Power Point Presentation on steriochemistry

1. Stereochemistry
S. Y. B. Sc.
Chemistry paper-II
Sec-I
Dr. D. R. Shinde
P.D.E.As.
Prof Ramkrishna More
Arts, Commerce and Science College,
Akurdi, Pune-44

2. • Stereochemistry refers to the 3-dimensional three dimensional
properties of molecules.
• Stereochemistry has its own language and terms that need to
be learned in order to fully communicate and understand the
concepts.
• Def. Isomers: The compound having same molecular formula
but different structures.
• e.g. Molecular formula: C3H6O, Different isomers possible
with this formula are
H3C
C
CH3
O
H3C CH2
C
O
H
Aceton Propanealdehyde
H2C CH CH2OH
prop-2-en-1-ol

3. Do the compounds have the same molecular formula
No
Not isomers
Yes
Isomers
Do the compound have same connectivity
Constitutional isomers
No Yes
Stereoisomer's
Configurational isomers Conformational isomers
Does C in compound
is SP3 hybrid
Geometric isomers Optical Isomers

4. Representation of molecule
Molecules possess three dimensional structure. However, on
paper, we have represent them as two dimensional. To view
three dimensional structure on paper some methodologies are
used.
1) Wedge and Dotted line presentation:
In this method the atoms in the plane are
presented with simple line. The atom
pointing behind the plane is shown by
dotted line. The atom pointing above the
plane is shown by wedge line.
C
H
H
H
H
In plane
of paper
Pointing forward -
out of plane of paper
Pointing
backward out of
plane of paper
e.g. In CH4 molecule, two H and
central C atom lie in a plane of paper.
From remaining two H, one point
slanting behind the plane while
another point slanting above the
plane.

5. C
H
CH3
HO
C2H5
C
COOH
NH2
H
H

6. 2) Newman’s Projection Formula
• In Newman’s Projection Formula molecule is viewed end-on, i.e. along the
C-C bond.
• The two carbon atoms joined by the bond are represented as two
superimposed circles; however for representation only one circle is drawn.
• The centre of the circle represents the front carbon atom C whilst the circle
represents the back carbon atom. The line joining the two carbon atoms is
not visible.
• Bonds attached to the front C carbon atom are drawn as lines from the
centre and the bonds attached to back C atom drawn from the
circumference of the circle. The projected angle between each bond is kept
as 120o.
• The ethane molecule, with the help of Newman’s Projection Formula, can be
drawn as below.
Front C atom
Back C atom
H
H H
H
HH

7. CH3
H H
CH3
HH
n-butane, staggered conformation

8. 3) Sawhorse Projection Formula:
• In Sawhorse Projection Formula a side view of the molecule is taken.
• The two carbon atoms are joined by a larger diagonal line which is
taken to be in the plane of the paper. The remaining bonds are
projected on paper by small lines drawn out from the terminal ends
of the diagonal line.
• a) The heavy wedged bond projects the bond towards the viewer,
b) the dashed bond projects the bond away form the viewer and
c) the normal bonds lie in the plane of the page.
• The eclipsed and the staggered conformational form of the Ethane
molecule can be represented with the help of Sawhorse Projection
Formula of staggered and eclipsed ethane as shown in Fig.
C
CH
H
H
H
H
H
Eclipsed Ethane
C
C
H
H
HH
H H
Staggered ethane

9. Optical Isomerism:
Optical isomerism is observed in organic compounds consisting
of Chiral C atom.
The chiral c atom is –
i) sp3 hybrid or tetrahedral
ii) It consists of four different groups of atoms or atoms bonded
to it. It is also called as asymmetric C atom.
C
H
I
Cl
Br
C
H
CH3
HO
C2H5
The chiral C atom
shown by * on its
head

10. Mirror
Mirror Image
Original
Molecule

11. Optical activity is related to Plane-Polarized Light (PPL).
PPL is different from normal light.
Photons of normal light vibrate in all possible planes passing
through a point i.e. in infinite number of planes while all
photon in a beam of PPL vibrate only in one plane.
From ordinary light, PPL is obtained by passing ordinary light
through a prism called as Nicol Prism or polarizer.

12. When Plane-Polarized Light is passed through an Achiral
Compound plane of PPL do not changes.
When Plane-Polarized Light is passed through a chiral Compound
then plane of PPL rotate by certain degree.

13. • The compound which is able to rotate plane of polarized light
by certain degree is called as optically active substance.
• Optical isomers: The compounds having same molecular
formula but different optical rotations are called as optical
isomers.
Optical Isomers
Disteriomers
Optical isomers which are not
mirror images of each other
Enantiomers
Optical isomers which are non super-
imposible mirror images of each other
CHO
C
C
CH2OH
OHH
HOH
CHO
C
C
CH2OH
OHH
OHH
C
COOH
CH3
NH2H C
COOH
CH3
HNH2
mirror plane

14. Enantiomers are the non superimposible mirror
images of each other
Disteriomers are not the mirror images of each other

15. Configuration at Chiral C atom:
• The orientation of groups or atoms in space around chiral C
atom is called as configuration
• Scientist Cahn, Ingold and Prelog prescribed the method for
assigning configuration at Chiral C atom called as absolute
configuration.
Rules:
i) Represent the molecule in Fscher representation.

16. 16
16
Fischer Projections - representation of a three-dimensional
molecule as a flat structure. A tetrahedral carbon is represented by
two crossed lines:
Vertical line is going back behind the plane of the Paper i.e. away
from you
Horizontal line is coming out of the plane of the paper i.e. toward
you
lactic acid
CO2H
CH3
HO H
H3C
CO2H
OH
H
CO2H
CH3
H OH
H3C
CO2H
H
OH
OHH
CO2H
CH3
HHO
CO2H
CH3

17. ii) Assign priority sequence to the four groups attached to chiral
C atom.
a) Consider directly bonded atoms to the chiral C atoms.
Assign their At. No. The atom with highest Atomic Number
has 1 priority or highest priority. Assign priority sequence 1
to 4 depending on their At. No. in decreasing order.
eg: 1-chloroethanol,
CH3CHCl(OH).C
CH3
Cl
OHH
The atomic numbers of the atoms are; Cl
(17), O (8), C (6) of CH3, and H(1)
Therefore, the priority order given to the
four groups would be
-Cl > –OH > -CH3 > -H.
1 2 3 4
C
CH3
Cl
OHH
1
2
3
4

18. • In case of alkyl group, the group having large size has highest
priority.
• For example in 2-butanol, the four different groups attached to
chiral C aoms are –CH3, -C2H5, -OH and H. The highest
priority group would be –OH and the lowest priority group
would be H.
• Between the groups –CH3 and –C2H5, the –C2H5 group is larger
in size than –CH3. Therefore –C2H5 highest priority than –CH3.
C
C2H5
CH3
HOH
The priority sequence is
-OH > –C2H5 > -CH3 > -H 1
2
3
4

19. In order to decide priority for groups having double or triple
bonds;
Each bond is duplicated in case of a double bond and
Triplicated for a triple bond.
Thus, the consideration is as follows
C is equivalent to C
C
C
C
C
C
C
C
O C C
O
O
is equivalent to
is equivalent to

20. Group priority orders for some groups are
C
CHO
COOH
OHCH3
2
1
3
4
C
NH2
COOH
CH3
H
1
2
3
4
C
CHO
OH
CH2OH
H
1
2
3
4
C
C
CH3
HC
H
1
2
3
N
CH2
4

21. v. If it is not then, perform the allowed manipulations of the
Fischer projection to place the lowest priority group at the
bottom.
2. If one group of a Fischer projection is held steady, the other
three groups can be rotated clockwise or counterclockwise.
CO2H
CH3
H OH
CO2H
H
HO CH3
hold
steady
(R) (R)
iv. To assign absolute configuration, in the Fischer projection
the lowest priority group should be present at the bottom.

22. • Manipulation of Fischer Projections
Fischer projections can be rotated by 180° only! But not by 90˚
CO2H
CH3
H OH
(R)
90 °
H
OH
CH3HO2C
(S)
°
• A 90° rotation inverts the stereochemistry and is illegal!
CO2H
CH3
H OH
CO2H
CH3
HHO
(R) (R)
CO2H
CH3
OHH
CO2H
CH3
HO H
(S) (S)
180 ° 180 °

23. OH
C CH3
H
C2H5
1
2
3
4
Rotate other three
group clockwise
direction
CH3
C H
OH
C2H5
Step-1:
CH3
C H
OH
C2H5
1
2
3
4
Step-2:
Hold C2H5
Group Steady
Assign Priority
orders to the
groups
CH3
C H
OH
C2H5
1
2
3
4
Number 1 to 4 indicate priority order

24. C
CH3
COOH
H OH C
CH3
COOH
H OH 1
2
3
4
C
H
COOH
HO CH33
2
4
1
Step-2: Rotate other
three group
anti-clockwise direction
Step-1: Assign
Priority orders
to the groups
C
CH3
COOH
H OH 1
2
3
4
Hold -COOH
Group Steady

25.  After Manipulation of the formula assign R or S onfiguration
at Chiral C atom:
 If the priority of the groups 123 are clockwise then
assign the center as R configuration
 If 123 are counterclockwise then assign the center as S
configuration
OH
C CH3
H
C2H5
1
2
3
4
The priority of the groups 123 is
anti- clockwise.
Therefore configuration is S

26. C
H
COOH
HO CH33
2
4
1
The priority of the groups 123 is
clockwise
Therefore configuration is R.
C
H
COOH
H3C NH21
2
4
3
The priority of the groups 123 is
Anti-clockwise
Therefore configuration is S.

28. Three stereoisomers of 2,3-butanediol
2R,3R 2S,3S 2R,3S
Optically inactive
It is Mesomer
CH3
CH3
OHH
HHOH OH
CH3
CH3
HHO H
CH3
CH3
OH
OHH
Optically active Optically active

29. Number of optical isomers to the molecule with chiral C atom = 2n
where, n = number of chiral C atoms in molecule
Ex: i) D-glucose has 4 chiral C atoms.
Therefore number of optical isomers = 24 = 16
ii) 2-3 butane diol has 2 chiral C atoms, hence number of optical
isomers = 22 = 4
CHO
C OHH
C
C
C
CH2OH
H
OHH
HHO
OH
1
2
3
4
5
6
D-Glucose: Carbon
2, 3, 4, 5 are chiral
CH3
C OHH
C
CH3
HHO
2-3 butane diol : Carbon 2,
3, are chiral

30. Relative Configuration
In a compound consists of large number of chiral C atoms then
configuration is assigned at each chiral C atom. Such process is
tome consuming and inconvenient. Hence the another method
called as relative configuration was prescribed. It is mostly used
to describe the isomers of carbohydraates.
Rules:
i) Represent the molecule in Fscher representation.
ii) Number the C atoms in the vertical chain according to rules
of IUPAC.
iii) Select a C atom in vertical chain which at longest distance
from carbonyl carbon.
iv) If at this Chiral C atom position of –OH group is right hand
side of vertical chain of C atoms then configuration D.
v) If at this Chiral C atom position of –OH group is left hand
side of vertical chain of C atoms then configuration L.
Capital letter D and L do not same as that of small letter d
and l used for dextrorotatory and laevorotatory.

31. Examples: Glyceraldehyde
C
CHO
CH2OH
OHH
1
2
3
-OH group on right hand side of
vertical chain of C atoms hence
configuration D
C
CHO
CH2OH
HOH
1
2
3
-OH group on left hand side of
vertical chain of C atoms hence
configuration L
Carbon 2 to 5 are chiral. C-5 is at longest
distance from >C=O carbon. Hence
configuration is assigned at C-5. At this C atom
-OH group on right hand side of vertical
chain of C atoms hence configuration D
CHO
C OHH
C
C
C
CH2OH
H
OHH
HHO
OH
1
2
3
4
5
6

32. Mesomer: The Compound consisting of 2 chiral C atoms but do not show
optical activity. Such molecule have plane of symmetry.
Example: Meso-Tartaric Acid. This molecule is symmetric along horizontal
plane of symmetry (see fig). C-2 and C-3 are the chiral C atoms. The
configuration at C-2 and C-3 is opposite to each other [C-2 R and C-3 S].
Hence optical rotation at these centers is opposite to each other. Thus optical
rotation by one C is exactly cancelled by other C atom and result is no optical
rotation.
C
COOH
C
OHH
1
2
3
COOH
OHH
4 Plane of symmetry
Racemic Mixture: The mixture of enantiomers in equal concentration is optically
inactive.
Such mixture is called as racemic mixture.
Optical rotation by two enantiomers is opposite to each other. Thus optical rotation
by one enantiomer is exactly cancelled by other and result is no optical rotation.
Imp Note: Optical rotation is
additive and vector property

33. Stereochemistry of Cyclohexane
•Cycloalkanes are saturated cyclic hydrocarbons
• Have the general formula (CnH2n)
•Some of the cycloalkanes are represented below

34. Bayer’s Strain Theory
• Cycloalkanes are less flexible than open chain alkanes
• Bayer believed that cycloalkanes, including cyclohexane, have
planar structures.
• Carbon in cycloalakanes is sp3 hybrid. Thus expected C-C-C bond
angele is 109° 28’.
• The expected C-C-C bond angles in different cycloalkanes are as
follows: these angles are presented by considering that these
molecules are planar.

35. • All cycloalkanes, except cyclopentane, become unstable
because of angle strain in the molecule. According to Bayer,
angle strain arises in molecule since C atom is SP3 hybrid and
around it bond should have 109˚28′, but it is either less or
greater than this value.
• Angle Strain = 109˚28′ - bond angle in cycloalkane
• The angle strain are 49 for cyclopropane, 19 for cyclobutane,
1 for cyclopentane and 11 for cyclohexane.
• Except cyclopentane, in all other cyclic alkanes the bond angle
is either very less or very greater that sp3 hybrid bond angle.
109 28’
60
49
109 28’
9019
108
109 28’
1
120
109 28’
11

36. • According to Bayer’s strain theory due to angle strain
cycloalanes are unstable.
• But actually it has been observed that except cyclopropane
and cyclobutane other cycloalkanes are highly stable. This
clear from the combustion energies per -CH2- group.
• Combustion energies per -CH2- group for acyclic (open
structure) alkanes compounds 158.4 K cal per mole
-CH2-[ ]n + 3/2nO2  nCO2 + nH2O + Heat
• They are, 166.4 for cyclopropane, 164 for cyclobutane, and
very close to 157.4 K cal per mole for other cycloalkanes.
(see tablw on next slide)

37. 4-37
Ring Strain in Cycloalkanes
• The relative stabilites of cycloalkanes are determined by
measuring their heats of combustion.
Ring Size Heat per CH2
kcal/mol
Ring Strain per
CH2, kcal/mol
Total Ring
Strain, kcal/mol
Long-Chain
Alkane
157.4 0.0 0.0
3 166.6 9.2 27.6
4 164.0 6.6 26.4
5 158.7 1.3 6.5
6 157.4 0.0 0.0
7 158.3 0.9 6.3
8 158.6 1.2 9.6
• The more heat per CH2, the less stable the alkane.

38. • Interpretation of Bayer is incorrect since except cyclopropane
and cyclobutane all other molecules are nonplanar. As planarity
of molecules is loosed bond angle become equal to 109˚28’.
Thus other than cyclopropane and cyclobutane all other
cycloalkane do not consists of angle strain.
cyclopropane cyclobutane
cyclopentane cyclohexane

39. cyclopropane
cyclobutane
cyclopentane
cyclohexane

40. Steriochemistry of Cyclohexane
Molecular Formula: C6H12
It has hexagonal but nonplanar structure.
The hexagonal ring in two forms can be
represented as follows:
1
2 3
4
56
1
2 3
4
56
or
• This structure has chair like shape hence it is called as chair
conformation of cyclohexane.
• In these structure C-2, C-3, C-5 and C-6 are in horizontal plane
while C-1 and C-4 are one is to above side of horizontal plane
while another lie lower side of the horizontal plane
• With hydrogen atoms the structure of cyclohexane can be shown
as follows:

41. C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H

or
• With hydrogen atoms the structure of cyclohexane can
be shown as follows:

42. • In the structure presented here, one can easily observe that
on each C atom one H atom (shown by blue colour) is
orientated along vertical axis, either in upward or
downward direction. They are called as axial H atoms.
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
HAxial H atoms

43. H
H
H
H
H
HH
H
H
H
H
H
H
H
H
H
H
H
Equitorial
H atoms
• In the structure presented here, one can easily observe that
on each C atom one H atom (shown by red colour) is
orientated in horizontal plane, either slanting upward or
downward direction. It is called as equatorial H atoms.

44. • As seen earlier Cyclohexane exists in two Chair conformations
which are interconvertible with each other.
H
H
H
H
H
H H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
• When one conformation of cyclohexane changes to another,
axial H atoms in one chair conformation becomes equatorial in
another conformation. Likely equatorial H atoms in one chair
conformation becomes axial in another conformation (see
above diagrams carefully).

46. • As seen earlier Cyclohexane exists in two Chair conformations
which are interconvertible with each other. When one
conformation changes to another conformation, changes take
place in series, and other less stable conformations of
cyclohexane are formed. This is represented below.
Chair
Conformation Half-Chair
Conformation
Boat
Conformation
Twist Boat
Conformation
Chair
Conformation
Half-Chair
Conformation
Boat
Conformation
Two chair forms exists
in equilibrium with
each other

47. 1) Chair Conformation:
• It is most stable conformation of cyclohexane.
• In chair conformation H atoms on adjutant C
are in staggered position (see Newmans
projection) hence minimum steric interactions
exists between them. This gives rise minimum
energy and highest stability to this
conformation.
• Angle strain is absent in chair conformation.
H
H
H
CH
H
C H
h
H
HH
H H
Newmans projection
of chair conformation
2) Half-Chair Conformation:
• It is least stable conformation of cyclohexane.
• In this conformation there is angle strain in
molecule at C4 carbon atom.
• Angle strain increases energy of molecule and
hence it is least stable.

48. 3) Boat Conformation:
• In boat conformation H atoms on adjutant C
are in ecliped position (see Newmans
projection) hence maximum steric interactions
exists between them.
• The flag pole interactions between H atoms
also increases the energy of conformation.
• Both of these interactions increases the energy
of this conformation and make it less stable.
• Angle strain is absent in boat conformation.
• It is less stable than chair conformation of
cyclohexane but more stable than half chair
conformation.
H
H
C
H
H
H
H
CH
H
H HH H
H H
Flag pole Hatoms

49. 4) Twist Boat Conformation:
•It formed twisting of boat conformation due to
steric interaction between flag pole H atoms.
•In twist boat conformation H atoms on adjutant
C are in ecliped position but moves slightly away
from each other. Hence steric interactions exists
between them but less than boat conformation.
• The flag pole interactions between H atoms
decreases in this conformation.
• Angle strain is absent in boat conformation.
•It is less stable than chair conformation of
cyclohexane but more stable than boat
conformation.
Stability order:
Chair conformation> Twist boat conformation > boat conformation >
Half Chair conformation

50. Substituted Cyclohexane:
1) Monosubstituted Cyclohexane
a) Methyl cyclohexane:
• It can exists in axial and equatorial conformation.
• Axial Conformation is less stable than equitorial
conformation
• In axial conformation methyl H are close to axial H
at C3 and C5. hence there exists steric interaction
between then called as 1,3 diaxial interaction.
• 1,3 diaxial interaction decreases the stability of axial
methyl cyclohexane. Hence, it less stable.
• In equitorial conformation methyl H are away from
H atoms of cyclohexane. Hence there exists
minimum steric interaction between them, which
increases the stability of equitorial methyl
cyclohexane.
• Hence most of the methyl cyclohexane exists in
equitorial conformation.
H3C
Equitorial - Methyl cyclohexane
C H
HH
H
H
Axial - Methyl cyclohexane
1 2 3

51. Substituted Cyclohexane:
1) Di-substituted Cyclohexane
• In di-substituted we may have 1-2, 1-3 and 1-4 di-substitued
cyclohexane.
1
2
1,2 Axial i.e. a, a
conformation
1
2
1 equitorial,2 Axial
i.e. e, a conformation
1
2
1Axial 2 equitoriali.e.
a, e conformation
1,2 equitorial i.e. e,e
conformation
• According to the positions of groups in space, in 1-2, di-
substituted we have following 4-conformations.

52. • 1-2 di-sustituted Conformations can be clsssified as cis and trans.
• In Cis conformation both substituents lie in same plane i.e. either in
upper plane or lower plane.
• IF one substituent is axial and another is equatorial then it lie in same
plane, hence it cis conformation.
• IF both substituent are axial or equatorial then they lie in different
plane, hence it trans conformation.
1
2
1,2 Axial i.e. a, a
conformation
1,2 equitorial i.e. e,e
conformation
Trans Conformations
1
2
1 equitorial,2 Axial
i.e. e, a conformation
1
2
1Axial 2 equitoriali.e.
a, e conformation
Cis- Conformations