Download Background: Polarimetry is the measurement and interpretation of

Background:
Polarimetry is the measurement and interpretation of the electromagnetic waves
can be diffracted by some compound. A sensitive and non destructive method used to
determine the optical activity of substance is a polarimeter instrument. To study this
instrument we will focus on stereoisomerisms.
Stereoisomerism:

Occurs when two or more compounds have the same structural and molecular
formulas but they differ in spatial arrangements of their bonds.

Types of steroisomerism: geometric and optical isomerisms. Here in this lab we
will talk about optical isomers (enantiomers).

Enantiomer: two optical isomers (D, L) called enantiomer. The two enantiomers
rotate light by the same amount but in opposite direction.
Mirror
L-Amino acid
D-Amino acid
Figure 1: D and L amino acids
Note: Any substance to have stereoisomer must contain chiral atom (it is atom
which connect to four different groups). Every single chiral atom has two
enantiomeric forms.
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To test the optical isomerism, we will use the polarimeter instrument. It is a device
that measures the angle of plane polarized light that is rotated when passing through a
solution.
Non-polarized light
Plane polarized light
0°
α
90°
Light source
Polarizing filter
Sample tube
Analyzer
Viewer
Figure 2: Components of the polarimeter instrument. It is consists of light source, polarizing
filter "prism", sample tube, and analyzer to measure the angle of rotation.
Or it is an instrument which is used to determine optical activity of a compound.
Optical active compounds:
Any substance to be optically active must have chiral atom and able to rotate plane
polarized light (The light that pass through prism) to the right and the other rotates it
to the left.
If it rotates light to right
substance has D "dextro", (+)
If it rotates light to left
substance has L "levo",
(-)
D and L called
enantiomers “mirror”
image"
Racemic mixture: is a mixture which contains equal amounts of each enantiomer.
(50% D and 50% L) and is optically inactive because the rotation cancel each other.
Specific rotation:
It is physical parameter (index) that is specific for each substance. We use Biot's law
to calculate specific rotation.
λ
[α] =
T
λ
αT
C.L
[  ]: Specific rotation
T: Temperature,
C: Concentration by g/ml
: Optical rotation by degree
Wavelength 
l: tube path length by dm
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Optical rotation "angle of rotation" depends on the temperature and wavelength of
light used "usually standardized to 20oC and 589.3 nm".
Specific rotation [  ]
Optical rotation 
Is the number that is calculated from Biots law.
Is the number that is read from polarimeter.
Is constant for each substance and not affected Is affected by sample conc.
by sample conc.
Amount of optical rotation is affected by:
1. Molecular structure.
2. Concentration and the temperature of the solution.
3. Wavelength of the light.
Sample tube
Scale for reading
Eyepiece
Light source
Knob
Switch On/Off
Scale for reading
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Applications:
1. Measuring specific rotation and optical rotation are used to determine the
purity of products regards to how much D and L (it is used in quality control).
2. Pharmaceutical: only one isomer is pharmaceutically active, so production of
highly pure compounds increases the quality and cost of the product.
3. Food industry: To check the purity of raw material as flavor, fragrance and
essential organic oil.
4. Chemistry: Use optical and specific rotation to identify biopolymers, natural
polymers, and synthetic polymer.
5. Important of optical isomers in biological system:
a) In human body some compound isomer (enantiomer) is optically active and
some is not active or has harm effect; for example, human body is only able to
deal with D-sugars and L-amino acids. Also, some drugs that are used to treat
nausea in pregnant women contain the other isomer that has bad effect on
fetus.
b) Racemic mixture is not found in biological system (for example, when alanine
is isolated from biological sample only one of the two isomers is present, but
when alanine is synthesized in lab, a mixture of two isomers is produced) this
property is used to detect the presence of life on other planet.
c) Different isomer has different effects; for example, cocaine and codeine have
diverse effect. Another example is limonene, one isomer has smell of orange
and the other one has smell of lemon.
Taste for example, in some amino acids; one isomer tastes sweat and the other
one tastes bitter.
Materials:
 Dissolve glucose in D.W (5g/50ml)
 Dissolve fructose in D.W (5g/50ml)
Instrument:
Polarimeter
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Methods:
1. Put glucose or fructose solution (fresh) in the sample tube.
2. Switch on the polarimeter instrument.
3. Look through eyepiece.
4. Use knob to rotate either clockwise or anti-clockwise the plane polarized light.
If you use water (not optically active), then rotate the analyzer there you will find
no effect on the plane polarized light and then no light gets.
If use glucose (optically active), then rotate the analyzer there you will find effect
on the plane polarized light and rotated until no light goes through the second
lens. In this place take the angle (optical rotation) to determine the specific
rotation.
Light region in center
Completely dark
Dark region in the left and right
Dark region in the center
Light region in the left and right
5. Take the reading every 10 min then every 1 or 4 hours during 24 hours.
6. Take the reading of optical rotation in the last hour.
References:
 Chemistry in Context: Hill and Holman, Nelson, ISBN 0-17-438401-7
 Chemistry: A. and P. Fullick, Heinemann, ISBN 0-435-57080-3
 Higher Still Support: Advanced Higher Chemistry - Unit 3: Organic
Chemistry, Learning and Teaching Scotland, ISBN 0-333-18153-0
 Organic Chemistry: J. McMurray, Brooks/Cole Publishing, ISBN 0-534-16218-5
 Chemical Ideas: Salters advanced chemistry, Heinemann, ISBN 0 435 630148.
 http://en.wikipedia.org/wiki/Polarimetry
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Result Sheet
Name: ---------------
Compound name: -----------
Time
Optical rotation
Calculate the specific rotation of glucose.
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