1. The document describes different types of isomers including structural isomers, stereoisomers, enantiomers, and diastereomers. 2. It provides examples of compounds with one and two chiral carbon atoms, and explains that compounds with a chiral carbon can exist as non-superimposable mirror images called enantiomers. 3. Compounds with two chiral carbons can exist as enantiomers or diastereomers, which differ in their physical properties, or can be meso compounds that are superimposable on their mirror images.

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)]