Ambroxide/Ambroxan is a typical chiral compound used in fragrance. Its specific optical rotation (-21° to -24°) is not only a quality control indicator but also a key indicator of its natural origin. However, people sometimes cann't distinguish between optical rotation and specific optical rotation, so this article mainly distinguishes the differences and related information between them.
Both optical rotation and specific optical rotation are concepts used to describe the ability of optically active substances (also called optically active substances) to rotate the vibration plane of plane-polarized light. They are closely related, yet distinctly different:
| Optical rotation (Observed Optical Rotation / α) | Specific optical rotation (Specific rotation[α]) | |
| Definition |
Under specific experimental conditions (specific temperature, wavelength, solvent concentration, sample tube length), the actual rotation angle of the vibration plane of a beam of plane-polarized light after passing through a sample solution (or pure liquid) containing an optically active substance. It is a measured value. |
To eliminate the influence of concentration and sample tube length on optical rotation, a standardized optical rotation is defined, called specific optical rotation. It is the optical rotation produced by an optically active substance at a specific temperature(T) and under a specific wavelength (λ)of light, with unit concentration and unit optical path length. It is a characteristic constant of a substance. |
| Symbol | Usually represented by the Greek letter α | Usually represented by [α], with the temperature and wavelength indicated in the lower right corner, such as [α]25D represents the specific optical rotation measured using the D line of a sodium light lamp at 25°C. |
| Unit | Degree (°) | Degree (°) |
| Features |
The direction of rotation is indicated by symbols: It is a value obtained by direct measurement. |
It is a calculated value, calculated from the measured optical rotation α, concentration c, and tube length l. It is an important physical constant and characteristic property of optically active substances, similar to the melting point, boiling point, refractive index, etc.
It eliminates the influence of concentration and tube length, enabling standardized comparison of the optical rotation abilities measured at different concentrations and tube lengths. But it still depends on temperature and wavelength, so when reporting specific optical rotation, it is necessary to specify the temperature T and the wavelength λ of the light source (usually the sodium D line, marked as D) during the measurement. |
| Note | Sample concentration (c): The higher the concentration, the greater the optical rotation (directly proportional). Sample tube length (l): The longer the path the light passes through (the longer the sample tube), the greater the optical rotation (directly proportional). Temperature (T): Temperature changes may affect the molecular structure and solvent properties, thereby affecting the optical rotation. Wavelength of light (λ): Usually, the D line of a sodium lamp (589.3 nm) is used, and the optical rotation varies at different wavelengths (optical dispersion). Solvent: The optical rotation of the same substance may vary in different solvents. |
Calculation formula: [α]ᴛᴌ = α / (l * c) [α]ᴛᴌ: specific optical rotation at temperature T and wavelength λ (unit: °·mL·g⁻¹·dm⁻¹ or °·cm²·g⁻¹, often abbreviated as °) α: experimentally measured optical rotation (°) l: sample tube length (dm, decimeter, 1 dm = 10 cm) c: solution concentration (g/mL, grams per milliliter) (for pure liquids, "c" is the density g/mL) Unit: usually written as °·mL·g⁻¹·dm⁻¹ or °·cm²·g⁻¹, but often abbreviated as °. Positive and negative sign: The sign is the same as that of the optical rotation α. |
| Conclusion | The optical rotation cannot be used directly to compare the inherent optical rotation power of different substances or as a characteristic constant of substances. It is only applicable to a specific sample under specific experimental conditions. | The specific optical rotation can be used directly to compare the inherent optical rotation power of different substances or as a characteristic constant of substances. |
In other words, the optical rotation (α) is the value directly read on the polarimeter when you experiment. This value may vary if the solution you prepare is more concentrated or less diluted, or the tube containing the solution is longer or shorter. It describes the performance of the specific sample in your hand under the specific experimental conditions.
Not all flavor and fragrance raw materials need to be labeled with specific optical rotation. Chiral compounds such as Ambroxide must be tested, and specific optical rotation is the core quality control indicator. As a leading innovator in aroma chemicals, Xi'an App-Chem Bio (Tech) Co., Ltd. (APPCHEM) specializes in premium-grade Ambroxide for global partners.
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