Stereoisomerism
Stereoisomers - Compounds that have the same
molecular formula and the same connectivity, but
different arrangement of the atoms in 3-dimensional
space. the phenomenon as stereoisomerism.
Stereoisomers cannot be converted into each other
without breaking bonds.
Classified into three types:-
1 Geometrical isomerism.
2.Optical isomerism.
3.Conformational isomerism.
OPTICAL ISOMERISM
Any substance that rotates the plane polarized light is
optically active and the phenomenon related to it is
called optical isomerism.
Optical isomers:- are molecules that have the same
structural and molecular formulae, but are non-
superimposable mirror images of each other.
Plane polarized light
Ordinary light consists of electromagnetic waves that
oscillate in all planes perpendicular to the direction in
which the light travels.
Plane-polarized light is light whose vibrations take
place in only one of these possible planes.
Ordinary light truned into plane-plarized light by
passing it through a lans made of the material know as
Polaroid (pieces of calcite- a particular crystalline
form of CaCO3. Called-Nicol prism).
Optical Activity
Optical isomers have the ability to rotate the plane
polarized light. This property is often referred as
Optical Activity.
Polarimeter.
A Polarimeter is an instrument that allows plane-polarized
light to travel through a sample tube containing an organic
compound. After the light exits the sample tube, an analyzer
slit is rotated to determine the direction of the plane of the
polarized light exiting the sample tube.
There are two possible results.
When a solution of known concentration of an optically active material is
placed in the polarimeter, the beam of light is rotated either to the right
(clockwise) or to the left (anti-clockwise).
Rotation of plane of polarized light can be of two types.
• Dextrorotatory : If the compound rotates the plane of polarization to
the right(clockwise) it is said to be dextrorotatory (Latin: dexter-right)
and Dextrorotatory is indicated by + sign, or ‘d’.
• Laevorotatory : If the compound rotates the plane of polarization to
the left(anticlockwise) it is said to be laevorotatory (Latin: laevus-left)
and Dextrorotatory is indicated by (─) minus sign is denoted by (-) or ‘l’
For example, -Morphine, is levorotatory, and (+)-sucrose is
dextrorotatory.
A compound that does not change the plane of polarized light is said to be optically inactive.
Specific Rotation
The extent of rotation depends on the number of optically active
molecules encountered by the light beam.
This number, in turn, depends on sample concentration and
sample path length.
If the concentration of sample is doubled, the observed rotation
doubles.
If the concentration is kept constant but the length of the sample
tube is doubled, the observed rotation doubles.
In addition, the angle of rotation depends on the
wavelength of the light used.
To express optical rotations in a meaningful way so that
comparisons can be made, we have to choose standard
conditions.
The specific rotation, [α]D , of a compound is defined as
“the observed rotation when light of 589.6 nanometer
(nm; 1 nm = 10-9 m) wavelength is used with a sample
path length (l) of 1 decimeter (dm; 1 dm = 10 cm) and a
sample concentration (c) of 1 g/cm3”.
When optical rotation data are expressed in this standard way, the specific
rotation, [α]D , is a physical constant characteristic of a given optically active
compound.
For example, (+)-lactic acid has [α]D = +3.82, and (─)-lactic acid has [α]D =
─3.82. That is, the two enantiomers rotate plane-polarized light to the same
extent but in opposite directions.
specific rotation is as much a property of a compound as its metling
point,boiling point, density, or refractive index.
ENANTIOMERISM
The relationship between two stereoisomers having
molecules that are mirror images of each other;
Enantiomers have identical chemical and physical
properties in an achiral environment but form different
products when reacted with other chiral molecules and
exhibit optical activity.
Enantiomers
Optically active chiral compounds that are
non-super imposable mirror image of each
other are called enantiomers.
Enantiomers
Enantiomers are isomers whose 3D-arrangment of atoms give rise to the
formation of non-superimposable mirror image.these are alaso called chiral
compound, enantiomorphs or antipodes.
A molecule with one asymmetric carbon shows two enantiomers which are
mirror image of each other.
Enantiomers are optically active and they rotate the plane polarized light
either in left or right direction.
So enantiomers have identical physical and chemical properties except the
direction of optical activity. Generally enantiomers have equal but opposite
optical rotatory power.
The enantiomer rotate plane polarized light to right direction called
dextrorotatory and optical activity denoted as “+” whereas its mirror image
rotate plane polarized light to left direction called levorotatory and optical
activity denoted as “-”.
Properties of enantiomers
The main properties of enantiomers are given as follow
Enantiomers always exist in pair. Enantiomers are non-super imposable
mirror image to each other.
Enantiomers have same physical properties (like boiling point, melting
point, solubility, density, viscosity, refractive index etc.)and chemical
properties in achiral environment
Each enantiomers have opposite behavior with respect to plane polarized
light,
if one of them will rotate the plane polarized light towards right hand
direction then definitely the other will rotate the plane polarized light
towards left hand direction.
Each enantiomers shows the same chemical reactivity with achiral
reagent; however they have different reactivity with chiral reagent.
Example
Glyceraldehyde molecule is a chiral molecule.
It has a pair of enantiomers with same
physical properties except their behavior
towards plane polarized light
DIASTEREOMERS:
Diastereomers are those stereoisomers that are not mirror
image of each other.
Diastereomers are non-enantiomeric stereoisomers with
two or more stereo centers.
The pair of stereoisomer that differs in the arrangement of
atoms/groups bonded with at least one stereo centre is
called diastereomers.
categories
Diastereomers can be divided into two categories:
(a) Configurational Diastereomers: A relationship
between stereoisomers of a chiral molecule, but ones that
are not enantiomers.
(b) Cis-Trans Diastereomers: These are stereoisomers,
that usually arise due to restricted rotation within a
molecule; commonly at a carbon-carbon double bond.
Configurational Diastereomers
Configurational diastereoisomerism occurs in molecules
that have more than one chiral center.
Example
Example D-Galactose, D-Glucose and D-
Mannose are the non-mirror image
stereoisomer of each other. Therefore are
called diastereomers.
PROPERTIES OF DIASTEREOMERS
The main properties of diastereomers are given as follows:
All the stereoisomers except enantiomers are diastereomers.
Diastereomers have different physical properties like boiling
point, melting point, density, solubility, density, viscosity,
refractive index etc.
Diastereomers have different chemical properties like rates of
reactions, reactivity even in achiral reaction medium.
This difference in physical and chemical properties of
diastereomers is very useful in the separation of enantiomers
from their mixture.
ELEMENTS OF SYMMETRY
All optically active molecules/object are chiral and they
exhibit enantiomerism (Figure ).
A chiral molecule is that which cannot be superimposed
on its mirror image; however, both the non-super
imposable isomers are called enantiomers.
Fig:-Non super imposable mirror image relationship of right and left hands. (b) Ball
and stick model of tetravalent chiral carbon atom.
Elements of symmetry are a simple tool to
identify whether a molecule is chiral or not.
The necessary condition for optically active
molecule to be chiral is that, the molecule
should not possess any kind of symmetry
elements.
The elements of symmetry are generally
categorized as follows:
(i) Simple axis of symmetry (Cn) (ii) Plane of
symmetry (σ) (iii) Centre of symmetry (Ci) (iv)
Alternating axis of symmetry (Sn)
CHIRAL AND ACHIRAL MOLECULES
Mirror image and the property of non-
superimposability is called chirality. mirror image is
called achiral (non-dissymmetric or unsymmetrical).
or groups is called an asymmetric carbon atom or a
chiral atom. A chiral atom is indicated by an
asterisk (*).
A chiral molecule is a type of molecule that has a
non-superposable mirror image. The feature that
is most often the cause of chirality in molecules is
the presence of an asymmetric carbon atom. The
term "chiral" in general is used to describe the
object that is non-superposable on its mirror image.
