Why should one subtract the optical rotation value of a blank from that of the s

Question

Why should one subtract the optical rotation value of a blank from that of the solution when calculating optical rotation if the blank itself is not chiral?

A. All solvents are chiral, therefore subtracting its effects is not necessary.

B. The solvent, even if it is not chiral, can affect the flow of light through the solution and this effect must be subtracted from the observed value.

C. Subtracting the optical rotation value of the blank from that of the solution eliminates any differential temperature effects.

D. None of the answers shown are correct.

E. This procedure is necessary to remove the effects of chemical reactions taking place between the compound of interest and the solvent.

Explanation / Answer

Chiral molecules exhibit circular birefringence, which means that a solution of a chiral substance presents an anisotropic medium through which left circularly polarised (L-CPL) and right circularly polarised (R-CPL) propagate at different speeds. A linearly polarised wave can be thought of as the resultant of the superposition of two circularly polarised waves, one left-circularly polarised, the other right-circularly polarised. On traversing the circularly birefringent medium, the phase relationship between the circularly polarised waves changes and the resultant linearly polarised wave rotates. This is the origin of the phenomenon known as optical rotation, which is measured using a polarimeter. Measuring optical rotation as a function of wavelength is termed optical rotatory dispersion (ORD) spectroscopy.

]T = /(l * d)

In this equation,

l is the path length in decimeters

d is the density of the liquid in g/mL for a sample at a

temperature T (given in degrees Celsius) and wavelength (in nanometers).

If the wavelength of the light used is 589 nanometer (the sodium D line), the symbol “D” is used. The sign of the rotation (+ or -) is always given.

For solutions, a different equation is used:

[]T = 100 * /(l * c)

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