Experiment:
Determination of Specific Rotation
The rotation of an optically active compound is determined by use of an instrument called a
polarimeter. This instrument shines a beam of plane-polarized light on a solution. An optically
active sample rotates the plane of this plane polarized light, either to the left (levorotatory) or to
the right (dextrorotatory). The amount of observed rotation for a given solution is directly
proportional to:
•
•
the concentration of the optically active compound
the length of the tube holding the solution.
If either of these two factors is changed, the observed rotation will also change. The specific
rotation is defined as:
T
[a]
D
=
a measured
×C
(for a solution)
or
[a]
T
D
=
a measured
×d
(for a pure liquid)
Where,
ameasured = the degree of rotation measured with the polarimeter
(+) for dextrorotatory and (-) for levorotatory
ℓ = the path length of the sample holder in decimeters (usually 1.00 or 2.00 dm)
C = the concentration of the sample in terms of grams solute/mL of solvent
d = the pure liquid density in grams/mL.
The symbol [a]
T
is used for specific rotation, where the D stands for the sodium D-line (589
D
nm) and the T refers to the temperature at which the determination was made. The specific
rotation is a constant for an optically active substance at a given temperature.
 Pre-lab Preparation
1.
Read the procedure below.
2.
Carefully review how to use the polarimeter. Write a brief explanation of what you
should see when the polarimeter is adjusted to the correct angle to take a reading.
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3. One of the most difficult aspects of obtaining a good value is correctly reading the
Vernier scale. Read the procedure (step 4) below and then practice this using the
diagram of a Vernier scale shown here. Write in your notebook the value represented,
which should have one digit after the decimal and correct units and sign.
10
10
0
10
0
4. Using the internet, look up the specific rotation for sucrose and write it in your
notebook.
Experimental Procedure
! Safety Considerations
! None.
1.
Measure out an assigned amount of sucrose. This value will range from approximately
1 - 5 grams. Record the exact mass and then carefully transfer the sucrose from the
weighing paper to a 25.0 mL volumetric flask, which will be set out for you. (If you spill
any sucrose, you must begin again.)
2.
Add a few milliliters of de-ionized water to the volumetric flask and swirl to begin
dissolving the sugar. Continue adding water until you are a few milliliters from the
etched line denoting the 25 mL level. From that point, add water dropwise until the
bottom of the meniscus is just touching the line. (If you go past the line, you must begin
again after rinsing the flask with water.) Stopper, invert, and shake the volumetric flask
at least 20 times to insure complete uniformity of the sample.
3. Obtain one of the 2 decimeter polarimeter tubes and pour the solution carefully into the
tube. Tip up the end of the tube in each direction to release any air bubbles.
4.
Place the cylinder into the polarimeter and take a reading. This is done by turning the
pointer while looking in the eye piece through the sample. You will see that there are
semicircles visible in your sample. When you have the correct value, both half-circles
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will look equally dim. A slight adjustment in either direction will make one half go dark
and the other light. This intermediate value, where both halves look equal in intensity, is
the correct setting. (You should try adjusting it to the extremes of rotation to get a feel
for what you are seeing. When you are far away from the correct value, the sample may
look completely dark or completely light. This is not correct, even though both halves
look equally dark. The correct setting is the point where adjusting in either direction
causes one half-circle to becomes light and the other dark. Experiment with this to be
sure you are comfortable.)
5.
Read the angle of rotation through the upper eyepiece of the polarimeter. This eyepiece
magnifies the polarimeter scale. If the zero line of the movable scale lies to the left of the
zero of the fixed scale, the angle of rotation is negative and the sample is levorotatory. If
it lies to the right, the angle of rotation is positive and the sample is dextrorotatory. Each
line to the left or right of zero on the fixed scale represents one degree of rotation. Using
the Vernier scale on the polarimeter allows you to determine the angle to the nearest tenth
of a degree. The example below will show how to read the angle of rotation including
use of the Vernier scale.
The “marker” that gives you the number before the decimal is the zero line on the bottom
scale, which slides past the fixed, upper scale. For example, in the example below, the
zero line of the sliding scale is between 12-13° on the main scale (first arrow). This
means our reading must be 12.something. The number after the decimal is obtained by
looking along the sliding scale to see which of the 10 lines matches up perfectly with any
of the lines in the upper scale. This line on the sliding scale represents the digit after the
decimal. Here, the 6th line on the sliding scale (second arrow) matches with a line on the
upper scale, so our reading is 12.6°. Notice that we don’t care which line on the upper
scale matches; that is not relevant to our reading.
20
10
0
30
10
A levorotatory sample would be read using the left-side of the scales. Be careful to
realize that the numbers are increasing to the left. Test yourself by taking a reading for
the scale shown in the pre-lab.
5.
On the board provided, share you data with the rest of the class. You will need to provide
your sample mass, path length and polarimeter reading. Be sure to collect every else’s
data in your notebook as well.
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 Post-Lab and Report Requirements
1.
Make a table of all of the data obtained from this experiment by all of the other groups in
the class. The table should contain the masses of sucrose used, the length of the
polarimeter tubes, and the experimental rotation of the sucrose samples.
2.
From these data, calculate the specific rotation for your sample and for those of all of the
other groups in the class. Show your calculations.
3.
Calculate the average value for the specific rotation of sucrose.
4.
Compare the average value the class measured with the reference value. Calculate the
percent error and comment on the quality of the class data.
5.
How many chiral centers are present in each of the following compounds? Draw the
structures and label the chiral centers with an *.
OH
O
CH3
O
CH3
HS
CH3
HOOC
NH2
H
OH
CH3
H
H
H
O
penicillamine
cortisone
6.
How many stereoisomers are possible for each of the above compounds?
7.
Draw the enantiomer and a diastereomer of each of the above compounds, if one exists.
(Note that dots and wedges are always needed to show 3D structure.) Clearly label the
structures you draw as an enantiomer or diastereomer of the compound shown above.
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