1. 1
Organic Chemistry
Course Number: PCH 1120-217
Lecture # 7
Sunday September 22, 2013
Structural Isomerism-Stereoisomerism with
One and Two Chiral Carbons
Prof. Oludotun A. Phillips
Room # 2-81, 2nd Floor Pharmacy Building
Email: dphillips@hsc.edu.kw
Tel: 24986070

2. 2
Learning Objectives
At the end of the class students should be able to:
 describe structural and stereo-isomerism.
 identify chiral carbon / stereocenter and achiral
carbon.
 draw structures of stereoisomers.
 identify enantiomers and diastereomers.
 distinguish between diastereomers and meso
compounds.
 describe optical activity in stereoisomers.

3. 3
Relationship Among Isomers
Geometric isomers
Rigidity in a molecule leads to isomerism

4. 4
Structural (Constitutional) Isomers
 Constitutional isomers: compounds with the same
molecular formula but different connectivity
(order of attachment) of their atoms.
 There are two constitutional isomers with molecular
formula C4H10.
CH 3 CH2 CH2 CH3 CH3 CHCH3
CH3
B u ta n e
(b p -0 .5 ° C )
2 -M e th y l p ro p a n e
(b p -1 1 .6 °C )

5. 5
Structural (Constitutional) Isomers
 The potential for constitutional isomerism is
enormous:
4,111,846,7 63
4,347
75
3
1
Constitutional
Isomers
Molecular
Formula
C H 4
C 5 H 1 2
C 1 0 H 2 2
C 1 5 H 3 2
C 3 0 H 6 2
36,797,588C 2 5 H 5 2

6. 6
Assignment
Submit to me during the next class:-
Questions:
1. How many constitutional isomers would you expect
for each of the molecular formulas:
C5H12, C6H14, C2H6O and C4H10O?
2. Draw all the possible constitutional isomers for the
molecular formulas above.

7. 7
Stereoisomerism
 Stereoisomers: compounds with the same
molecular formula and same connectivity (order of
attachment) of their atoms, but different 3D
(3-dimensional) orientations of their atoms in
space.
 Stereoisomerism relates to the three dimensional
structure of the molecule in space.
 Stereochemistry is the study of the orientation of
molecules in space, relating how the atoms are
arranged in space relative to each other.

8. 8
Stereoisomerism
 Three aspects of Stereochemistry:
1. Chirality of molecules: with right- or left-handed
arrangement of the atoms around a carbon atom.
 Chiral (from the Greek: cheir; hand) molecules are
compds that are non-superimposable on their
mirror images are stereoisomers; e.g Enantiomers
2. Geometric isomers: relates to how rigidity in the
molecule can lead to isomerism.
3. Conformation of molecules: relates to how the change
in the shapes of molecules can lead to isomerism.

9. 9
Stereoisomers with One Chiral Carbon Atom
 A carbon with four different bonded groups is referred to
as a Chiral Carbon (a Stereocenter, Chiral center;
Stereocenter, Stereogenic center):
I C C l
F
B r
H 3C C C H 2 C H 3
C H 2 C H 2C H 3
C H 2 C H 2C H 2C H 3
C hiral C arbons
(Stereocenter)
all four groups
are different
H 3C H 2C C C H 3
F
H

10. 10
Stereoisomers with One Chiral Carbon Atom
 On the other hand, a carbon with all the four or two of
the groups the same is referred to as an Achiral
carbon:
H C H
H
H
achiral
Carbons
all the four groups
are the sam e
H3C C CH3
CH3
CH3
H3CH2C C CH2CH3
CH2CH3
CH2CH3

11. 11
Stereoisomers with One Chiral Carbon Atom
 A Chiral carbon can exist in either of two 3D structural
arrangements, because of the tetrahedral geometry.
 The Two 3D representations are non-superimposable,
mirror-image configurations referred to as
Enantiomers.
 Configuration refers to the orientation of the groups
around the chiral carbon (stereocenter).
 For example the two structural representation of Lactic
acid, shown in next slide are in 3D, tetrahedral
geometry of the chiral carbon:…

12. 12
Stereoisomers with One Chiral Carbon Atom
The two structural forms
are referred to as a pair
of Enantiomers.
 Structure (i) is different from Structure (ii), they are both
non-superimposable mirror images.
 Each structural form exists independently: e.g.
 isomer (i): L(+)-lactic acid - is produced in muscles of the
body during exercise and responsible for the soreness.
 while isomer (ii): D(+)-lactic acid is found in sour milk.
C O 2 H
C
H
H 3 C
O H
C O 2H
C
H
C H 3
H O
Lactic A cid: m irror im ages,
both structures are different
Structure (i) Structure (ii)
(S) (R)

13. 13
Stereoisomers with One Chiral Carbon Atom
 Enantiomers of
2-butanol shown below:
 To confirm that the mirror image of 2-butanol is not
superimposable on the original; rotate the mirror image by 180o
O H
C
H3 C
CH 2 CH 3
H
HO
C
CH3
H
CH3 CH2
O ri g in al m o l e cu l e M i rror i m ag e
O rigin al m o le cu le
OH
C
H 3 C
CH2 CH3
H
OH
C
CH3
H
CH3 CH2
OH
C
H3 C
H
CH2 CH3
M irro r im ag e Th e m irro r im ag e
ro tate d b y 180°
180°
rotate th e
m irro r im ag e
b y 180° ab o u t
th e C -O H b on d

14. 14
Stereoisomers with One Chiral Carbon Atom
 now try to fit one molecule (original on mirror image rotated
180o) on top of the other so that all groups and bonds match
or align exactly:
 The original and mirror images are non-superimposable….
 they are different molecules with identical physical properties
 They are enantiomers (non-superimposable mirror images)
OH
C
H3 C
CH 2 CH3
H
OH
C
H3 C
H
CH2 CH3
T h e o rigin al m o le cu le
T h e m irro r im ag e
tu rn e d b y 180°

15. 15
Stereoisomers with One Chiral Carbon Atom
Enantiomers:
 are non-superimposable on their mirror images
 are chiral molecules, having a center of chirality (the
carbon with all the four different groups attached is
the center of Chirality) – i.e. show handedness
 contain stereogenic center, stereocentre, chiral
center or asymmetric carbon atom.
 do not have a plane of symmetry

16. 16
Stereoisomers with One Chiral Carbon Atom
 The Physical properties of Enantiomers, such as
melting point, boiling point, and refractive index are
identical.
 Enantiomers differ in only one physical property, the
direction in which they rotate Plane-Polarized light.
 Hence, Enantiomers are Optically active molecules.
 Racemic mixture or Racemate: a mixture of equal (1:1)
parts of enantiomers.
 Racemic mixtures are Optically Inactive.

17. 17
Plane of Polarization of Plane Polarized Light
 Ordinary light waves - oscillate in all E-vector planes
perpendicular to its direction of propagation.
 Plane-Polarized Light waves – oscillate in a single E-
vector plane to its direction of propagation.

18. 18
Schematic diagram of a polarimeter
Measurement of Optical Activity – Polarimeter:
 Polarimeter: instrument for measuring the rotation
of plane-polarized light by a chiral compound.
 Optically active: indicates that a compound rotates
plane-polarized light.

19. 19
Rotation of Plane Polarized Light
Optical Activity:
 Dextrorotatory: clockwise rotation of plane-polarized
light
 Levorotatory: counterclockwise rotation of plane-
polarized light
 Specific rotation (): the observed rotation of an
optically active substance at a concentration of 1g/100mL
in a sample tube 10cm long; for a pure
liquid, concentration is in g/mL (density):
 Specific rotation ()= ________observed rotation “degrees”__________
length of sample tube “dm” X conc of sample “g/cm3”

20. 20
Stereoisomers with One Chiral Carbon Atom
 Enantiomerism: is the presence of a carbon with
four different groups bonded to it.
 A compound having one chiral carbon with four
different attached groups will exist in two non-
superimposable mirror-image forms.
 However, If a compound and its mirror image are
superimposable, they are identical and there is
no possibility of enantiomerism.
 Such molecules are referred to as Achiral (without
chirality):

21. 21
Isomers without Chiral Carbon Atom
 A carbon having the two or more identical groups around
it is referred to as Achiral carbon:
 E.g. Propanoic acid:
 Here two groups on the center carbon are
similar, therefore both compounds are identical.
CO2H
C
H
H
CH3
Propanoic acid: Both structures are identical
Structure (i) Structure (ii)
CO2H
C
H
H
H3C

22. 22
Stereoisomers with Two Chiral Carbon Atoms
1. Molecules with Two Dissimilar Chiral Carbon Atoms
– will give Enantiomers and Diastereomers:
 e.g. 2,3,4-Trihydroxybutanal have two
stereocenters.
 Maximum possible number of stereoisomers = 2n;
(n= number of chiral carbons)
 22 = 4 stereoisomers are possible
HO CH2 -CH -CH- CH
O H O H
O
* *

23. 23
Stereoisomers with Two Chiral Carbon Atoms
1. Molecules with Two Dissimilar Chiral Carbon Atoms:
Enatiomers and Diastereomers:
 Four Stereoisomers of 2,3,4-Trihydroxybutanal are
possible:
 Structures AC, AD, BC and BD are Diastereomers.
C
C
H O H
C H O
O H
C H 2 O H
H
C
C
HH O
C H O
H O
C H 2 O H
H H
C H 2 O H
H O
C
C
H O H
C H O
C
C
HH O
C H O
H
C H 2 O H
O H
A p a i r o f e n a n ti o m e rs
(E ry th ro s e )
A p a i r o f e n a n ti o m e rs
(T h re o s e )
A. B. C. D.

24. 24
Stereoisomers with Two Dissimilar Chiral Carbon Atoms
 Diastereomers are stereoisomers having more
than one chiral carbon atom, but they are not
mirror images.
 Diastereomers differ in physical properties such as:
 melting points,
 boiling points,
 densities refractive indices, and
 if they are chiral, specific rotations can differ.

25. 25
Stereoisomers with Two Chiral Carbon Atoms
2. Molecules with Two Similar Chiral Carbon Atoms -
gives Enatiomers, Diastereomers and Meso
Compounds:
 Meso compound: an achiral compound having two or
more stereocenters that is superimposable on its mirror
image.
 e.g. as may be found in
tartaric acid
 Have two stereocenters; hence the expected number of
stereoisomers 2n = 22 = 4; but only three stereoisomers
truly exist
CC
O H
CO O H
O H
H
H O O C
H

26. 26
Stereoisomers with Two Similar Chiral Carbon Atoms
 Meso compounds (E and F) are achiral compounds therefore they are
optically inactive molecules (they are the same molecule).
 Compounds (G and H) are Enantiomers.
 While compounds (E and G), and (E and H) are Diastereomers.
E. F. G. H.
C
C
H OH
COOH
OH
COOH
H
C
C
HHO
COOH
HO
COOH
H
C
C
H OH
COOH
H
COOH
HO
C
C
HHO
COOH
H
COOH
OH
A pair of enantiomersA meso compound
(plane of symmetry)
C
C
H
COOH
OHH
COOH
OH
180o
identical C
C
HO
COOH
OHH
COOH
H
different
180o
Has a Plane of Symmetry!!

27. 27
Rotation of Plane Polarized Light
Optical Activity:
 Like melting point and boiling point, Specific
rotation is a physical property of a compound e.g.
DD
H 3 C
C
O H
H
CO O H
CH3
C
H O
H
COO H
[ ]
21
= -2.6°= + 2.6°
21
[ ]
(R )-(-)-L actati c ac i d(S)-(+ )-Lacti c aci d

28. 28
Questions - Stereoisomerism
Submit to me during the next class:-
 How many stereoisomers are possible for the
carbohydrate molecule 2-Deoxyribose shown below?
 Draw all the possible stereoisomers of 2-Deoxyribose
shown above, and identify the enantiomers and
diastereomers.
CH2
OH
H
C
OH
H
C
OH
CH2 C H
O
2-Deoxyribose
[A carbohydrate that is a structural component of
the genetic material Deoxyribonucleic acid (DNA)]