updated Friday, April 01, 2016

Polarimeter

Most physical properties of enantiomers i.e., melting point, boiling point, refractive index, etc. are identical. However, they differ in a property called optical activity, in which a sample rotates the plane of polarization of a polarized light beam passing through. This effect was first discovered in 1808 by E.L. Malus (1775-1812), who passed light through reflective glass surfaces. Four years later, J.B. Biot (1774-1862) found that the extent of rotation of the light depends on the thickness of the quartz plates that he used. He also discovered that other compounds i.e., turpentine and sucrose solutions were capable of rotating the light. He attributed this "optical activity" to certain features in their molecular structure (asymmetry). Based on his research, he designed one of the first polariscopes, and formulated the basic quantitative laws of polarimetry. In 1850, Wilhelmy used polarimetry to study the reaction rate of the hydrolysis of sucrose. In 1874, van't Hoff proposed that a tetrahedral environment of the carbon atom could explain the phenomenon of optical activity. Today, polarimetry is used routinely in quality and process control in the pharmaceutical industry, the flavor, fragrance and essential oil industry, the food industry, and the chemical industry. The optical purity of the product can be determined by measuring the specific rotation of compounds like amino acids, antibiotics, steroids, vitamins, lemon oil, various sugars, and polymers and comparing them with the reference value (if the specific rotation of the pure enantiomer is known).

Compound	[α]D (in o)
(1R)-(+)-Camphor	+44.26
Sucrose	+66.47
Cholesterol	-31.50
D-(+)-Glucose	+52.70
D-(-)-Fructose	-92.00
Morphine	-132.00
L-Proline (in water)	-84.00

How does it work? Normal monochromatic light contains light that possesses oscillations of the electrical field in all possible planes perpendicular to the direction of propagation. When light is passed through a polarizer (i.e., Nicol prism, Polaroid film) only light oscillating in one plane will leave the polarizer ("picket fence model"). This linear polarized light can be described as a superposition of two counter-rotating components, which propagate with different velocities in an optical active medium. If one component interacts stronger than the other with a chiral molecule, it will slow down and therefore arrive later at the observer. The result is that the plane of the light appears to be rotated because the two vectors are not canceling each other anymore due to the phase shift.

In a polarimeter (figure 2), plane-polarized light is introduced to a tube (typically 10 cm in length, figure 3) containing a solution with the substance to be measured. If the substance is optical inactive, the plane of the polarized light will not change in orientation and the observer will read an angle of [α]= 0^o. If the compound in the polarimetry cell was optical active, the plane of the light would be rotated on its way through the tube. The observed rotation is a result of the different components of the plane polarized light interacting differently with the chiral center. In order to observe the maximum brightness, the observer (person or instrument) will have to rotate the axis of the analyzer back, either clockwise or counterclockwise direction depending on the nature of the compound. For clockwise direction, the rotation (in degrees) is defined as positive ("+") and called dextrorotatory (from the Latin: dexter=right). In contrast, the counterclockwise direction is defined as negative ("-") and called levorotatory (from the Latin laevus=left). Unfortunately, there is no direct correlation between the configuration [(D/L) in Rosanoff, (R/S) in Cahn-Ingold-Prelog nomenclature] of an enantiomer and the direction [(+) or (-)] in which they rotate plane-polarized light. This means that the R-enantiomer can exhibit a positive or negative value for the optical rotation depending on the compound. In some cases, the solvent has an impact on the magnitude and the sign as well i.e.,(S)-lactic acid exhibits an optical rotation of [α]= +3.9^o in water and [α]= +13.7^o using 2 M sodium hydroxide solution as solvent because the observer looks at a different species (lactate).

The observed specific rotation [α]_obs depends on the length of the tube, the wavelength that it is used for the acquisition, the concentration of the optical active compound (enantiomer), and to a certain degree on the temperature as well. However, the temperature effect is very difficult to specify since it differs for each compound. For instance, the [α]-value for α-pinene only slightly increases in the range from 0 ^oC to 100 ^oC (at λ=589.3 nm), while it is almost cut in half for ß-pinene. These two compounds only differ by the position of the alkene function.

Generally the following equation is used to calculate the specific optical rotation from experimental data:

α_obs = observed optical rotation
c = the concentration of the solution in grams per milliliter
l = the length of the tube in decimeters (1 dm=10 cm)

Example 1:

A student obtained the following specific optical rotation from his measurement.

This notation means that the measurement was conducted at 25 ^oC using the D-line of the sodium lamp (λ=589.3 nm). A sample containing 1.00 g/mL of the compound in a 1 dm tube exhibits an optical rotation of 3.5^o in clockwise direction. Note that the instrument used in Chem 30BL and Chem 30CL can provide the specific optical rotation, which already corrects the optical rotation for the cell dimensions and the concentration. The optical rotation is raw data, which does not include these corrections. It is very important to pay attention which mode was used to acquire the data!


As mentioned earlier, polarimetry can be used to determine optical purity of enantiomers.

Example 2:

The observed specific optical rotation of a compound is [α]= +7.00^o. The specific optical rotation for the pure enantiomer is .

Percent optical purity

The sample consists of 75 % of the racemic form (=equimolar mixture of both enantiomers, α=0^o) and an excess of 25 % of the enantiomer in question (62.5 % and 37.5 %).

The instrument used below allows you to calculate the specific rotation, if you know the concentration of the solution. The cell used for the measurement has a pathlength of 10.0 cm.