This is for UG students. In this unit concept of stereochemistry is explain in easy way. The content are shown below: -Stereochemistry -Isomerism and their classification -stereochemistry and their classification -Geometrical Isomerism -Optical isomerism -Confirmational Isomerism

1. STEREOCHEMISTRY
Mr. Nilkesh K. Dhurve
Assistant Professor
Department of Chemistry
Shri Pundlik Maharaj Mahavidyalaya Nandura
Dist.-Buldana
E-mail: nilkamaldhurve94@gmail.com

2. CONTENTS OF STEROCHEMISTRY
Optical isomerism
Geometrical isomerism
Conformational isomerism

4. Hands, like many objects in the world
around us, are mirror images that are not
identical.
Other molecules are like socks. Two socks
from a pair are mirror images that are
superimposable. A sock and its mirror
image are identical.

5. Due to these differences in stereochemistry, humans can metabolize starch for energy
but we cannot digest cellulose.
Essential for plants and animals Toxic to tissue

6. STEROCHEMISTRY
Stereochemistry is about the three-
dimensional shape of molecules and how
these affect or control their reactivity.
Or
Stereochemistry is the branch of chemistry
that involves “the study of the different
spatial arrangements of atoms in
molecules”.

7. Isomerism
The compound having same molecular formula but different physical or
chemical properties are called as isomers of each other and the phenomenon is called as
isomerism.
(Greek word: isos = equal and meros = parts)
Stereoisomers
• Different IUPAC names
• Same or different functional groups
• Different physical properties
• Different chemical properties
• Identical IUPAC names (except for a prefix
like cis or trans)
• Always have the same functional group(s).
• Differ in configuration(three-dimensional
arrangement).

9. Constitutional Isomerism

10. Structural/Constitutional Isomerism
The compounds having the same molecular formula, but different structural
formulas are called as Structural or Constitutional isomers of each other and the
phenomenon is called as Structural or Constitutional Isomerism.

11. Functional Isomerism
The compounds that have the same chemical formula but different functional
groups attached to them. Such compounds is called as functional isomers and the
phenomenon is known as Functional Isomerism.

12. Positional Isomerism
The compounds that have the same chemical formula but the positions of the
functional groups or substituent atoms are different. Such compounds is called as
positional isomers and the phenomenon is known as Positional Isomerism.

13. Chain/Nuclear Isomerism
The compounds that have the same chemical formula but differ in the
branching of carbon chain. Such compounds is called as chain isomers and the
phenomenon is known as Chain Isomerism.

14. Metamerism Isomerism
Isomers differing in distribution of alkyl groups around a central atom are
called as Metamerism.
This type of isomerism arises due to the presence of different alkyl chains on
each side of the functional group.

15. Tautomerism Isomerism
A tautomer of a compound refers to the isomer of the compound which only
differs in the position of protons and electrons.
Typically, the tautomers of a compound exist together in equilibrium and easily
interchange.
It occurs via an intramolecular proton transfer.
An important example of this phenomenon is Keto-enol tautomerism.

16. Stereoisomerism

17. Stereoisomerism
Two or more different compounds having (with) the same molecular formula,
but different structural arrangement of their atoms or group of atoms in the three
dimensions, 3-D’s (in space) are called as stereoisomers and this phenomenon is
known as stereoisomerism.
Or
Isomers having (with) the molecular formula, but different spatial arrangements
of atoms or group of atoms in space are called as stereoisomers and this phenomenon is
known as stereoisomerism.

18. Stereoisomerism

19. Drawing Conventions in Stereochemistry
Don’t confuse dashed lines with
the dotted line (-------),which we
will use for partially made or
partially broken bonds in
transition states.
Correct Way Incorrect Way

20. Drawing Conventions in Stereochemistry

21. Drawing Conventions in Stereochemistry
Generally used for saturated chains.
Enantiomers of 2-butanol

22. Drawing Conventions in Stereochemistry
Newman Projection Formula

23. Drawing Conventions in Stereochemistry
Fischer Projection Formula
• The main carbon chain must be in the
vertical plane and pointing away from the
observer
• The other two substituents must be in the
horizontal plane and pointing towards the
observer.

24. Drawing Conventions in Stereochemistry

25. Drawing Conventions in Stereochemistry

26. Optical Isomerism

27. Optical Isomerism
The isomers that can rotate the plane of plane polarized light are called as
optical isomer and the phenomenon is known as Optical Isomerism.
Plane Polarized Light
A beam of light consists of electromagnetic waves oscillating in an infinite
number of planes. When a light beam passes through a polarizing filter, it is converted
to plane-polarized light whose electromagnetic waves oscillate in a single plane.

28. Plane-polarized light interacts with chiral molecules. This interaction can be measured
by an instrument called a polarimeter.
Polarimeter
Schematic Diagram of a Polarimeter

29. Optical Activity
The property of compound to rotate plane of plane polarized light in clockwise
or anticlockwise directions is called as optical activity.

30. Specific Rotation
The optical activity of a pure chiral substance is reported as its specific rotation,
symbolized by []D.
It is the number of degrees of rotation of a solution at a concentration measured in
gmL–1 in a tube 1 dm (10 cm) long.
where α = observed rotation
l = length
c = concentration
For example, one of the enantiomers of 2-iodobutane
has [a]D = –15.15. It is called (–)-2-iodobutane. The
other enantiomer is (+)-2-iodobutane, [a]D = +15.15.

31. Dextro-rotatory
• The optical isomer which rotate the plane polarized light in clockwise direction or to
right direction are called as dextro-rotatory compounds.
• In nomenclature, use either ‘d’ or ‘+’ prefix. (Do not confused with ‘D’)
Levo-rotatory
• The optical isomer which rotate the plane polarized light in anti-clockwise direction
or to left direction are called as levo-rotatory compounds.
• In nomenclature, use either ‘l’ or ‘-’ prefix. (Do not confused with ‘L’)

32. Asymmetric Carbon Atom
An asymmetric carbon atom (chiral
carbon/chiral center/stereogenic center) is a carbon
atom that is attached to four different types of atoms or
groups of atoms.
• It is represented by asterisk (*) .
• Le Bel-van't Hoff rule states that the number of
stereoisomers of an organic compound is 2n, where
n represents the number of asymmetric carbon
atoms.

33. Asymmetric Carbon Atom

34. Chiral Molecules
• An object that is not superimposable on its
mirror image is chiral (Greek chiron, hand).
• In each pair, two molecules will rotate the
plane of plane polarized light equally but in
opposite direction(racemic mixture).
• A chiral object cannot have a plane or centre
of symmetry.

35. Achiral Molecules
• An object that can be superimposed on
its mirror image is achiral.
• None of these molecules will rotate the
plane of plane polarized light.
• Achiral object have a plane or center of
symmetry.

36. Symmetry Elements
A)Plane of Symmetry(Imaginary Plane)
Bromochlorofluoromethane
does not have a plane of
symmetry. Therefore, it is
chiral, and it exists as a pair
of non-superimposable
mirror image isomers.
Bromochloromethane and
Dichloromethane have a
plane of symmetry, and
therefore, it can be
superimposed on its mirror
image. It is achiral.
The presence or absence of a plane of symmetry tells us whether an object is chiral or achiral.

37. Symmetry Elements
B)Center of Symmetry/ Centre of inversion(Imaginary Point)
It is imaginary point in the centre of molecule, such
that, straight line are drawn through this point meet identical
atoms or group of atoms at the same distance from the
centre, such a compound is said to have centre of
symmetry & it is optically inactive (achiral).

38. Symmetry Elements
B)Axis of Symmetry 1)n-Fold Simple Axis of Symmetry(Proper Axis of Symmetry)
Rotation through 3600/2 = 1800 about the axis leads to an arrangement identical to the original.
Therefore, it possesses two fold axis of symmetry.
Rotation through 3600/3 = 1200
about the axis leads to an
arrangement identical to the
original. Therefore, it possesses
three fold axis of symmetry.

39. Symmetry Elements
B)Axis of Symmetry 2)n-Fold Alternating Axis of Symmetry(Improper Axis of Symmetry)
Rotation --------> Reflection

40. Enantiomers/Enantiomorphs
• Two stereoisomers related as nonsuperimposable
mirror images are called enantiomers (Greek
enantios,opposite + meros, part).
• A substance is chiral.
• The Chiral compounds (stereoisomers) having
nonsuperimposable mirror image relationship are
called enantiomers or enantiomorphs and this
phenomenon is called as enantiomerism.

41. Diastereomers/Diastereomorphs
• Two stereoisomers that are not mirror images of
each other are called as Diastereomers or
Diastereomorphs.
• This phenomenon is known as diastereomerism.

42. Configuration

43. Configuration
The special (specific or particular or definite) three dimensional arrangements of
atoms and group of atoms around the asymmetric C-atom that characterizes a particular
stereoisomer (special structures) are called as configuration.
• Configuration can be changed only by breaking and making of bonds.
• Configuration of isomers has independence existence & hence it can be separated.
A. Absolute Configuration (R/S)
B. Relative Configuration (D/L)

44. An absolute configuration refers to the spatial arrangement of the atoms of
a chiral molecular entity (or group) and its stereochemical description. e.g. R or S,
referring to Rectus, or Sinister, respectively.
• Absolute configurations for a chiral molecule (in pure form) are most often obtained
by X-ray crystallography.
Absolute Configuration

45. Relative Configuration
Relative configuration: The position of atoms or groups in space in relation to
(i.e., relative to) something else in the molecule. Compare with absolute configuration,
which is independent of atoms or groups elsewhere in the molecule.
• E/Z configuration
• Cis/trans configuration
• D/L configuration
Cis-2-pentene Trans-2-pentene
(Z)-2-pentene (E)-2-pentene

46. D-L Configuration
The D/L system (named after Latin dexter and laevus, right and left)
• D and L system was used to specify the
configuration at the asymmetric carbon atom.
• In general, the absolute configuration of a
substituent (S) at the asymmetric centre is
specified by writing the projection formula
with the longest carbon chain vertical and
lowest (lower) number of carbon at the top.
• The D-configuration is then the one that has
the substituent (S) on the bond extending to
the “right” of the asymmetric carbon,
whereas the L-configuration has the
substituent (S) on the “left”.

47. D-L Configuration

48. D-L Configuration

49. R-S Configuration
R or S, referring to Rectus, or Sinister, respectively.
Cahn–Ingold–Prelog System of Configurational Nomenclature
1. Atoms: Rank the four atoms bonded to a chiral carbon atom in order of decreasing atomic number;
the lower the atomic number, the lower the priority. Isotopes are ranked in order of decreasing mass.
For example, 2H (deuterium) > 1H. (I > Br > CI > F > O > N > C > 2H > 1H)
2. Groups of atoms: If a chiral atom is attached to two or more identical atoms, move down the chain
until a difference is encountered. Then apply rule 1. Using this rule, we find that the priority of alkyl
groups is (CH3)3C- > (CH3)2CH- > CH3CH2- > CH3-
3. Multiple bonds: If a group contains a double bond, both atoms are doubled. That is, a double bond is
counted as two single bonds to each of the atoms of the double bond. The same principle is used for a
triple bond. Thus, the order is
The priority order for common functional groups containing oxygen is
—CO2H (carboxylic acid) > —CHO (aldehyde) > —CH2OH (alcohol)

50. R-S Configuration
Multiple Bond
C, H, H
Vinyl Group
H, H, H
Isopropyl group

51. R-S Configuration

52. R-S Configuration

53. Racemisation or Racemization
The process in which pure enantiomeric form (+) or (-) get converted into the
racemic mixture (±) is called as racemisation.
Or
Racemisation is the process of conversion of optically active form of the compound (+)
or (-) into the optically inactive racemic (±) mixture (form).
Racemization can be brought about by the action of heat, light or chemical reagent.

54. Resolution
The process of separation of racemic mixture (±) into its two pure enantiomers
[(d or +) & (l or -)] is known as resolution.
The following are various methods used for resolution of racemic mixtures:
1. Mechanical methods
2. Biochemical method
3. Chemical method
4. Chromatographic method
• Enantiomers have the same physical properties, such as boiling point or solubility,
they cannot be separated by distillation or crystallization.
• Diastereomers have different physical properties, they can often be separated on the
basis of solubility differences.
The optically pure compound used to form the diastereomeric mixture is called the
resolving agent.

55. Resolution
Chemical method is easy for the resolution. It is based on the principle that
diastereomers have different physical properties.

56. Resolution
Chromatographic method

57. Geometrical Isomerism

58. Geometrical Isomerism
The different compounds having same molecular formula but different three
dimensional arrangement of the atoms or group of atoms about the double bond
(>C=C<) are called Geometrical isomers. This phenomenon is called Geometrical
isomerism.
cis – isomer
Similar group lie
on the same side
trans – isomer
Similar group lie on
the opposite side
• Trans isomers of compounds
are usually more stable than
Cis isomers.
• Geometric (Cis and Trans)
Isomers result from
restriction rotation.

59. Geometrical Isomerism
Geometrical Isomer
Geometrical Isomer
No Geometrical Isomer

60. E-Z Nomenclature
E = Opposite (German word: Entgegen, meaning opposite) &
Z = Together /same side (German word: Zusamen, meaning together / same side)
In E &Z system, the atoms or group of atoms of higher priority attached to the end of
the double bond (>C=C<) are selected as per in accordance with the Cahn, Ingold and
Prelog (CIP) sequence rules of the ‘R & S’ system.
When the atoms or group of atoms of higher priority are on the same side of the
double bond (>C=C<); then the isomer is “Z”-form

61. E-Z Nomenclature
When the atoms or group of atoms of higher priority are on the opposite side of the
double bond (>C=C<); then the isomer is “E”-form.

62. E-Z Nomenclature

63. Conformational Isomerism

64. Conformational Isomerism
Conformational isomerism is a form of stereoisomerism in which
the isomers can be interconverted just by rotations about formally single bonds (refer to
figure on single bond rotation).
• The isomers which differ only by rotation about one or more single bonds (C-C) are
called as rotational isomers or conformational isomers or conformations.

65. Conformational Isomerism

66. Differences between Configuration and Confirmation

67. Baeyer’s Strain Theory

68. Baeyer’s Strain Theory
• A theory which explains specific behavior of chemical compounds associated with bond
angle strain.
• The four valencies of carbon are arranged symmetrically by forming the angles of 109028’.
• Adolf Von Baeyer was honored with a Nobel Prize for the discovery of the strain theory in
1905.

69. Conformation of Ethane

70. Conformation of Ethane

71. Conformation of n-Butane

72. Conformation of n-Butane
Stability of n-Butane:
Fully eclipsed < Partial eclipsed < Gauche < Anti or Skew Confirmation

73. Conformation of Cyclohexane
Chair Confirmation Boat Confirmation

74. Conformation of Cyclohexane
Twist form or Confirmation

75. Conformation of Cyclohexane

76. Conformation of Cyclohexane