Stereochemistry by Dr. P. R. Padole

By
Dr. P. R. Padole
Department of Chemistry
Shri Shivaji Science College, Amravati.

Contents of Stereochemistry:
Optical isomerism:1
Geometrical isomerism:2
Conformational isomerism:3
Unit Test4

Stereochemistry:
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.

Stereochemistry:
Two socks from a pair are mirror images
that are superimposable.
P q
Mirror
Dis-symmetry
M M
Mirror
Symmetry

Stereochemistry:
(Greek word: Stereos = Solid)
Defination:
The branch of chemistry which deals with
the study of structure of molecule in three
dimensions is called stereochemistry.

Why we need
stereochemistry?
 Cis, butanoic acid “maleic acid” essential for plants and ani
 trans, butanoic acid “fumaric acid” toxic to tissue
Cis-Isomer
Maleic Acid
Trans-Isomer
Fumaric Acid
m.pt. 403 k. m.pt.560 k.
HOOC
C C
H
H
COOH
HOOC
C C
H H
COOH

Molecules with the Same Molecular Formula
Whether superimposable?
Yes No
Homomers
(Identical)
Isomers
(Different compounds with the same Molecular Formula)
e.g.: Socks, Ball,
A, O, M, etc
e.g.: Hands, Shoes, P, F, J, etc
Whether same constitution / structure?
Are the molecules named the same
except for prefixes such as:
cis, trans, R or S?Yes No
Stereoisomers
(isomers with a difference
in 3-D arrangement only)
Structural / Constitutional
isomers
[isomers having atoms bonded to
different atoms(structural formula)]Whether non- superimposable mirrer images?
Yes No
Diastereomers
(not mirror images)
Enantiomers
(mirror images)
1) Chain isomerism
(differeng in their chains)
e.g. CH3CH2CH2CH3
& (CH3)2CHCH3
2) Positional isomerism
(differeng in position of
Functional group)
e.g. CH3CH2CH2-OH
& CH3CH(OH)CH3
3) Functional isomerism
(Isomerism differeng in their
Functional group)
e.g. CH3CH2-OH
& CH3-O-CH3
4) Metamerism 5) Tautomerism

Isomerism:
(Greek word: isos = equal and meros = parts)

Q.1) The different compounds which have same molecular formula but differ in their physical and
chemical properties are called as __________. (S-13, ½ Mark)
Q.2) Different compounds having same molecular formula are called as ____. (W-14, ½ Mark)
Defination:
1) Different compounds having (with) the same molecular
formula are called isomers and this phenomenon is known as
isomerism.
Or
2) The different compounds which have same molecular formula but
differ in their physical and chemical properties are called as
isomerism.
Or
3) Isomers are different compounds with the same molecular
formula.
The isomers may have different physical as well as chemical properties.

Types of isomerism:
Isomers
(Different compounds with the same Molecular Formula)
e.g.: Hands, Shoes, P, F, J, etc
Whether same constitution / structure?
Are the molecules named the same
except for prefixes such as:
cis, trans, R or S?Yes No
Stereoisomers
isomers
different atoms(structural formula)]
same molecular formula
different names
Constitutional isomers
n-butane isobutane
same molecular formula
same name except for the prefix
Stereoisomers
cis- trans-

Structural / Constitutional Isomerism:
Q.1) Which of the following is not the type of structural isomerism? (S-13, ½ Mark)
(a) Chain isomerism (b) Optical isomerism (c) Functional group isomerism (d) Position isomerism
Defination:
Compounds or isomers having the same molecular
formula, but different structures (formula) are called
as Structural or Constitutional isomers.
This phenomenon is known as Structural or
Constitutional isomerism.
isomers
different atoms(structural formula)]
1) Chain isomerism
(differeng in their chains)
e.g. CH3CH2CH2CH3
& (CH3)2CHCH3
2) Positional isomerism
(differeng in position of
Functional group)
e.g. CH3CH2CH2-OH
& CH3CH(OH)CH3
3) Functional isomerism
(Isomerism differeng in their
Functional group)
e.g. CH3CH2-OH
& CH3-O-CH3
4) Metamerism
5) Tautomerism

Chain /Nuclear Isomerism:
Q.1) Explain: (i) Chain isomerism with suitable examples. (W-12, 2 Mark)
Q.2) Define Chain Isomerism. (S-14, 1 Mark)
Q.3) n-Butane and iso-butane are chain isomers of each other. (S-15(old), ½ Mark)
Q.4) What is chain isomerism? Explain it in alkanes taking suitable examples. (S-16, 4 Mark)
Defination:
Isomers differing in their structure of carbon chains
are called as chain isomers and this phenomenon is
known as chain isomerism.
H3C
H2
C
C
H2
CH3
H3C
CH
CH3
CH3
n-butane iso-butane
&
(M.F.=C4H10) (M.F.=C4H10)
H3C
H2
C
C
H2
H2
C
CH3
H3C C
CH3
H3C
CH
C
H2
CH3
CH3
CH3
CH3
n-pentane neo-pentaneiso-pentane

Positional Isomerism:
Q.1) Define the terms : (i) Position isomerism. (S-12, 2 Mark)
CH2CH3
CH2 CH3CH3
OH
CH2 OH
n-propyl alcohol Isopropyl alcohol
CH3 -CH2 -CH CH2
CH3 -CH CH3CH
1-butene 2-butene
o-Xylene m-Xylene p-Xylene
CH3
CH3
CH3
CH3
CH3
CH3

Functional Isomerism:
Q.1) Which type of isomerism is shown by the following pair? (W-13, 1 Mark)
CH3-CH2-OH & CH3-O-CH3
O
CH2CH3 C CH3CH3CHO &
AcetonePropionaldehyde

Metamerism:
Defination:
Isomers differing in distribution of alkyl groups around
a central atom are called as Metamerism.
This is special type of structural isomerism.
For example,
(1) Diethyl ether and methyl propyl ether (Molecular formula = C4H10O)
C2H5-O-C2H5 & CH3-O-C3H7
Diethyl ether Methyl propyl ether
(2) Diethyl Ketone and methyl propyl Ketone
(Molecular formula = C5H10O)
O
C2
H5 C2
H5
O
CH3 C3
H7
C
diethyl ketone
C&
methyl propyl ketone

Tautomerism:
Defination:
Isomers differing in the arrangement of atoms, but
which exist simultaneously in dynamic equilibrium with
each other are called as Tautomerism.
For example,
CH3
-CH2
-N
O
O
CH3-CH=N
OH
O
CH3
-C-CH2
-COOC2
H5
O
CH3
-C = CH-COOC2
H5
OH
nitro ethane iso-nitro ethane
e.g.1)
e.g.2)
Ethyl acetoacetate Ethyl acetoacetate
(Keto form) (Enolic form)

Stereoisomerism:
Q.1) The isomerism exhibited by different arrangements of atoms or groups in a space is called
as stereoisomerism. (W-13, ½ Mark)
Q.2) What are stereosiomers? (S-15(old), 1 Mark)
Defination:
1) 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
2) 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.
OR
3) The isomerism exhibited by different arrangements of atoms or
groups in a space is called as stereoisomerism.

Examples of Stereoisomerism:
Cis-Isomer
Maleic Acid
Trans-Isomer
Fumaric Acid
m.pt. 403 k. m.pt.560 k.
HOOC
C C
H
H
COOH
HOOC
C C
H H
COOH
Non-superimposable
(because spatial arrangemnet)
a
be
d
*C
a
eb
d
*C

Type of Stereoisomerism:
Stereoisomerism
Optical isomerism Geometrical isomerism

Optical rotation of an optically active compound can be measured
by the apparatus, called, “Polarimeter”.

Plane of polarized light (PPL):
Q.1) Plane polarized light is one which: (S-12, ½ Mark)
(a) passes through some plane (b) which vibrates only in one plane
(c) which is reflected from a plane surface (d) which consists of only one wavelength
Defination:
Light whose rays vibrate only in a one (single) plane
(direction) is called as plane of polarized light (PPL).
Plane of polarized light (PPL) is obtained by passing the ordinary light through Nicol prism ,
called as Polarizer.

Optical active substances (compounds):
Q.1) What do you mean by optically active compound? (W-14, 1 Mark)
Q.2) Explain the term: Optically active compounds. (W-15, 2 Mark)
Q.3) What are optically active compound? (W-16, 1 Mark)
Defination:
Substances (compounds) which rotated the plane of polarized
light (PPL) are called as Optical active substances(compounds).
OR
Substance (compound) that rotates the plane of polarized light
(PPL) is called as Optical active substance (compound).

Optical Activity:
Defination:
The property of substance to rotate the plane of
polarized light (PPL) is called as optical activity.
This phenomenon is called optical isomerism.
With chiral compounds, the plane of polarized light is rotated through an angle “”.
The angle  is measured in degrees (°), and is called the observed rotation.
The rotation of polarized light can be clockwise or anti-clockwise.

(a)Dextrorotatory compound:
(substance or isomer): (Latin: dexter = right)
Q.1) Define: (i) Dextro isomer. (W-11, 2 Mark)
Defination:
The substance which rotates the plane of polarized light
to the clockwise direction (to the right position) is
known as dextrorotatory compound (substance or
isomer).
The rotation is denoted by d or (+).
e.g.: d or (+) -Lactic acid
20
20
Light
source
Ordinary
light
Nicol Prism
(Polarizer)
Plane
polarised
light
(PPL)
Sample Tube
Solution of
opically active
compound
(Chiral Compound)
PPL
rotated
clock-wise
(right side)
The plane of polarization is changed
Light rays (PPL)
oscillate in a single
(one) plane
Light rays
oscillate (vibrate)
in all plane
Viewer
(Eye)
*
COOH
C
CH3
OHH
d or (+)-Lactic Acid
+2.2o

(b) Laevorotatory compound:
(substance or isomer): (Latin: laevus = left)
Q.1) Define: (ii) Laevo isomer (W-11, 2 Mark)
Defination:
The substance which rotates the plane of polarized light
to the anti-clockwise direction (to the left position) is
known as levorotatory compound (substance or
isomer).
The rotation is denoted by l or (-).
e.g.: l or (-) -Lactic acid
20
20
Light
source
Ordinary
light
Nicol Prism
(Polarizer)
Plane
polarised
light
(PPL)
Sample Tube
Solution of
opically active
compound
(Chiral Compound)
PPL
rotated
anti-clock-wise
(left side)
The plane of polarization is changed
Light rays (PPL)
oscillate in a single
(one) plane
Light rays
oscillate (vibrate)
in all plane
Viewer
(Eye)
*
COOH
C
l or (-)-Lactic Acid
H3C
HO
H
-2.2o

Dis-symmetry is an essential condition for optical activity.
P q
Mirror
Dis-symmetry
M M
Mirror
Symmetry
O
Mirror
M M
Mirror
SymmetrySymmetry
O A A
Mirror
Symmetry
Superimposable mirror images
Achiral or Symmetric
e.g.1) e.g.2) e.g.3)
a
C
b
d
e
a
C
b
d
e
MirrorObject image
Optical isomers

Dis-symmetry is an essential condition for optical activity.
P q
Mirror
Dis-symmetry
M M
Mirror
Symmetry

Do you know?
Racemic Mixture (± form):

Q.1) An equimolar mixture of dextrorotatory and levorotatory forms (d & l forms) is called as a racemic
mixture. (W-11, ½ Mark)
Q.2) Explain why: Racemic mixture is optically inactive. (S-12 & W-14(old), 2 Mark)
Q.3) Racemic mixture is optically inactive. (S-14, ½ Mark)
Q.4) Racemic compounds are: (W-14, ½ Mark)
(a) Dextrorotatory (b) Laevorotatory (c) Optically inactive (d) All of these
Defination:
1) An equimolar mixture of dextrorotatory and
levorotatory forms (d & l forms) is called as a racemic
mixture or a recemate.
Or
2) An equal amount (50:50) of two enantiomers (d & l
forms) is called as a racemic mixture or a recemate.
A racemic mixture is optically inactive
( i.e, PPL remains unchanged), is due to external compensation, because
optical rotation of one form (+ θ) get cancelled by the optical rotationof
another form (- θ) and no rotation is observed.

External Compensation: The process of racemic mixture in which optical
rotation of one form get cancelled by the optical rotation of another form
and no rotation is observed is called external compensation.
e.g. d-Lactic acid & l-Lactic acid
For example:
Racemic mixture (± form) of Lactic acid:
*
COOH
C
CH3
OHH
*
COOH
C
H3C
HO
H
= + 2.2o = - 2.2o
+
Racemic Mixture of Lactic acid
Optically inactive External compensation

Asymetric Carbon Atom (C*):
Q.1) Explain the term: (i) Asymmetric carbon atom. (S-12, 2 Mark)
Q.2) Define & explain with example: (i) Asymmetric carbon atom. (W-13, S-15(o) & S-16, 2 Mark)
Q.3) What is asymmetric carbon atom? (W-14(old), 1 Mark)
Q.4) What is meant by Asymmetric carbon atom? (W-15, 1 Mark)
Defination:
A carbon atom bonded (attached) to four different
atoms or group of atoms is commonly called as
asymmetric carbon atom.
An asymmetric carbon atom in formula indicated by an asterisk (*).
All organic compounds containing one asymmetric
carbon atom are optically active.
C
a
be
d
*
CH3
COOH
OH*H C
Asymmetric
C - atom
Lactic acid

pramodpadole@gmail.com
Element of Symmetry:
1
Plane of
symmetry:
2
Centre of
symmetry:
3
Axis of
symmetry:
Optically inactive compound (Achiral) shows three types of symmetry.
n-fold simple
axis of symmetry:
n-fold alternating
axis of symmetry:
Imaginary plane Imaginary point
Imaginary axis

Plane of symmetry:
Q.1) Define / Explain the terms: (ii) Plane of symmetry. (S-12, S-13, S-14, W-14(old), W-15 & S-16, 2 Mark)
Defination:
It is imaginary plane which divides the molecule into two equal
parts in such a way that one part is the mirror image of the other,
such a compound is said to have plane of symmetry & it is
optically inactive (achiral).
For example,
2-chloropropane has a plane of symmetry while 2-chlorobutane does not have.
A plane of symmetry is a mirror plane that cuts a molecule in half, so that one half
of the molecule is a reflection of the other half.
C
H
Cl
CH3H3C
Plane of symmetry
(Achiral)
2-chloro-propane
C
H
Cl
C2H5H3C
No Plane of symmetry
(Chiral)
2-chloro-butane

Centre of symmetry:
 Defination: 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).
 For example: Ethane have a centre of symmetry while
trans -1,2-dichloro-ethene does not have.
Q.1) Explain with suitable example: (i) Centre of Symmetry. (S-13 & S-16, 2 Mark)
Cl H
ClH
CC
Centre of symmetry
trans 1,2-dichloro-ethene
o.

Axis of symmetry:
n-fold
simple
axis of
symmetry:
Axis of
Symmetry
Shows two types of axis of symmetry.
n-fold
alternating
axis of
symmetry:

a) n-fold simple axis of symmetry:
 Defination:
It is imaginary axis passing the molecule
such that when molecule is rotated about
it’s axis, then same arrangement is repeated
more than once, in one complete rotation
(i.e., 360o).
 This rotation is indistinguishable from the
original.
Q.1) Explain with suitable example: (i) n-fold simple axis of symmetry. (S-15, 2 Mark)
Q.2) What is n-fold simple axis of symmetry? Give its example. (S-17, 4 Mark)

a) n-fold simple axis of symmetry:
 For example,
(E)-1,2- dichloroethane has a simple axis of symmetry that passes through
the midpoint of the molecule and is perpendicular to the plane of
molecule.
e.g.2) Methyl chloride has an axis passing through its midpoint about which
a 3600/3 = 1200 rotation produces an orientation identical of the original.
Rotation through 3600/2 = 1800 about the axis leads to an arrangement identical
to the original.
Therefore, it possesses two fold axis of symmetry.

(b) n-fold alternating axis of symmetry:
Defination:
A molecule is said to have n-fold alternating axis of
symmetry, if, a molecule is rotated about an axis
passing through the molecule through ‘θ’ or ‘α’
degrees (i.e., angle of rotation) and the rotated
molecule is reflected in mirror, i.e, perpendicular
to the axis, then same arrangement or reflected
image is identical with the original structure.
 For example,
1,3-dichloro-2,4-difluro-cyclobutane is rotated through 180o
about the axis passing through the centre of molecule and
then reflected in a mirror perpendicular to the axis, an
arrangement superimposable on the original is obtained.

(b) n-fold alternating axis of symmetry:
Thus, this compound has two fold alternating axis of symmetry
Cl
HF
H
H
Cl
H
F
H
ClH
F
Cl
H
F
H
Cl
HF
H
H
Cl
H
F
Rotation by1800
about axis
mirror
Same
(Identical)
arrangement
Rotated
molecule
Reflection perpendicular
to the axis of rotation
Reflected Image
Original
e.g. 1,3-dichloro,2,4-difluoro-cyclobutane
Twofold alternating axis of symmetry

Asymetric Compound (Molecule):
 The term asymmetric denoted absence of any symmetry.
 Defination: A compound (molecule) having
no element of symmetry (i.e., plane of
symmetry, centre of symmetry or axis
of symmetry) is called as asymmetric
compound (molecule).
Non-superimposable
(because spatial arrangemnet)
a
be
d
*C
a
eb
d
*C

LOGO
Chiral
or
Chiral Molecules
or
Chiral Compound
or
Chiral Object:
Or
Dis-symmetric Compound:
Chiral (headedness) comes from the Greek word :
Cheir meaning hand.

Chiral or Chiral Compound:
 Defination:
 1) Compound (molecule) having no element of
symmetry except n-fold simple axis of symmetry
is called as Chiral or Chiral molecule of Chiral
compound or Dis-symmetric compound.
OR
 2) A molecule (object) that is non-
superimposable (not identical) on its mirror
image is said to be Chiral or Chiral molecule of
Chiral compound or Chiral object or Dis-
symmetric compound.
Q.1) A chiral molecule is optically active. (W-12 & W-14(old), ½ Mark)
Q.2) What is chiral molecule? (W-12, 1 Mark)

Chiral Molecules or Chiral Object:
Or Dis-symmetric Compound:
Note:
 Chiral comes from the Greek: Cheir, meaning
hand.
e.g. Left & right hands are mirror images of each
other, but they are not identical,
i.e, they do not superimposable on each other.

Non-superimposable (not identical)

Note: continue……..
2) All the asymmetric compound & Chiral
compound are optically active.
3) All the asymmetric compounds are Chiral.
4) Asymmetric molecule & Chiral molecule are
related but distinct.
 Because in the asymmetric molecule
absence of element of symmetry, but in Chiral
molecule may or may not be the presence of
n-fold simple axis of symmetry.
 Some Chiral compounds having no simple axis
of symmetry are strictly called as asymmetric
compound (molecule).

LOGO
Chiralilty
or
Dis-symmetry:

LOGO
Chiralilty or Dis-symmetry:
 Defination: The essential (necessary) condition
(property) for a compound having optically
active is called as Chirality or dis-symmetry.
Note: Dis-symmetry is an essential condition for
optical activity.
Q.1) What do you understand by Chirality? (W-09, 2 Mark)
Q.2) Explain the term: Chirality. (W-16, 2 Mark)
P
Mirror
Dis-symmetry
M M
Mirror
Symmetry
q

LOGO
Chiralilty or Dis-symmetry:
P
Mirror
Dis-symmetry
M M
Mirror
Symmetry
q

LOGO
Achiral
or Achiral Molecules
or Achiral Compound
or Achiral Object:
Or Symmetric Compound:

Achiral Or Symmetric Compound:
 Defination:
1) Compound (molecule) having element of
symmetry (like, plane of symmetry, centre of
symmetry or axis of symmetry) is called as Achiral
or Achiral molecule or Achiral compound or
symmetric compound.
OR
2) A molecule (object) that is superimposable
(identical) on its mirror image is said to be Achiral
or Achiral molecule or Achiral compound or
Achiral object or symmetric compound.

Achiral Or Symmetric Compound:
O
Mirror
M M
Mirror
SymmetrySymmetry
O A A
Mirror
Symmetry
Superimposable mirror images
Achiral or Symmetric
e.g.1) e.g.2) e.g.3)
C
H
Cl
CH3H3C
Plane of symmetry
(Achiral)
2-chloro-propane
Note: Achiral molecules contain a plane of symmetry, but Chiral molecule do not.

My student still waiting about Chiral & Achiral

 Everything has a mirror image. What’s important in Chemistry
is whether a molecule is identical to or different from its
mirror image.
 Some molecules are like hands. Left and right hands are
mirror images of each other, but they are not identical. If you
try mentally place one hand inside the other hand you can
never superimpose either all the fingers, or the tops and
palms. To superimpose an object on its mirror image means to
align all parts of the object with its mirror image.
 With molecules, this means aligning all atoms and all bonds.
 Left and right hands are chiral: they are mirror images that do
not superimpose on each other.

Two socks from a pair are mirror images that are
superimposable.
One sock can fit inside another, aligning toes and tops
and bottoms.
A molecule (or object) that is superimposable on
its mirror image is said to be achiral.

Stereoisomers
Whether non- superimposable mirrer images?
Yes No
Diastereomers
(not mirror images)
Enantiomers
(mirror images)

If you don't know :
 mirror images are the reflections of an
object.
 If two objects are superimposable, it means
you can not tell them apart, they are identical.
 If two objects are non-superimposable, then
you can always distinguish them.
 Bring these together, and it means we are
comparing an object with it's mirror image to
see if the object can be distinguished from it's
mirror image or not.

Enantiomers or Enantiomorphs or Enantiomerism:
(Mirror images)
(Enantiomers Greek word: Enantio = Opposite; Meros= Parts)
(Enantiomorphs Greek word: Enantio = Opposite; Morphs= forms)

S limonene (lemons) R limonene (oranges)
CH3
HCCH2
CH3
CH3
H C CH2
H3C

Enantiomers or Enantiomorphs or
Enantiomerism:
Q.1) Define: Enantiomers. (S-14, 2 Mark)
Q.2) What are enantiomers? (S-15, 1 Mark)
Q.3) Define and explain with the term: Enantiomers. (S-16, 2 Mark)
Q.4) Enantiomers have non-super imposable mirror images relationship. (W-16, ½ Mark)
Defination:
The Chiral compounds (stereoisomers) having non-
superimposable mirror image relationship are called
enantiomers or enantiomorphs and this phenomenon is
called as enantiomerism.
OR
When a compound and its mirror image are not
superimposable, they are different chiral compounds called
enantiomers. A chiral compound has no plane of symmetry
in any conformation.
For example: Two enantiomeric forms of lactic acid:

Enantiomers or Enantiomorphs or
Enantiomerism:
For example:
Two enantiomeric forms of lactic acid:
*
COOH
C
CH3
OHH
*
COOH
C
H3C
HO
H
+2.2o -2.2o
m.p. 299K m.p. 299K
Enantiomers of Lactic acid
Mirror
Enantiomers have non-super imposable mirror images relationship.

Properties of Enantiomers:
(i) Enantiomers (both d or l-forms) have identical physical
properties (like melting point, boiling point, densities, refractive
indices, solubility, etc.)
(ii) Hence, it cannot be separated by Fractional distillation or
Crystallization.
(iii) Enantiomers (both d or l-forms) have one or more asymmetric
(Chiral) C-atoms.
(iv) Enantiomers are optically active (Chiral).
(v) Enantiomers have identical chemical properties (except towards
optically active reagents).
(vi) They are sterioisomers that are mirror images.
(vii) Two enantiomers (d & l forms) rotates PPL to an equal extent but
in the opposite direction.
(viii) An equal amount (50:50) of two enantiomers (d & l forms) is
called a racemic mixture or a recemate.
A racemic mixture is optically inactive.

Mirror Objects – Carbon with 4 different substituents. We
expect enantiomers (mirror objects).
Reflect!
These are mirror objects. Are they the same thing just viewed
differently ?? Can we superimpose them?
We can
superimpose two
atoms. but not all
four atoms.
The mirror plane still relates the two structures. Notice that we can
characterize or name the molecules by putting the blue in the back, drawing
a circle from purple, to red, to green. Clockwise on the right and
counterclockwise on the left. Arbitrarily call them R and S.
RS
Arrange both
structures with the
light blue atoms
towards the rear….
Notice how the reflection
is done, straight through
the mirror!

LOGOwww.themegallery.com
(Not mirror images)
(Diastereoisomers Greek word: Diastereo = different; Meros= Parts)
(Diastereoisomorphs Greek word: Diastereo = different; Morphs= forms)
Diastereoisomers
or
Diastereoisomorphs
or
Diastereoisomerism:

LOGO
Diastereoisomers:
Defination:
 Stereoisomers having no mirror images
relationship are called Diastereoisomers or
Diastereoisomorphs and this phenomenon is
called as diastereoisomerism.
OR
When a compound and its mirror image are
superimposable, they are identical achiral
compounds called diastereoisomers.
An achiral compound has plane of symmetry in
one conformation.
Q.1) Explain Diastereomers with suitable examples. (W-13, 4 Mark)
Q.2) Any pair of stereosiomers which are not mirror Images are known as:
(a) enantiomers (b) diastereomers (c) d-l isomers (d) both (a) and (b)
(W-14(old), ½ Mark)
Q.3) Write note on: (i) Diastereomers (S-15, 2 Mark)

LOGO
Diastereoisomers:
For example:
 Tartaric acid having two asymmetric carbon atoms,

LOGO
Properties of Diastereoisomers:
(i) Diastereoisomers have different physical
properties (like melting point, boiling point,
densities, refractive indices, solubility, etc.)
(ii) Hence, they can be separated by
Fractional distillation or Crystallization.
(iii) Diastereoisomers may or may not have
asymmetric (Chiral) C-atoms.
(iv) Diastereoisomers may or may not be
optically active (Chiral or Achiral).
(v) Diastereoisomers have similar, but not
identical chemical properties.
(vi) They are sterioisomers that are not
mirror images.

Now Superimposable mirror objects:
Tetrahedral Carbon with at least two identical
substituents.
Reflection can interchange the two red substituents.
Clearly interchanging the two reds leads to the same
structure, superimposable! Remember it does not
make any difference where the mirror is held for the
reflection.
This molecule does not have an enantiomer; the
mirror object is superimposable on the original, the
same object.

LOGO
Distinguish between Enantiomers and Diastereoisomers:
S.No. Enantiomers Diastereoisomers
1.
The Chiral compounds (stereoisomers)
having non-superimposable mirror image
relationship are called enantiomers or
enantiomorphs and this phenomenon is
called as enantiomerism.
Stereoisomers having no mirror images
relationship are called Diastereoisomers or
Diastereoisomorphs and this phenomenon is
called as diastereoisomerism.
2.
Enantiomers (both d or l-forms) have
identical physical properties (like melting
point, boiling point, densities, refractive
indices, solubility, etc.)
Hence, it cannot be separated by Fractional
distillation or Crystallization.
Diastereoisomers have different physical
properties (like melting point, boiling point,
densities, refractive indices, solubility, etc.)
Hence, they can be separated by Fractional
distillation or Crystallization.
3.
Enantiomers (both d or l-forms) have one or
more asymmetric (Chiral) C-atoms.
Diastereoisomers may or may not have
asymmetric (Chiral) C-atoms.
4.
Enantiomers are optically active (Chiral). Diastereoisomers may or may not be
optically active (Chiral or Achiral).
5.
Enantiomers have identical chemical
properties
Diastereoisomers have similar, but not
identical chemical properties.
6.
They are sterioisomers that are mirror
images.
They are sterioisomers that are not mirror
images.
7. Example: Lactic acid Example: Tartaric acid
(W-12 & W-14, 3 Mark)

LOGO
Do you know?
What is configuration?

www.themegallery.com
Configuration:
 Defination:
 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.
OR
A particular three-dimensional
arrangement is called a configuration.
Q.1) What is configuration? (S-16, 1 Mark)
Dr P R Padole

Configuration……….
 Configuration can be changed only by breaking
and making of bonds.
 Configuration of isomers has independence
existence & hence it can be separated.
 The energy difference between two
configurational isomers is large.
*
COOH
C
OH
CH3
H
*
COOH
C
HO
H3C
H
Configuration of Enantiomers of Lactic acid
Mirror
Special or Specific 3-D arrangement around *C-atom
Dr P R Padole

Absolute configuration & Relative configuration:
Absolute configuration :
The actual (precise = accurate/exact)
arrangement of atoms in a space, of optical
active compound is called its absolute
configuration (R-S).
Relative configuration :
The configurational relationship between two
optically active compounds can be determined
by converting one into other by reaction.
OR
Relative configuration compares the three-
dimensional arrangement of atoms in space of
one compound with those of another compound.

Absolute configuration & Relative configuration:
For example:
When (-) 2-methyl-1-butanol treated with HCl, the
product obtained is (+) 2-methyl-1-chloro butane.
 The alcohol and chloride shows that whether two
compounds, i.e., product and reactant have similar or
opposite configuration is called as relative
configuration of different compound.
C
CH3
+ C +
(-)2-Methyl 1-Butanol (+)2-Methyl 1-Chloro Butane
CH2OHH
CH3
HCl H
CH3
CH2Cl
CH3
H2O

LOGO
Dee (D) and Ell (L) configuration (system) of
Nomenclature of Optical Isomers:
D and L system:
Dr P R Padole

D and L system:
 The oldest system of nomenclature of enantiomers is D
& L system.
 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, wheres the L-configuration has
the substituent (S) on the “left”.

D and L system:
C S
R3
R1
R2 C R2
R3
R1
S
Lowest number
Carbon at
Top
Longest Carbon chain
in Vertical position
SubstituentRight
side
Left
side
D-configuration L-configuration

D and L system:
In this system, the configuration of an enantiomer is
related to standard, Glyceraldehyde
(Absolute configuration).
L(-)-glyceraldehydeD(+)-glyceraldehyde
C
CH O
OH
CH2OH
H C
CH O
HO
CH2OH
H
Right
side
Left
side
D-configuration L-configuration

D and L system:
D (+)-Lactic Acid D(+)-Glyceric Acid
L (-)-PhenylanineL(-)-Alanine
C
COOH
OH
CH2OH
H
C
COOH
H
CH3
OH
C
COOH
H
CH2C6H5
H2N C
COOH
CH3
HH2N
*
RHS
RHS
LHS
LHS
e.g.-2) e.g.-3)
e.g.-4) e.g.-5)
*
**

D and L system:
L (-)-Lactic Acid
D (-)-Alanine
C
COOH
HO
CH3
H C
COOH
NH2H
CH3
RHS
LHS
(i)
* *
D-configurationL-configuration
(ii)
Note that: Small letters “d ” & “ l ” represent sign of rotation while capital
letters D & L represent configuration.

Disadvantage D and L system:
Note that:
 D & L nomenclature (system) creates confusion
in assigning the configuration to some compound.
For example: (+)-Tartaric Acid
COOH
CH OH
C
COOH
HO H
(+)- Tartaric acid
*
*
RHS
LHS
D
L
(+) Tartaric acid may be assigned L-configuration with respect of the
bottom chiral carbon,
or
D-configuration with respect of the top chiral carbon

LOGO
R and S System:
Cahn-Ingold-Prelog (CIP) Nomenclature:
R R = Rectus, i.e., Right handed
S S = Sinister ,i.e., Left handed

R and S System or Nomenclature or Configuration:
Sequence Rules:
Cahn-Ingold-Prelog (CIP) Nomenclature:
Q.1) Explain the R-S system of assigning the configuration of optically active
compounds. (W-14(old) & W-15, 4 Mark)
Q.2) What are the sequence rules for R & S configuration? (W-16, 4 Mark)
R & S system is a newer and
more systematic method is due
to Cahn, Ingold and Prelog (CIP)
in 1956 and is used to specify the
configuration of asymmetric
carbon compounds (isomers).
C
a
be
d
*
This system consists of the following two steps:
Step-1):
The four atoms or group of atoms attached to the asymmetric or
chiral carbon atom are assigned a sequence of priority (1, 2, 3 or 4)
according to the following set of sequence rules:

Sequence rules needed to
Assign Priority:
Rule-1):
 If the four atoms, directly attached to the asymmetric
carbon atom, are all different, the priority (1, 2, 3 or 4)
depends on their atomic number. The atom of higher
atomic number gets the highest priority.
For example: 1) bromo-chloro-iodomethane (CHClBrI)
Br
CI Cl
H
*
1
2
3
4
Bromo,chloro,iodo-methane
The priority order is as shown below:
S.No. Atom Atomic No. (Z) Priority
1. I 53 First (1)
2. Br 35 Second (2)
3. Cl 17 Third (3)
4. H 1 Forth (4)

www.themegallery.com Examples of Rule-1):
Rule-1):
For example:
SO3H
C
F
Cl
Br
H
1
2
3
4
C Cl
H
I
1 2
3
4
e.g.-2) e.g.-3)
Note that:
We consider the atom of the group which is directly
linked to the central asymmetric carbon atom.

www.themegallery.com Rule-2):
Rule-2): If two or more isotopes are
bonded (attached) to an asymmetric
carbon atom, assign priorities in order
of decreasing mass number.
For example:
S.No. Isotope Mass No. (A) Priority
1. T (Tritium) 3 ( 1 Proton + 2 Neutrons) First (1)
2. D (Deuterium) 2 ( 1 Proton + 1 Neutrons) Second (2)
3. H (Hydrogen) 1 ( 1 Proton) Third (3)
CH3
4
2
1
3
2 3
1
4
e.g.-1) e.g.-2)
C
Br
CH3
H
D
C D
H
T

Rule-3): If two or more groups attached to
the asymmetric carbon atom, have their first
atoms identical (same); then the priority order
depend upon the atomic number of the second
atom, and if the second atom is also identical;
then atomic number of the third atom along
the same chain determines the priority.
For example:
S.No. Group Priority
1. CH3CH2CH2- First (1)
2. CH3CH2- Second (2)
3. CH3- Third (3)

For example:
This C is bonded to 1 H's & 2 C -CH CH3
Higher priority group (2)
C CH2-CH3
OH
CH
H
*
1
2
3
4
H3C
CH3 This C is bonded to 2 H's & 1 C
CH3
-CH2 CH3
Lower priority
group (3)
For example: 2-methyl-3-Pentanol

Rule-4): If the first atoms of the two
groups have the same substituent of
higher atomic number; then the group
with more number of substituent gets
the higher priority.
 For example:
 –CH2Cl & –CHCl2
Thus, –CHCl2 has higher priority than –CH2Cl

Rule-5): A doubly or triply bonded atom
present in a group is considered equivalent to
two or three singly bonded atom.
C C=O
*
is equivalent to C
O
O
Consider this C bonded to 2 O's
Thus, e.g. -1)
e.g.-2)
C N is equivalent to C
N
N
N
Consider this C is bonded to 3 N's

 e.g.-3) The phenyl group, C6H5-, is handled as
if it had one of the Kekule structures.
CC
C
C
C
* is equivalent to
Other common multiple bonds:

www.themegallery.com Step-2):
 Decreasing order of their priority (1→2→3),
our eye moves (travel) in a clockwise
direction, then the configuration is specified
as “R” [Rectus (Latin word) meaning right
handed, clockwise] &
 on the other hand, if our eye moves in the
anticlockwise direction, the configuration is
specified as “S” (Sinister meaning left handed,
anticlockwise).

www.themegallery.com University problems on R & S system:
Q.1) According to the sequence rules of R-S system, the correct order of priority of
groups is ________. (W-12, ½ Mark)
(a) Cl > C2H5 > CH3 > H (b) Cl > H > CH3 > C2H5 (c) Cl > CH3 > H > C2H5
(d) Cl > H > C2H5 > CH3
Q.2) Arrange the following groups in proper priority order according to R-S
nomenclature system: (S-14, 4 Mark)
(i) –CHO, -COOH, -NH2, -OH
Ans: -OH > -NH2 > -COOH > –CHO
(ii) –H, -OH, -C2H5, -Cl
Ans: -Cl > -OH > -C2H5 > –H
nomenclature system: (S-15(old), 4 Mark)
(i) -C2H5, -OH, -H, -COOH,
Ans: -OH > -COOH > -C2H5 > -H
(ii) –CHO, -COOH, -NH2, -OH
Ans: -OH > -NH2 > -COOH > –CHO
nomenclature system: (S-16, 4 Mark)
(i) -COOH, -NH2, -H, -CH3
Ans: -NH2 > -COOH > -CH3 > -H
(ii) -C2H5, -OH, -H, -CH3
Ans: -OH > -C2H5 > -CH3 > –H
Q.5) Assign priorities to the following groups: (S-17, 4 Mark)
-CH3 , -OH, -CHO, -COOH
Ans: -OH > -COOH > -CHO > -CH3

LOGO
Racemization:
Q.1) Explain the term: Racemization.
(S-12, W-15 & W-16, 2 Mark)
Q.2) Explain: Racemization with suitable examples.
(W-12, 2 Mark)
Q.3) Define: Racemization. (S-14, 2 Mark)
Q.4) What is racemisation? (S-17, 1 Mark)

Company Logo
Racemisation or Racemization:
 Defination: 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).
{Note: Racemization can be brought about by the action of heat, light or
chemical reagent.}
 For example: Optically active (+)-1-chloro-2-methyl butane on
halogenation; to form racemic mixture.
C
CH2Cl
H3C-H2C
CH3
H
Cl2
C
CH2Cl
H3C-H2C
CH3
Cl C
CH2Cl
Cl
CH3
CH2-CH3
Racemization
+
Racemic mixture
of 1,2-dichloro-2-methyl butane
(+)-1-chloro-2-methyl butane
- HCl
1
234

LOGO
Resolution
or
Resolution of Racemic Mixture:
Separation of individual optically active components:

Resolution:
 Defination: The separation of individual optically active
components [(d /+) & (l /-)] from the racemic (±) mixture
(form) is known as resolution.
OR
 The process of separation of racemic mixture (±) into its
two pure enantiomers [(d or +) & (l or -)] is known as
resolution.
Resolution involves the separation of d-form and l-form of racemic mixture.
Q.1) What is meant by resolution? Explain the chemical method of resolution. (S-12 & S-17, 4 Mark)
Q.2) Define the term: Resolution. (S-13, 2 Mark)
Q.3) Discuss the chemical method of resolution of racemic mixture. (S-13, 4 Mark)
Q.4) Define: Resolution of racemic mixture. (S-14, 2 Mark)
Methods of Resolution of optical isomers
OR
Method of Resolution
1) Mechanical
Separation
2) Biochemical
Separation
3) Chemical
Method
4) Selective
Adsorption

Resolution of racemic mixture by Chemical Method:
 In chemical method, racemic mixture is treated with
optically active acid or base; to form mixture of
diastereoisomeric salts.
 Due to difference in their solubilities, two salts from the
mixture can be separated by means of Fractional
Crystallization.
 These salts are then treated with mineral (inorganic)
acid or alkalies to get individual enantiomers (original
active compounds, i.e., (+) & (-) form)
*
COOH
C
CH3
OHH
*
COOH
C
H3C
HO
H
= + 2.2o = - 2.2o
+
Racemic Mixture of Lactic acid
Optically inactive External compensation

(+) Acid.(-) Acid
Racemic Mixture
+ (+) Base
Optically
active
[(+) Acid.(+) Base] + [(-) Acid. (+) Base]
Mixture of Diastereoisomeric salts
Separated by Fractional Crystallization
[(+) Acid.(+) Base] salt + [(-) Acid. (+) Base] salt
H+
/OH-
H+
/OH-Hydrolysis Hydrolysis
(+) Acid
Enantiomer
(-) Acid
Enantiomer

Mixture of Diastereoisomeric salts
Separated by Fractional Crystallization
(Physical Method)
(+) Tartaric Acid
Enantiomer
(Dextro)
Racemic (±) Tartaric Acid + (+) Cinchonine
(Base)
Optically acive
[(+) Tartaric Acid . (+) Cinchonine] + [(-) Tartaric Acid . (+) Cinchonine]
[(+) Tartaric Acid . (+) Cinchonine] salt + [(-) Tartaric Acid . (+) Cinchonine] salt
Hydrolysis by H+H+
Hydrolysis by
(-) Tartaric Acid
Enantiomer
(Laevo)
{Note that:
1) Bases used for resolution are mainly alkaloids e.g. (i) Cinchonine, (ii) Quinonine, etc.
2) Similarly, acids commonly used for resolution are-(i) tartaric acid, (ii) camphor-sulphonic acid}

LOGO
Geometrical Isomerism:
Or
Cis-Trans Isomerism:
same molecular formula but different three dimensional arrangement

Do you know?
Geometrical Isomerism

Geometrical Isomerism: Or
Q.1) What are geometric isomers? Explain why maleic acid easily forms an anhydride than fumaric acid.
(W-11, 4 Mark)
Q.2) Define / Explain with example: (ii) Geometrical Isomerism. (W-13, W-14(old) & S-15(old), 2 Mark)
Q.3) Explain with suitable example: Cis-trans isomerism. (S-15 & W-16, 2 Mark)
Q.4) Write note on: (ii) Geometrical Isomerism. (S-15, 2 Mark)
Q.5) What is geometrical isomerism? Explain with suitable examples. (S-16, 4 Mark)
Defination: 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.
The geometrical isomer (or isomer) in which the similar group lie
on the same side about >C=C< bond is called cis – isomer.
The geometrical isomer (or isomer) in which the similar group lie
on opposite side about >C=C< bond is called trans – isomer.

Geometrical Isomerism: Or
Due to the existence of cis & trans forms of the isomers;
geometrical isomerism is also called as Cis-Trans isomerism.
Molecular formula = C2A2B2
C C
A
BB
A
Cis-isomer (form)
C C
B
AB
A
Trans-isomer (form)
Q.1) Assign cis-trans nomenclature to the following compounds. (S-13 & S-16, 2 Mark)
Q.2) Maleic and fumaric acids are geometrical isomers of each other. (S-17, ½ Mark)
Cis-Isomer
Maleic Acid
Trans-Isomer
Fumaric Acid
m.pt. 403 k. m.pt.560 k.
HOOC
C C
H
H
COOH
HOOC
C C
H H
COOH
CIS (Z)
Groups/atoms are on the
SAME SIDE of the double bond
TRANS (E)
Groups/atoms are on OPPOSITE SIDES
across the double bond

Explain why ?
Q.1) Explain why maleic acid easily forms an anhydride than
fumaric acid. (W-11, 4 Mark)
Note that:
Maleic acid readily forms cyclic anhydride whereas fumaric acid
does not.
Obviously maleic acid must be cis form and fumaric acid trans form.

GEOMETRICAL ISOMERISM
RESTRICTED ROTATION OF C=C BONDS
C=C bonds have restricted rotation so the groups on either end of the bond are
‘frozen’ in one position; it isn’t easy to flip between the two.
This produces two possibilities. The two structures cannot interchange easily
so the atoms in the two molecules occupy different positions in space.

LOGO
E & Z System of
Nomenclature:
E = Opposite & Z = Together /same side
{E = Opposite (German word: Entgegen, meaning opposite) &
Z = Together /same side
(German word: Zusamen, meaning together / same side)}

E & Z System of Nomenclature:
Q.1) Explain E-Z system of nomenclature with example. (W-15, 4 Mark)
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.
(i) 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
(German word: Zusamen, meaning together / same side).
C C
A
BB
A
Z- form or Z-configuration
Higher Higher
1 1
Lower Lower
2 2
C C
Br
FH
Cl
Z- form or Z-configuration
Higher Higher
1 1
Lower Lower
2 2
Atoms or group of atoms of higher priority are on the same side

E & Z System of Nomenclature:
(ii) 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 (German
word: Entgegen, meaning opposite).
E- form or E-configuration
C C
B
AB
A
Higher
Higher
1
1
Lower
Lower
2
2
Atoms or group of atoms of higher priority are on the opposite side
C C
F
BrH
Cl
Higher
Higher
1
1
Lower
Lower
2
2

Examples of E & Z System:
Q.1) Assign E-Z nomenclature to the following compounds: (S-12, 2 Mark)
Higher
Higher
Lower
Lower
E- form or E-configuration Z- form or Z-configuration
Higher Higher
Lower Lower
C C
Br
Cl
CH3
NH2
C C
Cl
Br
CH3
NH2
(i) (ii)
Q.2) Assign and explain E-Z nomenclature to the following compounds:
(W-13 & S-15(old), 2-4 Mark)
Higher
Higher
Lower
Lower
Higher
HigherLower
Lower
C C
Br
HOOC
Cl
CH3
C C
H3C
Br
CH3
H
(i) (ii)
&

By Dr. P. R . Padole
Important terms:
1
Torsional
strain:
Strain caused
by eclipsing
interactions.
2
Steric strain:
Strain
produced
when atoms
are forced too
close to each
other.
3
Angle strain:
Strain produced
when bond
angles deviate
from 109.5o
(for sp3
hybridized
atoms).

Baeyer’s Strain theory:
Stability
of cycloalkanes:

LOGO pramodpadole@gmail.com
Stability of cycloalkanes:

LOGO
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.
Q.1) On the basis of Baeyer’s strain theory, calculate angle strain in cyclohexane and explain their
relative stability. (W-11, 4 Mark)
Q.2) Discuss the Baeyer’s strain theory with its limitations. (W-14, 5 Mark)
Q.3) Explain the Baeyer’s strain theory. Give its limitations. (W-14(old), W-15 & W-16, 4-6 Mark)

LOGO
Stability of Cycloalkanes:
Ring Strain
 Rings larger than 3 atoms are not flat
 Cyclic molecules can assume nonplanar
conformations to minimize angle strain and
torsional strain by ring-puckering
 Larger rings have many more possible
conformations than smaller rings and are more
difficult to analyze

LOGO
Cycloalkane Formula
Deviation from normal
tetrahedral angle (Angle strain)
Cyclopropane
(C3H6)
½ (109.280 - 600) + 24.440
Cyclobutane
(C4H8)
½ (109.280 - 900) + 9.440
Cyclopentane
(C5H10)
½ (109.280 - 1080) + 0.440
Cyclohexane
(C6H12)
½ (109.280 - 1200) - 5.16
The + sign indicates that the C-C bonds have to be compressed to satisfy the
geometry of the ring.
The – sign indicates that the C-C bonds have to be widened to satisfy the geometry of
the ring.

LOGO
Stability of cycloalkanes:
 In the examples given above, the deviation from
the normal tetrahedral angle is maximum in the
case of cyclopropane.
 Thus, according to the Baeyer Strain Theory,
cyclopropane should be a highly strained
molecule and consequently most unstable.
 The cyclopropane ring should, therefore, be
expected to open up on the slightest provocation
and thus releasing the strain within it.
 This is actually so, Cyclopropane is known to
undergo ring opening reactions with Br2, HBr,
and H2 (in presence of Ni-catalyst); to form
open chain addition compounds.

Company
LOGO Conformations:

Company name
Conformations:
 Defination: 1) The molecules which are capable of
forming isomers by rotation about a single bond (C-C)
are termed as Flexible molecules and 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.
OR
 2) The different special arrangements of the atoms
obtained by the rotating the compound (molecule)
through any single bond are called Conformations.
Q.1) The various structural arrangements adopted by a molecule due to rotation about
a C-C single bond are known as Conformational isomers. (W-11, ½ Mark)
(a) Geometrical isomers (b) Conformational isomers (c) Optical isomers (d) None of these
Q.2) Define conformation. (S-14, 1 Mark)
Q.3) What is conformation? (S-15(old) & W-15, 1 Mark)

Company name
Conformations:
OR
3) The different spatial arrangements of the atoms in a molecule which can
be readily converted into one another by rotation around single bonds (C-
C) are called Conformations.
OR
4) The different arrangements of the atoms in a space that result from
rotation about a single bond (C-C) are called Conformations.
OR
5) The different spatial arrangements of the atoms in a molecule which are
readily inter-convertible by rotation around single bonds (C-C) are called
Conformations.
OR
6) The various structural arrangements adopted by a molecule due to
rotation about a C-C single bond are known as conformational
isomers.
OR
7) Conformations are different arrangements of atoms that are
interconverted by rotation about single bonds. A particular conformation is
called a conformer.

Company name
Conformations:
H H
H
H H
H
Rotation around / about
C-C bond
The C-H bonds are all alligned
Eclipsed Conformation
Rotation
H H
H
HH
H
Two different Conforamtions of Ethane
The location of the indicated atom
changes with rotation
The C-H bonds are not alligned
Staggered Conformation
Sawhorse Drawing
Conformations are different spatial arrangements of a
molecule that are generated by rotation about single bonds.

Company name
Eclipsed conformationEthane

Company name
Staggered conformationEthane

LOG
O
Distinguish between
Configuration
and
Conformations.

S.No. Configurations Conformations
1.
The special (specific or particular or
definite) three dimensional
arrangements of atoms and group of
atoms around the asymmetric C-atom
are called as configuration.
The different special arrangements of the
atoms obtained by the rotating the compound
(molecule) through any single bond (C-C) are
called Conformations.
2.
Configuration can be changed only by
breaking and making of bonds.
Conforamtions of a molecule are easily
changed (converted) by rotation around
(about) single C-C bond.
3.
Configuration of isomers has
independence existence. Hence it can be
separated.
Conforamtions of isomers has no
independence existence. It is easily inter-
converted and hence can not be separated.
4.
The energy difference between two
configurational isomers is large.
The energy difference between two
Conforamtional isomers is very small.
5.
Example: Configuration
Enantiomers of Lactic acid Example: Conforamtions of Ethane
*
COOH
C
OH
CH3
H
*
COOH
C
HO
H3C
H
Mirror H H
H
H H
H
Rotation
H H
H
HH
H
Eclipsed Conformation Staggered Conformation

Sawhorse projection formula:
Q.1) Draw Sawhorse projection for eclipsed and staggered conformations of ethane.
(W-12, W-14, & S-15, 2 Mark)
Q.2) Draw Sawhorse projection formulae for ethane molecule. (S-16, 2 Mark)
H H
H
H H
H
Rotation around / about
C-C bond
The C-H bonds are all alligned
Rotation
H H
H
HH
H
Two different Conforamtions of Ethane
Bond between two C-atoms is drawn diagonally
The C-H bonds are not alligned
Sawhorse Drawing
Front
Carbon
Upper Right hand
Carbon is taken as
Back C-atom
Lower Left hand
Carbon is taken as
Front C-atom
60o
C-C
Back
Carbon

COMPANY LOGO
Newman Projection Formulae:
Q.1) Draw Newman projection for eclipsed and staggered conformations of ethane.
(W-12, W-14 & S-15, 2 Mark)
Q.2) What is projection formula? Explain Newman projection formula with an example.
(S-15(old), 4 Mark)
Q.3) Draw Newman projection formulae for ethane molecule. (S-16, 2 Mark)
The conformation in which the H-atoms of
back Carbon are just behind those of the Front carbon
is known as Eclipsed Conformation
Rotation
60o
about C-C single bond
Point represented as
Front Carbon Circle represented as
Back carbon atom
(Rear)
H
HH
H H
H
120o 0o
A conformation with a 0o
(zero) torsional (dihedral) angle
HH
H
H
H H
60o
Point represented as
Front Carbon
The conformation in which the H-atoms of
two Carbons are as far apart as possible
is known as Staggered Conformation
A conformation with a 60o
torsional (dihedral) angle
Newman Projection Formulae for Conformation of Ethane

COMPANY LOGO
End-on representations for conformations are commonly drawn
using a convention called a Newman projection.
A Newman projection is a graphic that shows the tree groups
bonded to each of the carbon atoms in a particular C-C bond, as
well as the dihedral angle that separates them.
Rotating the atoms on one carbon by 60o converts an
eclipsed conformation into a staggered conformation,
and vice versa.

Conformations
of Ethane:
Q.1) Explain the conformational analysis of ethane with energy level diagram.
(W-14 & S-15, 4 Mark)
Q.2) Define conformation. Explain conformations of ethane with associated energy changes.
(S-14, 5 Mark)
Q.3) What is conformation? Explain the conformations of ethane.
(S-15(old), 4 Mark)
Q.4) Total number of conformations of ethane are: (W-15, ½ Mark)
(a) 2 (b) 3 (c) 4 (d) 6
Q.5) Explain the conformations of ethane with energy level diagram. (W-16,4 Mark)

LOGO
Do you know?
Conformations of Ethane:

LOGO
Conformations of Ethane:
 When an ethane molecule is rotated about
its C-C single bond by keeping one of the
carbon atom fix (remains Stationary)
through 60o; two extreme conformations are
obtained, such as Eclipsed & Staggered
conformations.
H-atoms are just behid.
So more repulsion between it's electron.
So, more energy.
Hence, Least Stable
Rotation
60o
about C-C single bond
H
HH
H H
H
120o 0o
HH
H
H
H H
60o
Newman Projection Formulae for Conformation of Ethane
H-atoms are as far apart
So less repulsion between it's electron.
So, less (low) energy.
Hence, Most Stable

LOGO
Ethane with energy level diagram:
H
H
H
H
H
H
Front
carbon
Back
carbon
Eclipsed
Staggered

LOGO
Steric Strain or
Vander-Waal’s Strain
or Steric Hindrance:
Strain produced when atoms are forced too
close to each other

Steric Strain or Vander-Waal’s Strain
Defination: 1) Steric Strain or Steric hindrance is the Strain
produced on a molecule, when it’s atoms or group of atoms
are large in size (e.g. –CH3 in n-Butane) & due to this they
are too close to each other, which causes repulsion
between the electrons of atoms or group of atoms.
OR
2) In a molecule, when atoms or group of atoms of large
size are brought closer than it,s Vander Waal’s radii, which
causes repulsion between the electrons of atoms or group
of atoms are called as Vander Waal’s Strain or Steric Strain.
H
H3C
H
CH3
HH
=600
CH3
H
H
H H
=1800
CH3
Methyl group as far apart
No Steric Strain
2 -CH3 group are only 60o
apart, i.e., close
Steric Strain

Steric Strain or Vander-Waal’s Strain
C C
CH3
H3C
H
H
H
H
HH
CH3
H H
H3C
C C
CH3
H
H3C
H
H
H
CH3H
H
H H
H3C

Do you know?
Conformations of n-Butane:

 Butane has three C-C single bonds and the
molecule can rotate about each them.
Q.1) Explain the conformations of n-butane and the associated energy changes with
suitable diagram. (S-12, W-13 & W-14(old), 6 Mark)
Q.2) Explain the conformations of n-butane with energy level diagram.
(S-13, W-15 & W-16, 4-6 Mark)
Q.3) Anti-staggered conformation of n-butane has dihedral angle, θ = __. (S-15, ½ M)
(a) 240o (b) 300o (c) 180o (d) 60o
Q.4) Draw Newmann and Sawhorse projection formulae for fully eclipsed conformation
of n-butane. (S-15, 2 Mark)
Q.5) Which of the following conformations of n-butane is least stable? (S-17, ½ Mark)
(a) Gauche (b) Anti (c) Eclipsed (d) Fully Eclipsed
Q.6) Explain the conformational analysis of n-butane with energy level diagram.
(S-17, 4 Mark)
CH3 CH2 CH2 CH3
1234

H
CH3
H
H
CH3
H
H3C
H H
H3C
H H
H3C
H H
CH3
H H
IV
VVI
I
II
III
=600
Fully or Completely
Eclipsed Form
Fully or Completely Staggered
or Anti Form
Partially Eclipsed Form
Partially Staggered
or Gauche / Skew form
[Gauche is French for"Left']
C
H3
H H
H3C
H
H
H3C H
H
HH
CH3H
H
CH3
H
HCH3
Rotation
60o
Rotation
60o
Rotation
60oRotation
60o
Rotation
60oRotation
60o
(Anti is Greek for "opposite of")
[Most Stable]
Steric Strain
=00
[Least Stable]
Partially Staggered
or Gauche / Skew form
[Gauche is French for"Left']
Partially Eclipsed Form
Conforamtion of n-Butane

Anti > Gauche > Partially Eclipsed > Completely Eclipsed
I III & V II & VI IV

Conformations of Cyclohexane:
Q.1) Draw chair and boat conformations of cyclohexane. Explain their stability.
(W-11 & W-13, 4-5 Mark)
Q.2) Explain why: Chair form (conformation) of cyclohexane is more stable than the
boat form (conformation). (S-12, S-14, W-14, W-14(old), S-15 & W-15, 4 Mark)
Q.3) Explain conformational analysis of cyclohexane with energy level diagram.
(S-17, 4 Mark)

Conformations of Cyclohexane:
Chair Conformation:
If a cyclohexane ring were flat …….
120o H
H
H
H

Conformations of
Cyclohexane:
Boat Conformation:
In boat form, adjacent H-atoms are in eclipsed position, more repulsion take place
between Flagpole H-atoms. Thus, due to more strain in boat form as compare to chair
form, Boat form (maximum energy) is less stable than Chair form (minimum energy).

Conformations of
Cyclohexane:
Twist form or Conformation:
Twisting
(More Stable)
Hs
Ha
Ha
Hf
as far apart
Near (closer)
Minimum strain
Twist form
He
He
The Twist form (conformation) of cyclohexane is less stable
than Chair form and is more stable than Boat form.

Boat
Half-chair
Chair
Half-chair
Twist-boat Twist-boat
Chair
Chair-Chair Interconversion of Cyclohexane
axial
equatorial
axial
equatorial

Stereochemistry by Dr. P. R. Padole

Stereochemistry by Dr. P. R. Padole

Recommandé

Recommandé

Contenu connexe

Tendances

Tendances (20)

Similaire à Stereochemistry by Dr. P. R. Padole

Similaire à Stereochemistry by Dr. P. R. Padole (20)

Plus de pramod padole

Plus de pramod padole (20)

Dernier

Dernier (20)

Stereochemistry by Dr. P. R. Padole