument that allows plane-polarized light to pass through a sample of the substance being studied. The angle by which the plane of polarization is rotated is called the angle of rotation, denoted by the Greek letter alpha (α). The specific rotation \([α]\) is a standardized measure, defined as the angle of rotation observed when light passes through a 1 dm path length of a solution at a concentration of 1 g/mL, at a specified temperature and wavelength. The formula for specific rotation is:

\[ [α] = \frac{α}{l·c} \]

where α is the observed angle of rotation in degrees, l is the path length in decimeters, and c is the concentration of the solution in grams per milliliter.

Types of Optical Activity

There are two types of optical activity: dextrorotatory and levorotatory. Substances that rotate the plane of polarization clockwise, as seen by an observer towards whom the light is traveling, are termed dextrorotatory and denoted by a positive (+) sign before their specific rotation value. Conversely, substances that rotate the plane of polarization counterclockwise are termed levorotatory and denoted by a negative (−) sign.

Factors Affecting Optical Rotation

Several factors can influence the degree of optical rotation, including:

• Wavelength of light (dispersion): Optical activity varies with the wavelength of the light used, a phenomenon known as optical rotatory dispersion (ORD).
• Concentration: The angle of rotation is directly proportional to the concentration of the optically active substance in the solution.
• Path length: The angle of rotation is directly proportional to the length of the path that the polarized light travels through the substance.
• Temperature: The specific rotation of a substance can vary with temperature.
• Solvent: The nature of the solvent can also affect the optical rotation of a substance.

Applications

Optical rotation is widely used in various fields, including:

• Chemistry: To determine the purity and concentration of enantiomerically pure samples.
• Pharmacy: In the pharmaceutical industry, optical rotation is used to identify and quantify chiral drugs.
• Food industry: To assess the concentration of sugars in solutions, such as in the production of high-fructose corn syrup.
• Biochemistry: To study the structure and behavior of biomolecules like proteins and DNA.

See Also

• Chirality (chemistry)
• Enantiomer
• Polarimeter
• Stereochemistry