Optical Rotation (α)
Definition
The angle in degrees through which a chiral solution rotates the plane of polarisation of a plane-polarised light beam; measured by rotating the analyser to the null (minimum) or equal-brightness condition.
- Symbol
- α (alpha)
- Unit
- Degrees
- Requires
- Chiral (optically active) compound
- Measured with
- Polarimeter
- Read at
- Null or equal-brightness point of the analyser
Common questions
Why is optical rotation useful when a compound might already be identified by other methods?+
It distinguishes between enantiomers that share identical mass spectra, infrared spectra and chromatographic retention on achiral columns, since enantiomers differ only in the direction they rotate polarised light. This matters for drugs like methamphetamine, where the d- and l- forms have different legal and pharmacological significance.
What factors can distort a specific optical rotation reading?+
Concentration, solvent, temperature, and the light wavelength used all affect the measured angle, so specific rotation values are only comparable when reported under matched conditions. Impurities that are themselves optically active can also skew the apparent rotation of the target compound.
Related terms
- Biot Equation
- The fundamental equation of polarimetry: [α]_T^λ = α / (c × l), relating observed rotation to specific rotation, concentration, and path length;...
- Chirality
- The property of a molecule that cannot be superimposed on its mirror image; a chiral molecule and its mirror image are enantiomers...
- Dextrorotatory (+)
- Rotating the plane of polarisation clockwise (to the right) as seen facing the light source; designated (+) or d-; the d-methamphetamine and...
- Enantiomeric Excess (Ee)
- A measure of chiral purity: ee (%) = (R - S)/(R + S) x 100, where R and S are the measured...
- Enantiomers
- Two molecules that are non-superimposable mirror images of each other; they have identical physical properties (melting point, boiling point, solubility) except for...
- Invert Sugar
- The equimolar mixture of glucose and fructose produced by hydrolysis (inversion) of sucrose; laevorotatory overall because fructose's negative rotation exceeds glucose's positive...
- Laevorotatory (-)
- Rotating the plane of polarisation anticlockwise (to the left) as seen facing the light source; designated (-) or l-; l-methamphetamine and natural...
- Laurent Half-Shade Polarimeter
- A classical polarimeter design that uses a half-wave plate covering half the aperture to create two adjacent half-fields; the null point (equal...
- OIML R 139
- International Organisation of Legal Metrology Recommendation R 139 (2014) governing metrological requirements for polarimeters used in official testing; specifies maximum permissible error...
- Saccharimeter
- A polarimeter calibrated in International Sugar Degrees (°Z) rather than angular degrees; used in the sugar industry for purity determination under the...
- Specific Optical Rotation ([α])
- The standardised optical rotation of a substance at a defined temperature and wavelength, calculated from the Biot equation: [α] = α /...