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POLARIMETRY
• T2
•Chapter # 5, “Organic Chemistry” (7th ed.)
by Solomons and Fryhle
Introduction:
• It’s a type of qualitative and quantitative
technique,used mostly for optically active
compounds
• the tendency of the molecules to rotate the plane
of plane polarized light (clockwise or
anticlockwise) and the extent of rotation is
measured
• these properties are unique for a molecule, thus
polarimetry can be used to identify and estimate
the compounds
Isomerism: different compounds that have
the same molecular formula are c/a isomers
1) Structural (constitutional) isomersdiffer because their atoms or functional
groups are connected in a different order
2) Stereoisomers- atoms are connected in
the same sequence but differ only in
arrangement of the atoms in space
Stereoisomers are subdivided into two types:
1) Diastereomers- stereoisomers whose
molecules are not mirror image of each
other
•These two compounds are isomers of each other
because they are different compounds and have
the same molecular formula
•Their atoms are joined in the same sequence,
therefore not a constitutional (structural) isomer
•They differ only in the arrangement of their
atoms in the space, therefore stereoisomers
•They are not mirror image of each other,
therefore not enantiomers but diastereomers
Another example of diastereomers
2) Enantiomers: stereoisomers whose
molecules are nonsuperposable mirror image
of each other
• enantiomers occur only with those
compounds whose molecules are chiral
3-D drawing of 2-butanol enantiomers
• a pair of enantiomers is
always possible for molecules
that contain one tetrahedral
atom with four different
group attached to it (chirality
center)
• in 2-butanol this is C2
• any two groups at the
tetrahedral atom that bear 4
different groups converts one
enantiomer into the other
Enantiomers of 2-butanol
Achiral molecule
When there are only three dissimilar groups
around the carbon atom (ie. the same group
occur twice), the molecule is
• symmetric
• superimposable on its mirror image
• achiral
Chiral and achiral molecule
Properties of Enantiomers: Optical activity
• when a beam of plane-polarized light passes
through an enantiomers, the plane of polarization
rotates
• separate enantiomers rotate the plane of planepolarized light equal amounts but in opposite
directions
• because of their effect on plane-polarized light,
enantiomers are said to be optically active
compounds
Plane polarized light:
A beam of light consists of two mutually
perpendicular oscillating fields:electric field and
magnetic field
In a beam of ordinary light (ex from bulb) the
oscillation of electric field are occurring in all
possible planes perpendicular to the direction of
propagation, c/a Unpolarized light
When an unpolarized light is passed through a
polarizer, the polarizer interacts with the electrical
field
•The resultant light which emerge from the
polarizer has their electric field vector oscillating
in only one direction
•Such light is c/a plane-polarized light
•Plane polarized light can be polarized in different
directions
Plane polarize light
The Polarimeter: a device used to measure
the effect of plane-polarized light on
optically active compounds
The components of polarimeter are:
• a light source - (usually a sodium lamp)
• a polarizer
• a tube for holding sample in the light beam- a
sample cell
•an analyzer- second polarizer, and
•a scale- to measure the rotation of plane
polarized light
schematic of a polarimeter
Schematic
of a
polarimeter
• if no or optically inactive sample is present in
the tube and the instrument is reading zero
(0o), the axes of plane polarized light and the
analyzer is exactly parallel
• the observer will detect maximum amount
(100 % transmittance) of light passing
through.
• if the sample is optically active the plane of
PPL will be rotated as it pass through the tube
• in order to detect the maximum brightness of the
light (ie. 100% transmittance) observer will have
to rotate the axis of the analyzer in either
clockwise or counterclockwise direction
• if the analyzer is rotated in a clockwise
direction, the rotation (α in degree) is said to be
positive (+), and such substance are c/a
dextrorotatory
• if the rotation is counterclockwise, the α is –ve,
and such substances are c/a levorotatory
Specific Rotation:
• extent of optical rotation depends on both the
sample path length and the analyte
concentration.
Specific rotation, [α], provides a normalize
quantity to correct for this dependence, and is
defined as:
[α] = α / c.l
where,
[α]= the specific rotation ; α = observed rotation
c = conc. of sample in gm/ml
l = length of the tube in decimeter (1dm = 10cm)
• [α] depends on the temperature and the
wavelength of the light used
• these quantities are also incorporated while
reporting [α]
25
[α]D =
o
+3.12
• means D line of a sodium lamp (λ=589.6nm) is
used for the light at a temperature of 25oC, and that
a sample containing 1.00g/ml of the optically
active substance, in a 1-dm tube, produces a
rotation of 3.12o in a clockwise direction
Specific rotation of enantiomers of 2-butanol
Racemic Mixture:
• an equimolar mixture of two enantiomers is c/a a
racemic mixture
• a racemic mixture is optically inactive and shows
no rotation of plane-polarized light
• it is often designated as being (±)
ex (±)-2-Butanol
Origin of optical activity:
• a beam of plane-polarized light a achiral molecule
(ex 2-propanol) in orientation (a) and then a second
molecule in the mirror-image orientation (b)
• resultant beam emerges from these two encounters
with no net rotation of its plane of polarization
• when a beam of plane-polarized light encounters
a molecule of (R)-2-butanol (chiral molecule) in
orientation (a) slight rotation of plane of
polarization results
•exact cancellation of this rotation requires that a
second mole be oriented as an exact mirror image
• this cancellation does not occur because the
only molecule that could ever be oriented as an
exact mirror image at the first encounter is a
molecule of (s)-2-butanol, which is not present
• as a result a net rotation of the plane of
polarization occurs
Biological importance of chirality
Application
• polarimetric method is a simple and accurate
means for determination of structure in micro
analysis of expensive and non-duplicable samples.
• it is employed in quality control, process control
and research in the pharmaceutical, chemical,
essential oil, flavor and food industries.
• it is so well established that the United States
Pharmacopoeia and the Food & Drug
Administration include polarimetric
specifications for numerous substances.
Research Applications
Research applications for polarimetry are found in
industry, research institutes and universities as a
means of:
• isolating and identifying unknowns, crystallized
from various solvents or separated by HPLC.
• evaluating and characterizing optically active
compounds by measuring their specific rotation and
comparing this value with the theoretical values
found in literature.
• investigating kinetic reactions by measuring
optical rotation as a function of time.
• monitoring changes in concentration of an
optically active component in a reaction mixture, as
in enzymatic cleavage.
• analyzing molecular structure by plotting optical
rotatory dispersion (ORD) curves over a wide
range of wavelengths.
• distinguishing between optical isomers.
Pharmaceutical Applications
Determines product purity by
measuring specific rotation and optical
rotation of: Amino acids, Amino
sugars, Analgesics, Antibiotics
Cocaine, Dextrose Diuretics Serums
Steroids Tranquilizers Vitamins
Utilizes polarimetry for incoming raw
materials inspection of: Camphors,
Citric acid, Glyceric acid Gums
Lavender oil, Lemon oil Orange oil
Spearmint oil
Ensures product quality by measuring the
concentration and purity of the
following compounds in sugar based
foods, cereals and syrups: Carbohydrates
Fructose Glucose Lactose Levulose
Maltose Raffinose Sucrose Various
Starches Natural monosaccharides
Analyzes optical rotation as a means of
identifying and characterizing: Natural
polymers, Biopolymers, Synthetic
polymers
Summary:
• enantiomers
• polarimeter
• origin of optical activity
•Biological importance of chirality
•Application
Next time Gas Chromatography!!!!