How to Solve Enantiomeric Excess Related Numerical Problems – Part 3
Enantiomeric excess (ee) is a purity quantification for chiral compounds in stereochemistry and reflects the degree to which a sample contains one enantiomer in greater amounts than the other.
A racemic mixture has an ee of 0%, while a single completely pure enantiomer has an ee of 100%. So, for one, a sample with 70% of one enantiomer and 30% of the other has an ee of 40%.
ee is used as one of the indicators of success of an asymmetric synthesis.
ee is defined as the absolute difference between the mole fraction of each enantiomer.
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Specific rotation ([α]),
an intensive property/quantity (magnitude/extent independent of the size of the
system) of a chiral chemical compound, is defined as the change in orientation
of monochromatic plane-polarized light, per unit distance–concentration product, as the light
passes through a sample of the compound in solution.
The observed rotation (α) of a sample of a compound can be used to quantify the ee of that compound using the following equations:
where l = Path
length in dm, c = Concentration in g/100 mL (%w/v),
T = Temperature in 0C (typically, standard room
temperature 20°C), and λ = Wavelength in nm (typically, 589
for the Sodium D line)
The concentration of
bulk sugar solutions is sometimes determined by comparison of the observed
optical rotation with the known specific rotation.
Compounds which rotate
the plane of polarization clockwise are dextrorotary (d or +),
while those rotate the plane of polarization counterclockwise are levorotatory (l
or ─). If a compound is able to rotate the plane of polarization, it is ‘optically active’.
The optical rotation is NOT directly
related to the absolute configuration.
The variance of specific rotation with wavelength, referred to as optical rotatory dispersion, can be used to find the absolute configuration of a molecule.
Question:
What is the observed
rotation of the product on hydration by oxymercuation-demercuration
of 3-Methylbut-1-ene using a 20% solution in Ethanol in a 1 dm
polarimeter cell? Assume the specific rotation of the pure (S)-enantiomer
to be +16º.
Solution infographic:
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Creation of a new chiral center may yield
a mixture of enantiomers (of 3-Methylbutan-2-ol in the above case) or
diastereomers.
For (S)-enantiomer,
Click/tap to enlarge the image
Omnifarious optical rotation❓ Let Chemaficionado know in the comments below or at mychemistryhomework@gmail.com❗
References:
(1) Brown, C.; Ford, M. Higher Level Chemistry, 2nd ed.; Pearson Education: Harlow, Essex, 2014.
(2) Ncert. Chemistry : Textbook for Class XI - Part.II; National Council Of Educational Research And Training: New Delhi.
(3) Optical Rotation; 2019. https://en.wikipedia.org/wiki/Optical_rotation.
(4) Wikipedia Contributors. Specific rotation. Wikipedia. https://en.wikipedia.org/wiki/Specific_rotation.
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