Laboratory practice session 04

OPTICAL METHODS FOR MEDICINES QUALITY CONTROL. POLARIMETRY

I. Questions and tasks for discussion:

1. Why doesn’t a racemic mixture show optical activity?

2. Does the following image show optically active molecules? Why?

Figure \(\PageIndex{1}\)

3. What would happen if your feet were not optical isomers of each other?

4. Your friend has tuberculosis. Assuming you like your friend and you want him to live, which isomer of ethambutol do you give him and why?

Figure \(\PageIndex{2}\)

 5. Draw each compound's enantiomer:

Figure \(\PageIndex{3}\)

II. Do the tasks:

Exercise \(\PageIndex{1}\)

Determine the optical purity of a racemic mixture: [α]_{pure samples} = +/- 23,1 °.

Answer

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The specific rotation, [α], of the racemate is expected to be 0, since the effect of one enantiomer cancels the other out, molecule for molecule.

Optical purity, ω% = 100% [α]_mixture / [α]_{pure sample} = 100% (0) / +23,1 ° = 0%.

Exercise \(\PageIndex{2}\)

Determine the enantiomeric excess of the racemic mixture.

Answer

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You would expect [R] = [S] = 50%.

ω% = 100% ([R] – [S]) / ([R] + [S]) = 100% (50 – 50) / (50 + 50) = 0%

Exercise \(\PageIndex{3}\)

Which isomer is dominant and what is the optical purity of a mixture, of (R)- and (S)-2-bromobutane, whose specific rotation was found to be -9,2 °? ([α]_{pure R-sample} = -23,1°).

Answer

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The negative sign tells indicates that the R enantiomer is the dominant one.

Optical purity, ω% = 100 [α]_mixture / [α]pure sample = 100 (-9,2) / -23,1 = +40%.

This indicates a 40% excess of R over S.

Exercise \(\PageIndex{4}\)

Given that (S)-2-bromobutane has a specific rotation of +23,1 ° and (R)-2-bromobutane has a specific rotation of -23,1 °, what is the optical purity and % composition of a mixture whose specific rotation was found to be +18,4 °?

URL: http://www.mhhe.com/physsci/chemistry/carey/student/olc/graphics/carey04oc/ch07/others/ch07ans.html

Answer

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The positive sign indicates that the (S)-isomer is in excess.

Optical purity, % = 100 [α]_mixture / [α]_{pure sample} = 100 (+18,4) / +23,1°= 80%.

This indicates a 80% excess of S over R. The 20% leftover, which is optically inactive, must be equal amounts of both (R)- and (S)-bromobutane. The excess 90% is all S so there is a total of 10% (R) and 90% (S).

III. Lab practice.

In this experiment a sugar solution of known concentration (С), but unknown identity will be prepared (note the units of concentration used). The observed rotation (α_obs) will be obtained by using a polarimeter. This data will be used to calculate the specific rotation [α] and the identity of the sugar will be hypothesized:

[equation]

where [α]= specific rotation of the compound;

α_obs = observed rotation of light in degrees;

C = concentration in grams per milliliter (g/ml);

l = cell length in decimeters (dm).

Name	Structure (Fisher, Haworth)	Specific Rotation [α]
D-Fructose		-86
D-Glucose		+98
D-Galactose		+82
Sucrose	Glucose-fructose	+64,5

Sample Preparation, Set Up and Procedure:

1. Weigh out approximately 0,5 g of unknown sample (m). Record all the digits of this mass in your notebook.
2. In 50 ml beaker, dissolve your material in approximately 10 ml of deionized water. Swirl the contents until all of the solid has dissolved.
3. Carefully transfer this solution to a 25 ml volumetric flask. Rinse beaker with approximately 2 ml of deionized water. Transfer this solution to volumetric flask. Repeat.
4. Carefully drop wise add deionized water to volumetric flask until bottom of meniscus is exactly on line. This is your solution.
5. In notebook, calculate the concentration in g/ml.
6. Obtain the α_obs by analyzing your solution in the polarimeter using the instructions mounted adjacent to machine and reviewed by your instructor. Repeat the measurement for 3 times.
7. Using the above equation, calculate the specific rotation [α].
8. Select the identity of your material from the list provided.
9. Fill out the relevant spaces in the data table.

№	αobs	αobs	l	m	C	[α]	Identification result
1
2
3

Conclusion: _____________________________________

IV. Self-test:

1. The Pharmacopoeia prescribes determination of the specific rotation [α]^t_λ for lots of pharmaceuticals. Which of the following equations is correct?
   1. [equation]
   2. [equation]
   3. [equation]
   4. [equation]
   5. [equation]

where [α]^t_λ = specific rotation in 10 dm/kg;

α^t_λ = rotation measured at t °C, at wavelength λ;

l = length of the cell in dm;

ρ_B = mass concentration of substance B in g/0,1 dm;

ρ_t = density of liquid at t °C.

2. Which parts of the polarimeter, if any, should be transposed?

1. 2 and 4
2. 3 and 4
3. 2 and 3
4. 4 and 5
5. the sequence of the parts is correct