Specific rotation

Polarimeter_(Optical_rotation).svg

Specific rotation is a physical property of a chiral chemical compound that quantifies its ability to rotate the plane of polarized light. This property is crucial in the field of optical activity and is used extensively in the study of stereochemistry and enantiomers.

Definition

The specific rotation ([α]) of a substance is defined as the observed angle of rotation (α) when polarized light passes through a sample of the substance, divided by the path length (l) of the sample in decimeters and the concentration (c) of the sample in grams per milliliter. The formula is given by:

[α] = α / (l * c)

Measurement

Specific rotation is measured using a device called a polarimeter. The sample is placed in a tube of known length, and the angle by which the plane of polarized light is rotated is observed. The specific rotation is then calculated using the formula mentioned above.

Factors Affecting Specific Rotation

Several factors can influence the specific rotation of a substance:

• Wavelength of the light used: Specific rotation is dependent on the wavelength of the light, commonly measured using the sodium D-line (589 nm).
• Temperature: The specific rotation can vary with temperature.
• Solvent: The choice of solvent can affect the specific rotation of a substance.
• Concentration: Although specific rotation is defined for a specific concentration, deviations can occur at different concentrations.

Applications

Specific rotation is used in various applications, including:

• Determining the purity of a chiral compound.
• Identifying unknown substances by comparing their specific rotation with known values.
• Studying the kinetics of chemical reactions involving chiral compounds.
• Quality control in the pharmaceutical industry.

Related Concepts

• Optical rotation
• Enantiomeric excess
• Chirality (chemistry)
• Polarimetry

See Also

• Polarimeter
• Optical activity
• Chirality (chemistry)
• Enantiomer