STEREOISOMERISM IN DETAIL

1. CONSTITUIONAL ISOMERS
ENANTIOMERS
STEREOISOMERS
DIASTEREOMERS
CIS/TRANS ISOMERS
CONFORMERS/ROTAMERS
STEREOISOMERISM

2. CONTENT:
◦ Introduction to Stereoisomerism
◦ Classification of stereoisomers
◦ Enantiomers
◦ Optical activity and Polarimeter
◦ Dextrorotatory and Levorotatory
◦ Diastereomers
◦ Types of diastereomers
◦ Geometric Isomers
i.Cis isomers
ii.Trans isomers
◦ Conformational isomers
i. Eclipsed conformer
ii. Staggered conformers
◦ Interesting facts

3. Introduction to Stereoisomerism:
◦ Stereoisomerism is a form of isomerism in which molecules have the same molecular formula and
sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their
atoms in space.
◦ The molecules that differ in three dimensions of their space having same molecular formula and
sequence of bonded atoms are called stereoisomers.

4. Classification:
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5. Enantiomers:
◦ Enantiomers are chiral molecules(isomers) that are mirror images of one
another and the molecules are non-superimposable on one another.
◦ Non-superimposable means that the molecules cannot be placed on top
of one another and give the same molecule.
◦ They have identical physical properties but differ in direction of rotation of
plane polarized.
◦ The mixture of equal quantities of enantiomers forms the optically inactive
compound called as racemic mixture.
◦ Examples:

6. Optical activity and Polarimeter:
◦ Optical activity is the ability of a chiral molecule to rotate a plane of plane
polarized light.
◦ Polarimeter is scientific instrument used to measure the optical activity specifically
the specific angle of rotation.
◦ Polarimeter consists of a light source, polarizing lens, sample tube and analyzing
lens.

7. Dextrorotatory and Leuvorotatory:
◦ On the basis of rotation of plane polarized, pair of enantiomers consists
i. dextrorotatory(+)
ii. levorotatory(-)
 If a compound rotates plane polarized light in the clockwise (+) direction, it is said to be
dextrorotatory.
 If compound rotates light in the counterclockwise (-) direction it is levorotatory.
It is determined by only use of polarimeters.

8. Diastereomers:
◦ Stereoisomers of the substance that are not mirror of each other are termed as
diastereomers.
◦ They have different physical properties like m.p, b.p, solubility,density etc.
◦ Diastereomers have different specific rotation but they may have same or opposite signs.

9. Types of diastereomers:
◦ Generally,diastereomers have two types;
i. Geometric Isomers
ii. Conformational isomers
Diastereomerism can occur at a double bond(geometric isomerism),
where the cis vs trans relative positions of substituents give two non-
superposable isomers.
Many conformational isomers are diastereomers as well.

10. Geometric Isomers:
◦ Stereoisomers, that is, pairs of molecules which have the same formula
but whose functional groups are rotated into a different orientation in
three-dimensional space are called geometric isomers.
◦ Geometric isomers occurs in alkenes and cycloalkanes.
◦ Geometrical isomerism is due to the restricted or hindered rotation
around the carbon-carbon double bond. Due to the hindered rotation
around carbon-carbon double bond, the relative positions of atoms or
group attached to the doubly bonded carbon atoms get fixed.

11. ◦ Cis isomer-have same substituents on the same side of the double
bond[Z-zusammen (together) with more complex molecules having high
priority groups on the same side).
◦ Trans isomer-have same substituents on the opposite side of the double
bond[E-entgegen(opposite)with more complex molecules having high
priority groups on opposite sides].

12. Conformational isomers:
◦ Conformational isomerism is a form of stereoisomerism in which the isomers can be
interconverted just by rotations about formally single bonds.
◦ Conformation-number of possible spatial arrangements of atoms in a molecule that result from
rotation of its constituent groups of atoms about single bonds.
◦ Conformer- a specific rotation
i. rotation about single bonds
ii. Amine inversion

13. ◦ Eclipsed conformer- in which atoms or groups are as close as possible to
the atoms or groups on an adjacent carbon in a single bond.
◦ Staggered conformer- in which atoms or groups are as faraway as
possible to the atoms or groups on an adjacent carbon in a single bond.
◦ Since eclipsed conformation has a greater potential energy due to
strain, it is less stable than the staggered conformation.

14. Interesting facts:
◦ Stereoisomers are critically important in biochemistry and medicine
because nearly every biological molecule - amino acids, sugars, fats,
enzymes, etc - has one or more stereoisomer.
◦ Our ability to taste and smell is regulated by chiral molecules in our
mouths and noses that act as receptors to "sense" foreign substances.
◦ The two enantiomers of the amino acid, leucine, for example, have
different tastes - one is bitter, whereas the other is sweet.
◦ Enantiomers also can smell different, as is known from the odors of the
two carvones. One has the odor of caraway and the other of spearmint.

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