Reminder: These notes are meant to supplement, not replace, the laboratory manual.
Stereochemistry & Polarimetry notes
History and Application:
Approximately 25% of all drugs are marketed as either racemates (mixtures of
two enantiomers) or mixtures of diastereomersi. The orientation around a chiral center
can have a dramatic impact on the pharmacological response of that drug in the human
body. A worst case scenario is one which the non-desired stereoisomer causes serious
toxicity. The drug Thalidomide, was prescribed to millions of European women to
suppress morning sickness associated with pregnancy during the late 50’s and early
60’s. Horrible birth defectsii, including missing limbs, resulted. The cause of these birth
defects was assigned to the S-(-)-enantiomer of the drug, which did not undergo clinical
trials. This drug was close to being allowed into the US. A new woman FDA reviewer,
Dr. Frances Kelsey questioned the supporting evidence, did not yield to pressure from
the pharmaceutical manufactures, and blocked its introduction into US marketsiii. Dr.
Kelsey received a Presidential award from JFK for her actions that saved tens of
thousands of babies in the United States. This tragedy brought about severe tightening
in the laws surrounding the testing and introduction of new drugs into the US. Chiral
synthesis and purification is a crucial aspect of all successful drug manufacture.
Thalidomide
Safety considerations for this experiment:
In this experiment aqueous sugar solutions will be prepared and their optical
rotations will be measured. The solutions are nontoxic and the polarimeters have no
special hazards. Standard good lab practice of wearing goggles and lab coats applies.
Terminology.
Chiral: A material which is not superimposable on its mirror image.
Chirality center: A tetradedral atom having four different groups attached. This is also
known as a stereocenter.
Cahn-Ingold-Prelog system: A system of prioritization of groups around a carbon center,
placement of the lowest priority in the back, then sequencing of the remaining
groups. Clockwise is defined as R, counterclockwise is defined as Siv.
R & S: Orientation around a chirality center as defined by the Cahn-Ingold-Prelog
system.
Enantiomers: a pair of nonsuperimposible mirror images.
Diastereomers: stereoisomers that are not mirror images of one another.
Meso: A compound with 2 or more chirality centers that is achiral (superimposable on its
mirror image) due to presence of an internal plane or point of reflective
symmetry. v
Stereoisomers: Two or more molecules with the same empirical formula, and the same
atomic connectivity but different spatial orientations.
Constitutional isomers: Two or more molecules with the same empirical formula but
different atomic connectivity. These two molecules will have different names.
Plane Polarized Light: electromagnetic radiation in which the orientation of the electric
fields are perfectly aligned.
Racemates: A mixture composed of equal amounts of R and S enantiomers.
Optically active: A material which will rotate plane polarized light.
+ and -: The direction in which an optically active material will rotate light. If the light is
rotated clockwise it is defined as +. There is absolutely no correlation between R
& S and + and -. A given R compound may be + or -. In a pair of enantiomers,
one compound will rotate plane polarized light in the + direction and the other
compound will rotate the light by the same amount, in the opposite direction.
There is no way to theoretically determine if a compound will rotate light + or -.
The direction of rotation must be experimentally determined.
Specific Rotation, []: A fixed physical property describing the rotation of plane polarized
light by a chiral compound. Other physical properties include melting point and
boiling point. The specific rotation describes how far and in what direction a
standard solution (1.00 g/mL solution) of that material in a standard tube (1.00
dm) will rotate light. A [] of +87.6 means that a pure enantiomer made up into a
1.00 g/mL solution in a 1.00 dm tube will rotate light in the clockwise manner by
87.6 degrees.
Observed Rotation, obs: This is the experimentally obtained rotation of a compound.
This value is dependent upon the way the experiment was carried out including
the solution concentration and the length of tube. If the tube length and the
solution concentration is known, the observed rotation may be converted to
specific rotation.
1. Chiral molecules have an asymmetrical center which responds to light as a lens and
rotates light. The ability to rotate light is termed optical activity. Enantiomeric
compounds rotate light by exactly the same amount but in opposite direction. The
degree to which a substance rotates light may be used to determine a) the
identity of the substance, b) the enantiomeric purity of a known substance or c)
the concentration of a known substance in a solution. In today’s experiment
optical rotation will be used to determine the identity of unknown substances.
2. Chiral molecules synthesized in the lab are notoriously expensive. For this
experiment we are using chiral molecules synthesized by mother nature. Each of the
molecules is a type of naturally occurring sugar.
Name (other
names)
D- Fructose
(D-Levulose)
Structure (Fisher and Haworth)
Specific Rotation []
-86
and
D-Glucose
+98
and
D- Galactose
+ 82
and
D-Allose
+15
Sucrose
and
glucose-fructose
+64.5
Maltose
glucose-glucose
+118
3. The glassware which is selected to measure a volume has a large impact on the
accuracy and precision of the measured volume.
Glassware
Error of Measurement
Cost per unit
50 mL Beaker
± 3 mL
$3.82
50 mL Grad. Cylinder
± 0.2 mL
$33.26
25 mL Volumetric Flask
± 0.02 mL
$30.07
Volumetric flasks are ten times more precise than 50mL graduated cylinders and more
than 100 times more precise than beakers. In today’s lab it is very important to
know the concentration of the solution precisely. The accuracy and precision of
the volume and mass measurements will have a direct and significant effect on
the correct identification of the unknown. Volumetric flasks will be used to
precisely and accurately measure 25.00 mL. (The last two zeros here are
important and significant.)
4. Volumetric flasks are good for measuring one volume only. The solvent has to be
added until the meniscus is exactly on the line. If the meniscus is slightly below
the line, add more solvent. If too much solvent is added and the meniscus is
above the line, there is no fix. The entire solution must be disposed of, the
container rinsed and the entire procedure repeated.
5. Remember CHEM 1010 Module 4, learning goal 7 how to properly make solutions
from pure compounds and water.
Step 1. Measure out the amount of solute which you need.
Step 2. Put approximately one half of the water needed into the container.
Step 3. Add the substance from Step 1 into the water in Step 2 and mix until all is
dissolved.
Step 4. Add water until the total volume to be made is obtained. Be careful when
using volumetric flasks. Do not add too much solvent.
Step 5. Mix the solution. If made in a volumetric flask, cap and invert to mix.
6. In order to observe rotation, the light which is passed through the solution must be
plane polarized. Ordinary light has waves which are oriented in all directions.
Plane polarized light is made up of waves which are oriented parallel to a defined
plane.
Ordinary light
Plane Polarized Light
7. When a beam of plane polarized light passes through a solution of optically active
material the light will rotate.
8. Each pure chiral material has a set specific rotation [] which is a fixed physical
characteristic for that material. The enantiomer will rotate the plane of polarized
light by exactly the same amount but in the opposite direction.
If an S compound has an [] of +87.6, then the R
enantiomer will have an [] of -87.6. Some R compounds rotate light in the +
direction, some R compounds rotate light in the – direction. There is no
relationship between R/S and +/-.
Racemic mixtures (equal parts of two enantiomers) will have no net rotation because
the equal but opposite rotations cancel each other.
9. The specific rotation ([]) of a compound is a fixed physical property of that
compound (as is its boiling point or melting point or density). The observed
rotation (obs) depends on the concentration of the sample in solution (c) in
grams per milliliter, and the length of the cell (l) in decimeters as well as the
specific optical rotation of the compound
[ ]
Doubling the concentration of a material in a solution will double the observed
rotation. Cutting the cell length in half will half the observed rotation. The specific
rotation [] takes the concentration and cell length into account and hence
remains the same.
10. The observed rotation measurement will be taken using one of four Atago Polax-2L
polarimeters available for organic students. The image on the left, shows the
machine in the closed position ready to take a reading. The machine on the right
is open showing the polarimeter sample cell resting on the support.
11. If a material was made up into a solution at other than 1.00 g/ml concentration, and
tested in a tube other than 1.00 dm in length, that observed rotation can easily be
normalized back to specific rotation conditions by utilizing the above formula.
12. A mixture of R and S enantiomers will rotate light in the direction of the enantiomer
present in excess and to a degree related to the amount of that excess. This is
described by the following equation.
e.e. = |R-S| / (R+S) * 100%=|
[ ]
[ ]
| * 100%
If the amount of the two enantiomers are known, the observed rotation may be
calculated from the enatiomeric excess. For example, if a pure R material has a
specific rotation of -16.70, what will be the specific rotation of a mixture of 40% R
and 60% S? This means there is a (60%-40%) 20% enantiomeric excess of S.
Using basic algebra, the 20% e.e. is then divided by 100 to result in 0.20, and
multiplied by the absolute value of []pure of |-16.70| or 16.70 to show this mixture
will rotate light by |3.35|. Knowing that the R material rotates in the negative
direction, and knowing that this mixture has an excess of the S enantiomer,
means that this material will rotate light in the positive or clockwise direction by
3.35 degrees.
e.e. = |R-S| / (R+S) * 100%= |60-40|/(60 +40) *100% = 20%
20% of |-16.70| = |3.35|, and an excess of S means 3.35o clockwise.
13. Of more practical use is the calculation of the enantiomeric excess given a specific
rotation of a mixture and a specific rotation for a pure enantionmer. If a mixture of
this R and S has a specific rotation [mix of +3.97, and pure R has a specific
rotation of -16.7 ([R= -16.7) then the composition of the mixture may be
determined as follows. The overall rotation is positive, and the specific rotation of
pure R was stated as – 16.70, therefore this mixture has an excess of the S
enationmer. Deciding which enantiomer is in excess needs no calculations and
should be done first.
Using the above equation the amount of excess of one entionmer can be calculated
[ ]
e.e.=| [ ]
| * 100% =|
| * 100%=23.8%.
]
This means that there is 23.8 excess S enationmer. This also means the rest of
the material (100.0 – 23.8 = 76.2%) is equally divided between the R and S.
Therefore in the balanced material
there is 76.2  2 or 38.1% R material, and 38.1% S. The total composition of S is
the amount of excess 23.8% plus the amount from the balanced, 38.1% or a total
of (23.8% +38.1%=) or 61.9% S material. The total mixture composition is then R
= 38.1%, S= 61.9%. Always double check by adding these two together, the
result should be 100 %. 61.9% (S) + 38.1% (R) = 100.0 %, and the difference
between the two (61.9-38.1) is 23.8%, the amount of excess.
Revised October 21, 2014 S. L. Weaver
References.
i
Hutt, A. J., Grady, J. O., J.Antimicrobial Chemotherapy, 1996, 37, 7-32
Chemical & Engineering News, Thalidomide
http://pubs.acs.org/cen/coverstory/83/8325/8325thalidomide.html (October 5, 2011)
iii
http://blogs.fda.gov/fdavoice/index.php/2014/07/dr-frances-kelsey-who-protected-americans-fromthalidomide-turns-100/ (October 21, 2014)
iv
David Klein, Organic Chemistry, Wiley New York, 2012, pp199-201
v
Ibid pp 216-217
ii