Plane polarized light: Understanding optical isomerism

Light that has been passed through a nicol prism or other
polarizing medium so that all of the vibrations are in the same
plane.
Plane polarized light
non-polarized polarized

OPTICAL ISOMERISM
The polarimeter
If the light appears to have turned to the right turned to the left
DEXTROROTATORY LAEVOROTATORY
A Light source produces light vibrating in all directions
B Polarising filter only allows through light vibrating in one direction
C Plane polarised light passes through sample
D If substance is optically active it rotates the plane polarised light
E Analysing filter is turned so that light reaches a maximum
F Direction of rotation is measured coming towards the observer
A B
C D
E
F

Polarization is a property of certain types of waves that describes the orientation of
their oscillations. Electromagnetic waves such as light exhibit polarization; acoustic
waves (sound waves) in a gas or liquid do not have polarization because the direction
of vibration and direction of propagation are the same.
Plane polarised light: A polarized light vibrating in a single plane perpendicular to the
direction of propagation.
Polarimeter: A polarimeter is an instrument used to measure the angle of rotation
caused by passing polarized light through an optically active substance.
The property of a substance of rotating the plane of polarized light is called Optical
activity and the substance possessing it is said to be Optically active.

The observed rotation of the plane of polarized light (determined with the help of
polarimeter) produced by a solution depends on (a) the amount of the substance in
tube (b) on the length of the solution examined ;
(c) the temperature of the experiment; and (d) the wavelength of the light used.
Specific rotation: It is defined as the number of degrees of rotation observed
when light passed through 1decimetre (10 centimetres) is of its solution having
concentration 1 gram per millilitre.
The specific rotation of a given substance can be calculated by the following
expression
Where [α]tºD a stands for specific rotation determined at tºC and using
D-line of sodium light; αabc is the observed angle of rotation; l is the length of the
solution in decimeters ; and c is the concentration of the active compound in grams
per milliliter.

Optical Isomerism
Definition
When two molecules only differ by the three-dimensional
position of the substituents around one or more atoms, they are
called optical isomers and this phenomenon is called optical
isomerism.
C
Br
H
Cl
CH3
C
Br
H
H3C
Cl
* *
The positions of Cl and CH3 around the
carbon differs in the two molecules
C
H
Br
Cl
C
F
H3C
C
H
Cl
Br
C
CH3
F
H
H
*2
*1 *1
*2
The positions of Cl and Br differ around carbon 1
and the positions of CH3 and F differ around
carbon 2 in the two molecules

Chirality
• The term ‘Chiral’ and therefore the term ‘Chirality’ comes
from a Greek word Kheir which means hands.
• An object is called chiral when its mirror image is
non-superimposable on the original and this phenomenon is
called chirality.
• If the mirror image is superimposable then the object is
called achiral.
Optical Isomerism Contd.

Chiral center
• In chemistry, an atom which is attached to non-identical
substituents and the mirror image is non-superimposable is
called a chiral center.
C
Br
H
Cl
CH3
*
C2H5
N
H
CH3
is this a chiral center?
C3H7
P
C6H5
CH3
is this a chiral center?Technically, for an atom attached to
non-identical substituents, the mirror image
should be non-superimposable.
But if the mirror image is not stable enough,
then practically that atom will not be
considered as a chiral center.

Tetrahedral center
• In chemistry, an atom which is attached to four substituents
is called a tetrahedral center.
• Most commonly, carbons show tetrahedral centers.
Chiral carbon
• A carbon which is attached to four different substituents is
called a chiral carbon.
C
Br
Cl H
CH3
C
Br
Cl H
H
a tetrahedral carbon
but not a chiral carbon
a tetrahedral carbon
and a chiral carbon

Elements of symmetry
• Any point, line or plane which divides an object into two equal parts
is referred to as an element of symmetry.
Plane of symmetry
• The imaginary plane which divides an object into two equal parts is
called a plane of symmetry.
• In chemistry, the plane of symmetry is an imaginary plane which
divides a molecule into two parts which are mirror image of each
other.
HO
COOH
H
HO H
COOH
Plane of symmetry

Conditions for optical isomerism
• Following conditions must be met if a molecule is to have
optical isomers:
– The molecule must have at least one chiral carbon.
– There should not be any elements of symmetry (specifically plane
of symmetry).
– The mirror image of the molecule must not be suporimposable
on the original.
• So, what about the following structures….
Cl
OH
H3C C
Cl
Br
CH3H3C C
Cl
Br
C2H5

HO OH
HO OH
H3C C C
H
OH
Cl
Br
C
H
OH
CH3
H3C C C
OH
H
Cl
Br
C
H
OH
CH3 CHO
COOH
H
CHO H
COOH
CH
COOH
OH
CHO H
COOH
CH
COOH
OH
CH OH
COOH
CHO
COOH
H
CH OH
COOH

Meso compounds
• The compounds which have the following criteria are
called meso compounds:
– They have one or more chiral carbons.
– There is a plane of symmetry.
– The mirror image of the molecule is superimposable on the
original.
HO
COOH
H
HO H
COOH
A meso compound
H
COOH
OH
H OH
COOH
H
COOH
HO
HHO
COOH
The mirror image
molecule
180o
rotated mirror
image molecule

Wedge and dash representation
• Wedge and dash projection is a method to represent the three-
dimensional (3D) structure of a molecule.
• In this method, three types of lines are used to denote bonds:
– Solid lines: Represent atoms/groups in the same plane (the paper).
– Wedged lines: Represent atoms/groups which are coming out of the
plane, towards the viewer.
– Dashed lines: Represent atoms/groups which are extending away from
the plane, away from the viewer.
CH3
Cl Br
H
CH3
Br
H
Cl

Fischer projection
• Fischer projection is an attempt to depict
three-dimensional molecules in two-dimensional paper.
• According to this method, the groups bonded by horizontal
bonds are coming towards the viewer and the groups
bonded by vertical bonds are going away from the viewer.
• In this projection, the longest chain is drawn vertically with
C1 at the top.
C
Br
H F
CH3
C
Br
H F
CH3
Br
H3C
H
F

• Types of optical isomerism
Optical
isomers
Enantiomer
Dextrorotatory Levorotatory
Diastereomers
Erythro Threo

Enantiomer
• Enantiomers are those optical isomers which are mirror
images of each other.
• Since there can only be one mirror image, there will
always be two and only two molecules which are
enantiomers of each other.
• These two enantiomers differ in one property - optical
activity. Based on optical activity, the enantiomers are
divided into:
– Dextrorotatory enantiomer: This is the enantiomer which
rotates the plane of plane-polarized light to the right.
– Levorotatory enantiomer: This is the enantiomer which
rotates the plane of plane-polarized light to the left.

• These two compounds fulfill
the conditions for optical
isomerism.
• They are also mirror images of
each other.
• Hence they are enantiomers.
CH3
Br H
Cl
CH3
H Br
Cl
CH3
Br H
C
CH3
H Br
C
F
H Br H Br
F
• So… What about these two
structures?
• Are they optical isomers?
• Are they enantiomers?
• So, can you tell me among the two structures on the left
side, which is dextrorotatory and which is levorotatory?
• Can you tell me, which is R isomer and which is S isomer?

Diastereomers
• Optical isomers which are not enantiomers are
diastereomers.
• Meaning, two optical isomers which are not mirror images of
each other are diastereomers.
• For diastereomers to exist, there must be at least two chiral
carbons in the structure.
• In diastereomers, the configuration of at least one chiral
carbon will be same.
• Diastereomers differ in many physical and chemical
properties.
• Two terms ‘erythro’ and ‘threo’ are associated with
diastereomers. Another two terms commonly used are ‘syn’
and ‘anti’.

CH3
CBr H
C
CH3
CH Br
C
F
H Br H Br
F
• These two compounds fulfill
the conditions for optical
isomerism.
• But they are not mirror
images of each other.
• Hence they are not
enantiomers, they are
diastereomers.
CH3
CBr H
C
CH3
CH Br
C
F
H Br H Br
F
Erythro: When the identical groups on
adjacent chiral carbons are on the
same side, the diastereomer is called
ERYTHRO.
Threo: When the identical groups on
adjacent chiral carbons are on the
opposite sides, the diastereomer is
called THREO.

Racemic mixture
• A racemic mixture is one in which two enantiomers are present in
the same amount.
• Since each enantiomer rotates the plane of the plane-polarized light
by the same degree but in opposite direction, there is no net rotation
in racemic mixture.
• Many optically active compounds exist as racemic mixture. e.g.
thalidomide, tartaric acid etcetera.
• Racemic mixtures are denoted by symbols like (±) or dl-. e.g. (±
tartaric acid).
CH
COOH
OH
CHO H
COOH
L-Tartaric acid
(50%)
CHO
COOH
H
CH OH
COOH
D-Tartaric acid
(50%)

• Physical properties of meso compounds, racemic mixture
and enantiomers
• The chemical properties of enantiomers, meso compounds
and racemic mixtures do not vary at all. However the physical
properties can vary.
• This is shown with tartaric acid below:
Compound Melting point (°C)
Optical
rotation [α]D
(degree)
Density
(g/mL)
Solubility at 20
°C (g/100 mL
H2O)
(+)-Tartaric acid 168 - 170 +12 1.7598 139.0
(-)-Tartaric acid 168 - 170 -12 1.7598 139.0
meso-Tartaric acid 146 - 148 0 1.6660 125.0
Racemate of tartaric acid 206 0 1.7880 20.6

Representation of optical isomerism
• In general optical isomerism is represented
based on two criteria:
• Based on optical activity
– d/l method (old).
– (+)/(-) method (modern).
• Based on configuration around chiral carbon.
– D/L method (limited use).
– R/S method (universal).

Optical isomers based on optical activity
• Based on the ability to rotate the plane of the
plane-polarized light, optical isomers are divided
into two types.
– Dextrorotatory: Rotates the plane to the right. It is
denoted by d- or (+).
– Levorotatory: Rotates the plane to the left. It is denoted by
l- or (-).

CH
COOH
OH
CHO H
COOH
This compound is denoted
()-tartaric acid because it's
specific optical rotation is 12o
On the other hand, it is denoted
L-tartaric acid because the OH group
on the carbon before terminal is on the left,
it has nothing to do with optical rotation
CHO
COOH
H
CH OH
COOH
This compound is denoted
()-tartaric acid because it's
specific optical rotation is 12o
On the other hand, it is denoted
D-tartaric acid because the OH group
on the carbon before terminal is on the right,
it has nothing to do with optical rotation
d/l or (+)/(-) denotation is placed on a compound after its optical
rotation is measured with a polarimeter. D/L or R/S denotion has
nothing to do with it.

D/L configuration
• D and L method is used to describe the position of the
atoms/groups around the chiral carbon. It doesn’t tell whether
the compound is dextrorotatory or levorotatory.
• This method was proposed by Rosanoff in 1906.
• This method uses the two enantiomers of Glyceraldehyde as
reference molecules.
• Any compound which looks like or degrades to D-glyceraldehyde
would be denoted by D- and any compound which looks like or
degrades to L-glceraldehyde would be denoted by L-.
CHO
C
CH2OH
HO H
This enantiomer is dextrorotatory,
Rosanoff designated this molecule
as D-glyceraldehyde
CHO
C
CH2OH
H OH
This enantiomer is levorotatory,
Rosanoff designated this molecule
as L-glyceraldehyde

D/L naming method
• It can be applied to compounds which are similar to
glyceraldehyde or degrades to glyceraldehyde.
• This method is applied to:
– Carbohydrates
– Derivative of carbohydrates (e.g. some carboxylic acids, aldehydes)
– Amino acids
• For this method, first Fischer projection of the compound
must be drawn.
• For carbohydrates and its derivatives, the position of the
OH group on the highest numbered chiral carbon is
looked at. If the OH group is on the left it is termed L-
and if it on the right then it is termed D-.

CHO
C
CH2OH
HO H
CHO
C
CH2OH
H OH
L-glyceraldehyde D-glyceraldehyde
CHO
2
C
1
C
3
C
4
C
5
CH2OH
6
HO H
H OH
HO H
HO H
L-glucose
CHO
2
C
1
C
3
C
4
C
5
CH2OH
6
H OH
HO H
H OH
H OH
D-glucose
CH2OH
2
C
1
C
3
C
4
C
5
CH2OH
6
HO H
H OH
HO H
HO H
CH2OH
2
C
1
C
3
C
4
C
5
CH2OH
6
H OH
HO H
H OH
H OH
D-sorbitolL-sorbitol
CH2OH
2
C
1
C
3
C
4
C
5
COOH
6
HO H
H OH
HO H
HO H
CH2OH
2
C
1
C
3
C
4
C
5
COOH
6
H OH
HO H
H OH
H OH
D-glucoronic acidL-glucoronic acid

L-lactic acid
CH3
C
COOH
HO H
CH3
C
COOH
H OH
D-lactic acid
L-erythrose D-erythrose
CHO
C
C
CH2OH
H OH
H OH
CHO
C
C
CH2OH
HO H
HO H
C
C
C
C
C
CH2OH
HO H
H OH
HO H
HO H
L-heptoglucose
C
C
C
C
C
CH2OH
H OH
HO H
H OH
H OH
D-heptoglucose
CHO CHO
HO H H OH

R/S configuration
• D/L method of expressing chiral carbon
configuration works on only a few types of
compounds.
• To express the configuration of chiral carbons in
other compounds, we need another method.
• This other method is the R/S method. This method
is universal, meaning that this method works on
any compound.
• In R/S method, the configuration of each chiral
carbon of the compound is described.

R/S naming method
• First, every chiral carbons in the molecule are identified.
• Then the configuration in each chiral carbon is
determined.
• To determine the configuration, the groups attached to the
chiral carbons are assigned priority 1, 2, 3, and 4
according to Cahn-Ingold-Prelog (CIP) rules.
• The group with priority 4 (lowest priority) is sent to the
back. Then it is identified which direction follows if one
goes from 1 → 2 → 3.
• If the direction is right (clockwise), the chiral carbon is at R
(R = rectus, meaning right) configuration.
• If the direction is left (anticlockwise), the chiral carbon is
at S (S = sinister, meaning left) configuration.

CIP rules with examples
• The group whose first atom (atom connected to the
chiral carbon) has highest atomic number is given
priority 1 and so on.
Br C
F
I
H
Priority 3
(atomic number = 9)
Priority 1
(atomic number = 53)
Priority 2
(atomic number = 35)
Priority 4
(atomic number = 1)
Anticlockwise direction, steering wheel to the right
So, it is at R configuration
R-Bromo-fluoro-iodo-methane

• If first atoms are identical, then second atom will be
looked at. If the second atoms are also identical, third
atom will be looked at and so on.
• If the first atoms are identical, second atoms are also
identical, then the group with greater number of high
atomic number second atoms is given higher priority.
The number of the chiral
carbon is written before
the configuration is
written
NC C
CH2OH
C2H5
CH3
Priority 1
Priority 2
Priority 3
Priority 4
(2S)-2-Hydroxymethyl-2-methyl-butyronitrile

• If there is any double or triple bond, then it is considered as
two single bonds or three single bonds respectively.
NC C
CH2OH
CHCl2
CH2Cl
Priority 1
Priority 2 Priority 3
Priority 4
(2S)-3,3-Dichloro-2-chloromethyl-2-hydroxymethyl-propionitrile
HOOC C
CH2OH
CHCl2
CH2Cl
Priority 2
Priority 1
Priority 3
Priority 4
(2R)-3,3-Dichloro-2-chloromethyl-2-hydroxymethyl-propionic acid

• It is important to note however that Fischer projection is not always
reliable, and one should convert the Fischer projection into wedge
and dash projection.
C
Br
H F
CH3
C
Br
H F
CH3
Br
H3C
H
F
If the configuration is determined from Fischer projection,
then this compound is (S)-1-Bromo-1-fluoro-ethane
But actually the configuration is R
C
Br
H F
CH3
CH3
Br
F
H
Now the configuration is R
Br
H3C
H
FWhen is looked with the H (4th priority)
away from the viewer, it looks like
Fischer projection in wedge and dash projection looks like following

A Simple trick
• If the lowest priority group (priority 4 group) is bonded by vertical bonds,
then we can use the Fischer projection to determine R/S configuration
directly.
• If the lowest priority group is bonded by horizontal group, then
determine the R/S configuration directly. The correct configuration is
the opposite of the configuration determined.
Br C
F
I
Cl
Lowest priority group is vertically bonded,
just figure out the configuration
Br
C F
I
Cl
Lowest priority group is vertically bonded,
figure out the configuration. The opposite of
that configuration is the correct one
S configuration From Fischer projection: S configuration
Actual: R configuration

• Find the configuration of following structures
H3C C
CH2OH
OH
C
Br
Cl
CH3
(2R, 3S)-3-Bromo-3-chloro-2-methyl-butane-1,2-diol
Cl
CHOOC CH3
H
(2S)-2-Chloro-propionic acid
COOH
CH OH
CH3
(R)-Lactic acid
CHO
C
C
C
C
CH2OH
H OH
HO H
H OH
H OH
(2R, 3S, 4R, 5R)-Pentahydroxyhexanal
CHO
C
C
C
C
CH2OH
HO H
H OH
HO H
HO H
(2S, 3R, 4S, 5S)-Pentahydroxyhexanal

The stereoisomers of
aldohexoses• Monosaccharides which contain six carbons and a aldehyde group are called
aldohexoses.
• Aldohexose contains 4 chiral carbons, so a total of 24=16 stereoisomers are
there.
CHO
C
C
C
C
CH2OH
HO H
H OH
HO H
HO H
CHO
C
C
C
C
CH2OH
H OH
HO H
H OH
H OH
CHO
C
C
C
C
CH2OH
H OH
H OH
H OH
H OH
CHO
C
C
C
C
CH2OH
HO H
HO H
HO H
HO H
CHO
C
C
C
C
CH2OH
HO H
H OH
H OH
H OH
CHO
C
C
C
C
CH2OH
H OH
HO H
HO H
HO H
CHO
C
C
C
C
CH2OH
H OH
H OH
HO H
HO H
CHO
C
C
C
C
CH2OH
HO H
HO H
H OH
H OH
CHO
C
C
C
C
CH2OH
HO H
HO H
H OH
HO H
CHO
C
C
C
C
CH2OH
H OH
H OH
HO H
H OH
CHO
C
C
C
C
CH2OH
HO H
H OH
HO H
H OH
CHO
C
C
C
C
CH2OH
H OH
HO H
H OH
HO H
CHO
C
C
C
C
CH2OH
HO H
H OH
H OH
HO H
CHO
C
C
C
C
CH2OH
H OH
HO H
HO H
H OH
CHO
C
C
C
C
CH2OH
H OH
H OH
H OH
HO H
CHO
C
C
C
C
CH2OH
HO H
HO H
HO H
H OH
D-glucose
(+53o
)
L-glucose
(-53o
)
D-mannose
(+14o
)
L-mannose
(-14o
)
D-allose
(+14o
)
L-allose
(-14o
)
D-altrose
(+33o
)
L-altrose
(-33o
)
D-gulose
(-20o
)
L-gulose
(+20o
)
D-iodose
(+15o
)
L-iodose
(-15o
)
D-galactose
(+80o
)
L-galactose
(-80o
)
D-talose
(+21o
)
L-talose
(-21o
)

The stereoisomers of
aldohexoses Contd.
CHO
C
C
C
C
CH2OH
HO H
H OH
HO H
HO H
CHO
C
C
C
C
CH2OH
H OH
HO H
H OH
H OH
CHO
C
C
C
C
CH2OH
H OH
H OH
H OH
H OH
CHO
C
C
C
C
CH2OH
HO H
HO H
HO H
HO H
CHO
C
C
C
C
CH2OH
HO H
H OH
H OH
H OH
CHO
C
C
C
C
CH2OH
H OH
HO H
HO H
HO H
CHO
C
C
C
C
CH2OH
H OH
H OH
HO H
HO H
CHO
C
C
C
C
CH2OH
HO H
HO H
H OH
H OH
CHO
C
C
C
C
CH2OH
HO H
HO H
H OH
HO H
CHO
C
C
C
C
CH2OH
H OH
H OH
HO H
H OH
CHO
C
C
C
C
CH2OH
HO H
H OH
HO H
H OH
CHO
C
C
C
C
CH2OH
H OH
HO H
H OH
HO H
CHO
C
C
C
C
CH2OH
HO H
H OH
H OH
HO H
CHO
C
C
C
C
CH2OH
H OH
HO H
HO H
H OH
CHO
C
C
C
C
CH2OH
H OH
H OH
H OH
HO H
CHO
C
C
C
C
CH2OH
HO H
HO H
HO H
H OH
D-glucose
(+53o
)
L-glucose
(-53o
)
D-mannose
(+14o
)
L-mannose
(-14o
)
D-allose
(+14o
)
L-allose
(-14o
)
D-altrose
(+33o
)
L-altrose
(-33o
)
D-gulose
(-20o
)
L-gulose
(+20o
)
D-iodose
(+15o
)
L-iodose
(-15o
)
D-galactose
(+80o
)
L-galactose
(-80o
)
D-talose
(+21o
)
L-talose
(-21o
)

Plane polarized light: Understanding optical isomerism

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