Specific rotation

Specific rotation is a property of chiral substances, which allows them to rotate the plane of polarized light. It is defined as the observed rotation divided by the path length and the concentration of the substance. The specific rotation is usually reported in degrees per decimeter per gram per milliliter (°/dm·g/mL).

Definition

The specific rotation α of a substance is defined as the observed rotation divided by the path length (in decimeters) and the concentration (in grams per milliliter). It is given by the formula:

α = θ / (l·c)

where:

• θ is the observed rotation in degrees,
• l is the path length in decimeters,
• c is the concentration in grams per milliliter.

Measurement

The specific rotation is measured using a polarimeter. The sample is placed in a tube of known length, and the rotation of the plane of polarized light passing through the sample is measured.

Applications

Specific rotation is used in the study of chiral substances, which are substances that cannot be superimposed on their mirror images. It is also used in the determination of the enantiomeric excess in a sample of a chiral substance.

See also

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

Specific rotation Resources
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