This document provides an overview of stereochemistry concepts including: - Recognizing different types of isomers such as constitutional and stereoisomers. - Analyzing the conformations of alkanes and cycloalkanes using Newman, Fischer, and sawhorse projections to identify stable chair and staggered conformations. - Explaining the concepts of chirality, stereogenic centers, and how to determine R/S and E/Z configurations in stereoisomers using wedge diagrams and Fischer projections.

1. STEREOCHEMISTRY

2. Learning objectives
• Recognize types of isomerism
• Recognize and differentiate alkane and cycloalkane
conformers
• Recognize, draw and analyze open alkane structures using the
different types of projections (Newman, saw-horse and
Fischer)
• Draw cyclic alkane structures (chair and boat confirmations.)
• Analyze and explain conformational stabilities of alkane and
cycloalkanes
• Recognize and explain the concept of chirality
• Define and explain key terms in optical stereochemistry
• Determine R and S sequence in organic molecule
• Determine trans and cis, E and Z sequence in organic
molecule
• Determine other stereoisomerism such as (+), (-); D & L and
meso.

3. Classification of isomers

4. 4
• Recall that isomers are different compounds with the same
molecular formula.
• The two major classes of isomers are constitutional isomers
and stereoisomers.
Constitutional/structural isomers have different IUPAC
names, the same or different functional groups, different
physical properties and different chemical properties.
Stereoisomers differ only in the way the atoms are
oriented in space. They have identical IUPAC names
(except for a prefix like cis or trans). They always have
the same functional group(s).
• A particular three-dimensional arrangement is called a
configuration. Stereoisomers differ in configuration.
The Two Major Classes of Isomers:
* REFER TO THE SLIDE #3

5. 5
A comparison of constitutional isomers and geometric stereoisomers
Stereochemistry
Stereoisomers may be geometric (cis/trans) or optical.
Optical isomers are chiral and exhibit optical activity.

6. Three main topics to be discussed here:
a) Stereoisomers - isomers that are different to each
other only in the way the atoms are oriented in space.
b) Geometric isomers – isomers that owe their
existence to hindered rotation about double bond
c) Conformation – different spatial arrangements of a
molecule that are generated by rotations about single
bonds.

7. Conformational analysis of straight chain alkanes
•Conformational analysis is the study of how conformational
factors affect the structure of a molecule and its properties
•Rotation about single bond produces isomer that differ in
conformation.
•Conformers have same connection, interconverts rapidly, thus
cannot be isolated.
•Can be represented in 2 ways:
sawhorse representation or Newman projection
H
H H
H
H H
HH
H
H
H
H
rotate
Conformation

8. Conformation of ethane, CH3CH3
Two conformational isomers or conformers.
Eclipsed form = all hydrogen atoms nearest to each other.
Staggered form = all hydrogen atoms are furthest apart.
H
H
H H
H
H H
HH
H
H
H
H
H
H H
H
H H
HH
H
H
H
Eclipsed Staggered
Newman projection
Ball-stick formula
sawhorse representation
rotate

9. Conformational analysis of ethane
• Only two conformers.
• Torsional strain: resistance to rotation.
• For ethane, only 3.0 kcal/mol (or 12 kJ/mol)

10. • Similar rotation about the C-C single bond as in ethane.
• There are two conformational isomer.
• The potential energy different are about 3.3 kcal/mol, almost
similar to ethane. Slight increase due to bulkier methyl group.
Conformation of propane

11. For butane, C4H10, two different structures can be drawn, n-
butane and isobutane.
The two molecules are different and also having different
physical properties:
Physical properties n-butane isobutane
b.p 0 o
C -12 o
C
m.p. -138 o
C -159 o
C
Density 0.622 0.604
Solubility in 100 ml water 1813 ml 1320 ml
Conformation of butane

12. n-Butane conformation
•Consider the two central carbon atoms in the molecule.
•6 different conformers can be formed.
•60° rotation along central C-C bond
CH3
H H
CH3
HH
CH3
H H
H
HH3C
CH3
H H
H
CH3H
Anti
conformation (IV)
Gauche
conformation (VI)
CH3
H H
H
H
H3C
Gauche
configuration (II)
Eclipsed
conformation (III)
Eclipsed
conformation (V)
Eclipsed
conformation (I)
CH3
H H
H3C
H
H
CH3
H H
H
CH3
H

13. Conformational Analysis of n-Butane

14. Formed by the linkage of carbon atoms to form a ring.
The most common are the five and six-membered rings.
When a carbon atom is bonded to four other atoms, bonding
orbital (sp3
) are directed towards the corner of tetrahedron.
The angle between each pair of orbital is 109.5o
with maximum
orbital overlapped.
In cyclopropane, this angle cannot be achieved, but instead
obtained 60o
angle. Thus, orbital overlapped is greatly reduced.
The molecule is unstable due to angle strain.
Conformation of cyclic compounds

15. Conformation of cyclohexane
The most stable ring structure is cyclohexane which can
achieved the maximum angle of 109.5o
with maximum
overlapped of bonding orbital.
Strain free – no angle or torsional strain.
There are several conformations of cyclohexane:
Chair
conformation
Boat
conformation
Twist-boat
conformation
Half-chair
conformation
Chair conformation – most stable

16. Conformational analysis of cyclohexane

17. Chair conformation is the most stable. Why???
H
H
H H
HH
Staggered
H
H
H
H
H
H
H
H
H
H
1
23
4
5 6
H
H
H
H
H
HH
H
Looking through C4/C5
and C2/C1
The conformation obtain
similar to staggered form in ethane
H
H
All C-H bond are staggered (similar to staggered form of
ethane)

18. Chair Conformer

19. Boat Conformer

20. Axial and equatorial bonds
In the chair conformation, there are two types of C-H bonds:
the axial bond and the equatorial bond.
Axial bonds Equatorial bonds Axial and equatorial bonds

21. This can be visualized by imagining the carbon atoms
located on a plane and there will be:
6 C-H bonds above the plane
6 C-H bonds below the plane
From this, 3 C-H axial bonds pointing upwards
3 C-H axial bonds pointing downwards
3 C-H equatorial pointing upwards
3 C-H equatorial bonds pointing downwards
Cis- when two group on the same face of the ring
Trans - when two group on the opposite face of the ring
(regardless whether axial or equatorial, or adjacent)

22. Axial and Equatorial Positions

23. Conformation of monosubstituted cyclohexane
Example: methylcyclohexane
•Axial and equatorial conformations are not equally stable
at room temperature.
•Substituents is always more stable in equatorial position
•Energy diff. is due to 1,3-diaxial interaction
CH3
H
H
H
H
H
H
H
H
H
1
23
4
5 6
H
H
H
H
H
CH3
H
H
Looking through C2/C3
and C6/C5
H
1,3-Diaxial interaction

24. Hydrogen atoms of the axial methyl group on C1 are too close
to the hydrogens three carbon away on C3 and C5
The degree of unstability is about 0.9 kcal/mol for each
interaction
1,3-Diaxial Interactions
ring flip

25. No 1,3-diaxial interaction when CH3 group is equatorial.
Hence, this conformation is more stable since equatorial bonds
are pointing towards the outside.
H
H
H
H
H
H
H
H3C
H
H
1
23
4
5 6
H
H
H
H
CH3
HH
H
Looking through C2/C3
and C6/C5
H
At room temperature, most methylcyclohexane conformation
(95%) occurs in which the methyl group is at uncrowded
equatorial position.

26. Conformation of di-substituted cyclohexane
Example: 1,2-dimethylcyclohexane
Cis-1,2-dimethylcyclohexane
-one CH3 equatorial and one CH3 axial in both chair conformation
-each has two 1,3-diaxial interactions and one gauche butane interaction
The two conformations are exactly equal in energy.
CH3
1
23
4
5 6 CH3
axial-equatorialequatorial-axial
CH3
1
2
34
5 6
CH3
cis-1,2-dimethylcyclohexane
H
H
H
H

27. CH3
1
23
4
5 6
CH3
axial-axial equatorial-equatorial
CH3
CH3
1
2
34
5 6
trans-1,2-dimethylcyclohexane
more stable
H
H
H
H
Trans-1,2-dimethylcyclohexane
- either both CH3 equatorial (diequatorial) or both CH3 axial (diaxial)
- the diequatorial conformation only has one gauche butane interaction
- the diaxial has four 1,3-diaxial interactions
Therefore, the diequatorial conformers is more favoured (has lower
energy), occurs exclusively.

28. Conformation of di-substituted cyclohexane
Example: cis-1,3-dimethylcyclohexane

29. 29
• Although everything has a mirror image, mirror images
may or may not be superimposable.
• A molecule or object that is superimposable on its
mirror image is said to be achiral (lacking-chirality).
• A molecule or object that is not superimposable on its
mirror image is said to be chiral.
• Generally, a chiral carbon atom is sp3
with four different
attachments.
Chiral and Achiral Molecules:
Stereoisomers

30. 30
• Some molecules are like hands. Left and right hands are
mirror images, but they are not identical, or superimposable.
Chiral and Achiral Molecules:
Stereoisomers

31. 31
• We can now consider several molecules to determine
whether or not they are chiral.
Chiral and Achiral Molecules:
Stereoisomers

32. 32
• A carbon atom with four different groups is a chiral center.
• The case of 2-butanol. A and its mirror image labeled B are
not superimposable. Thus, 2-butanol is a chiral molecule
and A and B are isomers.
• Non-superimposable mirror image stereoisomers like A and
B are called enantiomers .
Chiral and Achiral Molecules:
Stereoisomers

33. 33
• In general, a molecule with no stereogenic centers will not
be chiral.
• With one stereogenic center, a molecule will always be
chiral.
• With two or more stereogenic centers, a molecule may or
may not be chiral.
• Achiral molecules usually contain a plane of symmetry but
chiral molecules do not.
• A plane of symmetry is a mirror plane that cuts the
molecule in half, so that one half of the molecule is a
reflection of the other half.
Chiral and Achiral Molecules:
Stereoisomers

34. 34
Chiral and Achiral Molecules:
Two identical attachments on an sp3
carbon atom eliminates the possibility
of a chiral center.
Stereoisomers

35. 35
Summary of the Basic Principles of Chirality
• Everything has a mirror image. The fundamental question is
whether the molecule and its mirror image are
superimposable.
• If a molecule and its mirror image are not superimposable,
the molecule and its mirror image are chiral.
• The terms stereogenic center and chiral molecule are related
but distinct. In general, a chiral molecule must have one or
more stereogenic centers.
• The presence of a plane of symmetry makes a molecule
achiral.
Stereoisomers

36. 36
• To locate a stereogenic center, examine each tetrahedral
carbon atom in a molecule, and look at the four groups—not
the four atoms—bonded to it.
• Always omit from consideration all C atoms that cannot be
tetrahedral stereogenic centers. These include
CH2 and CH3 groups
Any sp or sp2
hybridized C
Stereogenic Centers:
Stereoisomers

37. 37
• Larger organic molecules can have two, three or even
hundreds of stereogenic centers.
Identifying of Stereogenic Centers:
Stereoisomers

38. 38
• To draw both enantiomers of a chiral compound such as 2-
butanol, use the typical convention for depicting a
tetrahedron: place two bonds in the plane, one in front of the
plane on a wedge, and one behind the plane on a dash.
Then, to form the first enantiomer, arbitrarily place the four
groups—H, OH, CH3 and CH2CH3—on any bond to the
stereogenic center. Then draw the mirror image.
Drawing Stereogenic Centers - the wedge diagram:
Stereoisomers

39. 39
Three-dimensional representations for pairs of enantiomers
Drawing Stereogenic Centers - the wedge diagram:
Stereoisomers

40. 40
• Stereogenic centers may also occur at carbon atoms that are
part of a ring.
• To find stereogenic centers on ring carbons, always draw the
rings as flat polygons, and look for tetrahedral carbons that
are bonded to four different groups.
Contains a plane of symmetry
Identifying of Stereogenic Centers:
Stereoisomers

41. 41
• In 3-methylcyclohexene, the CH3 and H substituents that are
above and below the plane of the ring are drawn with
wedges and dashes as usual.
Drawing Stereogenic Centers - the wedge diagram:
Stereoisomers

42. 42
• Identify the chiral carbons in the compounds below.
Stereochemistry
Identifying of Stereogenic Centers:

43. 43
• In a Fischer projection of a chiral carbon and its mirror
image:
horizontal bonds project toward the viewer and vertical
bonds project away from the viewer.
• The test for non-superimposability is to slide one on top of
the other or rotate 180o
and attempt the same.
• Fischer projections of the two enantiomers of 2-butanol:
Stereochemistry
Drawing Stereogenic Centers – the Fischer Projection:
CH3
CH2CH3
H OH
CH3
CH2CH3
HO H
The chiral carbon atom
is at the center of the
crossed lines.

44. 44
• Fischer projections of a compound with 2 chiral carbons, (two
pairs of enantiomers).
• The maximum number of optical isomers is 2n
.
(where n = the number of chiral carbon atoms.)
The pairs are diastereomerically related.
Stereochemistry
Drawing Stereogenic Centers – the Fischer Projection:
CH3
OH
OH
COOH
H
H
CH3
H
H
COOH
HO
HO
CH3
OH
COOH
H
H
CH3
OH
COOH
H
HHO
HO

45. 45
• However, there may be severaI different Fischer projections
for the same compound depending upon the direction from
which is is viewed.
Are these structures the same or different ?
Stereochemistry
Drawing Stereogenic Centers – the Fischer Projection:
CH3
CH3
CH3 CH3
CH2CH3
CH3CH2 CH2CH3
CH2CH3
CH2=CH
CH2=CH CH2=CH
CH2=CH
OH OH
OH
HO
a b c d
This is a good place to use your models.

46. 46
• The three dimensional arrangement about a tetrahedral
carbon atom is referred to as its configuration.
• Early workers in the late 1800s including Fischer used the
terms D and L to label the two molecules in a non-
superimposable mirror image pair.
• D and L assignments were chemically related to the
structures of glyceraldehyde.
• More recently Cahn, Ingold and Prelog developed the R and
S system of assignment which is more convenient.
Labeling Stereogenic Centers:
Stereochemistry

47. 47
• Since enantiomers are two different compounds, they need
to be distinguished by name. This is done by adding the
prefix R or S to the IUPAC name of the enantiomer.
• Naming enantiomers with the prefixes R or S is called the
Cahn-Ingold-Prelog system.
• To designate enantiomers as R or S, priorities must be
assigned to each group bonded to the stereogenic center, in
order of decreasing atomic number. The atom of highest
atomic number gets the highest priority (1).
Labeling Stereogenic Centers with R or S:
Stereochemistry

48. 48
• If two atoms on a stereogenic center are the same, assign
priority based on the atomic number of the atoms bonded to
these atoms. One atom of higher atomic number determines
the higher priority.
Stereochemistry
Labeling Stereogenic Centers with R or S:

49. 49
• If two isotopes are bonded to the stereogenic center, assign
priorities in order of decreasing mass number. Thus, in
comparing the three isotopes of hydrogen, the order of
priorities is:
Stereochemistry
Labeling Stereogenic Centers with R or S:

50. 50
• To assign a priority to an atom that is part of a multiple bond, treat
a multiply bonded atom as an equivalent number of singly bonded
atoms. For example, the C of a C=O is considered to be bonded
to two O atoms.
• Other common multiple bonds are drawn below:
Stereochemistry
Labeling Stereogenic Centers with R or S:

51. 51
Figure 5.6 Examples of assigning priorities to stereogenic centers
Stereochemistry
Labeling Stereogenic Centers with R or S:

52. 52
Stereochemistry
Labeling Stereogenic Centers with R or S:

53. 53
Stereochemistry
Labeling Stereogenic Centers with R or S:

54. 54
Stereochemistry
Labeling Stereogenic Centers with R or S:

55. 55
Figure 5.7 Examples: Orienting the lowest priority group in back
Stereochemistry
Labeling Stereogenic Centers with R or S:

56. 56
• For a molecule with n stereogenic centers, the maximum number
of stereoisomers is 2n
. Let us consider the stepwise procedure for
finding all the possible stereoisomers of 2,3-dibromopentane.
Stereochemistry
Diastereomers:

57. 57
• If you have drawn the compound and the mirror image in the
described manner, you have only to do two operations to see if the
atoms align. Place B directly on top of A; and rotate B 180° and
place it on top of A to see if the atoms align.
• In this case, the atoms of A and B do not align, making A and B
nonsuperimposable mirror images—i.e., enantiomers. Thus, A
and B are two of the four possible stereoisomers of 2,3-
dibromopentane.
Stereochemistry
Diastereomers:

58. 58
• Switching the positions of H and Br (or any two groups) on one
stereogenic center of either A or B forms a new stereoisomer (labeled
C in this example), which is different from A and B. The mirror image
of C is labeled D. C and D are enantiomers.
• Stereoisomers that are not mirror images of one another are called
diastereomers. For example, A and C are diastereomers.
Stereochemistry
Diastereomers:

59. 59
Figure 5.8 Summary: The four stereoisomers of 2,3-dibromopentane
Stereochemistry
Diastereomers:

60. 60
• Let us now consider the stereoisomers of 2,3-dibromobutane. Since
this molecule has two stereogenic centers, the maximum number of
stereoisomers is 4.
Meso Compounds:
• To find all the stereoisomers of 2,3-dibromobutane, arbitrarily add the
H, Br, and CH3 groups to the stereogenic centers, forming one
stereoisomer A, and then draw its mirror image, B.
Stereochemistry

61. 61
• To find the other two stereoisomers if they exist, switch the
position of two groups on one stereogenic center of one
enantiomer only. In this case, switching the positions of H and Br
on one stereogenic center of A forms C, which is different from
both A and B.
• A meso compound is an achiral compound that contains
tetrahedral stereogenic centers. C is a meso compound.
Stereochemistry
Meso Compounds:

62. 62
• Compound C contains a plane of symmetry, and is achiral.
• Meso compounds generally contain a plane of symmetry so
that they possess two mirror image halves.
• Because one stereoisomer of 2,3-dibromobutane is
superimposable on its mirror image, there are only three
stereoisomers, not four.
Stereochemistry
Meso Compounds:

63. 63
Figure 5.9 Summary: The three stereoisomers 2,3-dibromobutane
Stereochemistry
Meso Compounds:

64. 64
• When a compound has more than one stereogenic center,
R and S configurations must be assigned to each of them.
R and S Assignments in Compounds with Two or More
Stereogenic Centers.
One stereoisomer of 2,3-dibromopentane
The complete name is (2S,3R)-2,3-dibromopentane
Stereochemistry

65. 65
• Consider 1,3-dibromocyclopentane. Since it has two stereogenic
centers, it has a maximum of four stereoisomers.
Disubstituted Cycloalkanes:
• Recall that a disubstituted cycloalkane can have two substituents
on the same side of the ring (cis isomer, A) or on opposite sides
of the ring (trans isomer, B). These compounds are
stereoisomers but not mirror images.
Stereochemistry

66. 66
• To find the other two stereoisomers if they exist, draw the mirror
images of each compound and determine whether the compound
and its mirror image are superimposable.
• The cis isomer is superimposable on its mirror image, making the
images identical. Thus, A is an achiral meso compound.
Stereochemistry
Disubstituted Cycloalkanes:

67. 67
• The trans isomer is not superimposable on its mirror image,
labeled C, making B and C different compounds. B and C are
enantiomers.
• Because one stereoisomer of 1,3-dibromocyclopentane is
superimposable on its mirror image, there are only three
stereoisomers, not four.
Stereochemistry
Disubstituted Cycloalkanes:

68. 68
Figure 5.10 Summary—Types of isomers
Stereochemistry

69. 69
Figure 5.11 Determining the relationship between two nonidentical molecules
Stereochemistry

70. 70
• The chemical and physical properties of two enantiomers
are identical except in their interaction with chiral
substances.
• The physical property that differs is the behavior when
subjected to plane-polarized light ( this physical property
is often called an optical property).
• Plane-polarized (polarized) light is light that has an
electric vector that oscillates in a single plane.
• Plane-polarized light arises from passing ordinary light
through a polarizer.
Optical Activity
Stereochemistry

71. 71
• Originally a natural polarizer, calcite or iceland spar, was
used. Today, polarimeters use a polarized lens similar to
that used in some sunglasses.
• A polarizer has a very uniform arrangement of molecules
such that only those light rays of white light (which is
diffuse) that are in the same plane as the polarizer
molecules are able to pass through.
• A polarimeter is an instrument that allows polarized light
to travel through a sample tube containing an organic
compound and permits measurement of the degree to
which the light is rotated.
Optical Activity
Stereochemistry

72. 72
• With achiral compounds, the light that exits the sample tube
remains unchanged. A compound that does not change
the plane of polarized light is said to be optically inactive.
Optical Activity
Stereochemistry

73. 73
• With chiral compounds, the plane of the polarized light is
rotated through an angle α. The angle α is measured in
degrees (°), and is called the observed rotation. A compound
that rotates polarized light is said to be optically active.
Optical Activity
Stereochemistry

74. 74
• The rotation of polarized light can be clockwise or
counterclockwise.
• If the rotation is clockwise (to the right of the noon position),
the compound is called dextrorotatory. The rotation is
labeled d or (+).
• If the rotation is counterclockwise, (to the left of noon), the
compound is called levorotatory. The rotation is labeled l or
(-).
• Two enantiomers rotate plane-polarized light to an equal
extent but in opposite directions. Thus, if enantiomer A
rotates polarized light +5°, the same concentration of
enantiomer B rotates it –5°.
• No relationship exists between R and S prefixes and the (+)
and (-) designations that indicate optical rotation.
Optical Activity
Stereochemistry

75. 75
• An equal amount of two enantiomers is called a racemic
mixture or a racemate. A racemic mixture is optically inactive.
Because two enantiomers rotate plane-polarized light to an
equal extent but in opposite directions, the rotations cancel, and
no rotation is observed.
Racemic Mixtures
Stereochemistry

76. 76
• Specific rotation is a standardized physical constant for the
amount that a chiral compound rotates plane-polarized light.
Specific rotation is denoted by the symbol [α] and defined
using a specific sample tube length (l, in dm), concentration
(c in g/mL), temperature (250
C) and wavelength (589 nm).
Stereochemistry
Racemic Mixtures

77. 77
• Enantiomeric excess (ee) is a measurement of the
excess of one enantiomer over the racemic mixture.
Enantiomeric excess and Optical purity: ee and op
ee = % of one enantiomer - % of the other enantiomer.
• Consider the following example: If a mixture contains 75% of
one enantiomer and 25% of the other, the enantiomeric
excess is 75% - 25% = 50%. Thus, there is a 50% excess of
one enantiomer over the racemic mixture.
• ee is numerically equal to Optical Purity.
• The optical purity can be calculated if the specific rotation [α]
of a mixture and the specific rotation [α] of a pure enantiomer
are known.
op = ([α] mixture/[α] pure enantiomer) x 100.
Stereochemistry

78. 78
• Since enantiomers have identical physical properties, they cannot be
separated by common physical techniques like distillation.
• Diastereomers and constitutional isomers have different physical
properties, and therefore can be separated by common physical
techniques.
Physical Properties of Stereoisomers:
Figure 5.12 The physical
properties of the three
stereoisomers of tartaric
acid.
Stereochemistry

79. 79
• Two enantiomers have exactly the same chemical properties except
for their reaction with chiral non-racemic reagents.
• Many drugs are chiral and often must react with a chiral receptor or
chiral enzyme to be effective. One enantiomer of a drug may
effectively treat a disease whereas its mirror image may be ineffective
or toxic.
Chemical Properties of Enantiomers:
Stereochemistry

80. Geometric Isomerism
Isomeration due to hindered rotation about C=C double bonds.
A disubstituted alkene can have substituents either on the same
side (cis) or opposite side (trans)
Example:
The two isomers of 2-butene can have structure I with b.p +4o
C
and assign as cis configuration and structure II, b.p. +1o
C and
assign configuration trans.
H
CH3
H
H3C
H
CH3
H3C
H
cis-2-butene trans-2-butene
I II

81. The two isomers of 2-butene do not interconvert because
free roation about the C=C is not possible.
Cis-trans isomerism occurs whenever both double bonded
carbons are attached to 2 different groups.
H
H
H3C
H3C
H
CH3
H3C
H3C
Isobutylene 2-Methylbutene
Cl
Cl
H
H3C
1,1-Dichloropropane
No geometric isomerism
CC
D
D
A
B
CC
D
D
B
A
= CC
C
D
A
B
CC
C
D
B
A
=
same isomer different isomer

82. (E) And (Z) Systems
Describes the arrangement of substituents around a double bond
that cannot be described by cis-trans system.
Cahn-Ingold Prelog sequence rule:
1) For each double bond, rank its substituents by atomic number-
atoms with high atomic number receive higher priority.
Br (35) > Cl (17) > O (16) > N (14) > C (12) > H (1)
H
CH3
H3C
H
trans-2-butene
E-2-butene
Higher
Lower
Lower
Higher
Lower
Higher
Lower
Higher
E double bond
(entgegen)
higher priority
groups on opposite
sides
Z double bond
(zusammen)
higher priority
groups on same
side
H
CH3
H
H3C
cis-2-butene
Z-2-butene

83. 2) If a decision cannot be reached by ranking the first atom,
look at the second, third or fourth atoms away from the double
bond until first point of difference is found
C
H
H
H C
H
H
C
H
H
H
lower higher
C
H
H
CH3
lower
C
H
CH3
CH3
higher
O H
lower
O C
H
H
H
higher
C
H
CH3
NH2
lower
C
H
CH3
Cl
higher

84. 3) Multiple bonded atoms are equivalent to the same number
of single-bonded atoms
OC
H
this O is bonded to C,C
this C is bonded to O,O,H
= C
H O
O
C
this O is bonded to C,C
this C is bonded to O,O,H

85. Geometric isomers in cyclic compounds
Most of the cyclic structures in organic molecules are either five-
or six-bonded.
Atoms bonded in cyclic structures are also not free to rotate about
the single bond.
By looking at the relative positions of the substituents on the
ring, geometric isomer can also be assigned.
CH3
OH
CH3
OH
above the plane
below the plane
trans-3-Methylcyclohexanol

86. Other methods representing cyclic structures
CH3
CH3
CH2CH3
H3CH2C
C(CH3)3
C(CH3)3
cis-1,2-Dimethylcyclohexane trans-1,3-Diethylcyclohexane
cis-1,4-di-tert-butylcyclohexane
H3C
H3C
H
H
trans-1,3-Dimethylcyclopentane

87. Name the following compound. Determine the geometric
isomerism of the double bond.
Exercise 3
H3C
CH3
OH
CH3
CH3
H3C
Cl H3C
H3C CH3
CHO
OHCOOH
NH2
CH3
a)
b)
c)
d)
e)
f)