1. Department of P.G Studies and Research in Chemistry
Jnanasahyadri, Shankaraghatta-577451
SUBMITTED TO,
THE CHAIRMAN,
Department of Chemistry,
Jnanasahyadri,
Shankaraghatta.
PRESENTED BY,
SHILPASHREE G S,
M.Sc., 1st Semester
Department of Chemistry,
Jnanasahyadri, Shankaraghatta.
1
2. CONTENTS
INTRODUCTION
E-Z SYSTEM OF NOMENCLATURE
GEOMETRICAL ISOMERISM DUE TO C=C, C=N, N=N BONDS
GEOMETRICAL ISOMERISM IN CYCLIC SYSTEMS
DETERMINATION OF CONFIGURATION OF THE GEOMETRICAL ISOMERS
CONCLUSION
REFERENCES
2
3. INTRODUCTION
• Isomers having the same structural formula but different relative spatial
arrangement of atoms or groups around a double bond are called geometrical
isomers, cis-trans isomers or E-Z isomers and this phenomenon is known as
geometrical isomerism.
• It is generally observed in alkenes and cyclic compounds due to their restricted
rotation around carbon-carbon bond.
• Geometrical isomers are examples of diastereomers because they are not mirror
images of each other.
• Due to rigidity of double bonds towards free rotation, these isomers have two
spatial arrangements. In one arrangement, called cis-, two similar or identical
atoms or groups are on the same side of the double bond.
• In the other arrangement called trans-, two similar or identical atoms or groups
are on the opposite side of the double bond.
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4. E-Z SYSTEM OF NOMENCLATURE
Let us consider a molecule in which the two carbon atoms of a double bond are
attached with four different halogens.
When we say that Br and Cl are trans to each other we can also say with equal degree
of confidence that I and Cl are cis to each other. It is thus different to name such a
substance either the cis or the trans isomer. To eliminate this confusion, a more general and
easy system for designating configuration about a double bond has been adopted. This
method, which is called the E and Z system, is based on priority system originally developed
by Cahn, Ingold and Prelog for use with optically active substance.
If the higher priority atoms or groups are on opposite side of double bond, the
configuration is designated as E (entgegen = across). If the higher priority atoms or groups
are on the same side of the double bond, the configuration is designated as Z (zusammen =
together).
Br F
C C
Cl
I
4
5. H H
CH3
C
H3
C C
1
2
1
2
Z-2-Butene
H CH3
H
C
H3
C C
2
2
1
1
E-2-Butene
H H
COOH
HOOC
C C
2 2
1 1
Z-But-2-ene-1,4-dioic acid
(Maleic acid)
HOOC H
COOH
H
C C
2
2
1
1
E-But-2-ene-1,4-dioic acid
(Fumaric acid)
I Cl
F
Br
C C
1
2
1
2
Z-1-Bromo-2-chloro-2
-fluoro-1-iodoethene
I F
Cl
Br
C C
2
2
1
1
E-1-Bromo-2-chloro-
2-fluoro-1-iodoethene
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6. GEOMETRICAL ISOMERISM DUE C=C, C=N, N=N BONDS
Geometrical isomerism is shown by various groups of compounds the major class
of compounds that exhibit geometrical isomerism are classified as:
i. Cyclic compounds
ii. Compounds having double bond:
C=C, C=N, N=N
Geometrical isomerism in compounds containing C=C bond:
In compounds containing C=C bonds, due to the presence of one σ (sigma) and one
π (pi) bond in carbon- carbon double bond, rotation around C=C bond is not possible.
The restricted rotation around C=C bond is responsible for geometrical isomerism.
If two similar groups are on the same side of C=C bond, this is called cis- isomer;
whereas, if two similar groups are on opposite side of C=C bond this is known as trans-
isomer.
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7. Example:
Geometrical isomerism in compounds containing C=N and N=N bond:
In addition to alkenes compounds containing C=N (aldoxime and ketoxime) and
N=N (azo compounds) bonds also show geometrical isomerism. However, in these cases,
‘syn’ (for cis) and ‘anti’ (for trans) are more commonly used.
In aldoximes, when –H and –OH groups are on the same side of the double bond,
the isomer is known as ‘syn’ (analogous to cis) and when these groups are on opposite
sides of the double bond, the isomer is known as ‘anti’ (analogous to trans).
In ketoximes, the prefix ‘syn’ indicates that the first named group and hydroxyl
groups are on the same side of double bond whereas prefix ‘anti’ indicates that they are
on the opposite side of the double bond.
H
H
CH3
C
H3
C C
cis-2- butene
CH3
H
H
C
H3
C C
trans-2-butene
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9. GEOMETRICAL ISOMERISM IN CYCLIC SYSTEMS
• Cyclic compounds such as the disubstituted derivatives of cyclopropane, cyclobutane,
cyclopentane and cyclohexane can also show cis-trans isomerism.
• The basic condition for such isomerism is that there should be sufficient hindrance to
rotation about a linkage between atoms.
(a) Disubstituted cyclopropanes: Only two isomers exist since position isomerism is not
possible.
(b) Disubstituted cyclobutanes and cyclopentanes: They exist as two position isomers
i.e., 1,2 and 1,3 and each of them exists as cis- and trans- isomers.
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H
CH3
H
CH3
cis-1, 2- Dimethylcyclopropane
H
C
H3
CH3
H
trans-1, 2- Dimethylcyclopropane
11. (c) Disubstituted cyclohexanes: They exists as three position isomers i.e., 1, 2-, 1,3-
and 1,4- and each of them exists as cis- and trans- isomers.
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H
OH
H
OH
cis-Cyclohexane-1,2-diol
H
OH
OH
H
trans-Cyclohexane-1,2-diol
H
OH
H
OH
cis-Cyclohexane-1,3-diol
H
OH
OH
H
cis-Cyclohexane-1,2-diol
H
OH
H
OH
cis-Cyclohexane-1,4-diol
H
OH
OH
H
trans-Cyclohexane-1,-diol
12. DETERMINATION OF CONFIGURATION OF THE GEOMETRICAL
ISOMERS
Methods used for determination of configuration of geometrical isomers can be broadly
classified in the following two categories:
o Physical methods
o Chemical methods
I. Physical Methods
(a) Melting points and boiling points: In general the trans isomer has a higher melting
point and a lower boiling point than that of the corresponding cis- isomer.
Trans-isomers being symmetrical get closely packed in the crystal lattice due to which
they have strong inter-molecular forces of attraction and hence higher melting points.
C
H3
H
H
C C
CH3
b.pt. 277 K b.pt. 274 K
CH3
C
H3
H
C C
H
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13. (b) Dipole moment: In general, cis-isomers have the greater dipole moment. The cis-
isomer of 2-butene, for instance, has a significant dipole moment because the two methyl
groups are attached to the same side of double bond and the combined inductive effects
are additive.
Trans-2-butene, on the other hand, has one methyl group and one hydrogen on each
side of the double bond and hence the bond moments cancel each other.
µ=1.85 D µ=0
µ=0.4 D µ=0
Cl
H
H
C C
Cl
H
Cl
H
C C
Cl
C
H3
H
H
C C
CH3 C
H3
CH3
H
C C
H
13
14. II. Chemical Methods
(a) Methods of formation from cyclic compounds: Suppose we oxidize benzene or
quinone to an unsaturated dicarboxylic acid of m. pt. 403 K. From the structure of
benzene or quinone, it becomes clear that two carboxylic groups must be on the same
side(cis). Therefore, maleic acid i.e., the isomer having m.pt. 403 K, must be cis and the
other isomer fumaric acid (m.pt. 575 K) must be trans.
(b) Method of stereoselective addition reactions:
Hydroxylation of double bonds: Hydroxylation of alkenes by aq. KMnO4 or OsO4 is
stereo specifically cis. Thus cis-isomer will give meso-compound and trans- will form
racemic mixture, e.g., hydroxylation of maleic acid produces meso-tartaric acid and that of
fumaric acid gives racemic mixture. Hence, maleic acid is cis-isomer and fumaric acid is
trans-isomer.
O
O
[O]
COOH
COOH
H
C
C
H
m.pt. 403 K
or
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16. CONCLUSION
Geometrical Isomerism is a form of stereo isomerism describing the orientation of
functional groups within a molecule.
It arises when atoms or groups are arranged differently in space due to restricted
rotation of a bond in a molecule. It will not be possible if one of the unsaturated
carbon atoms is bonded to two identical groups.
Important application of geometrical isomerism in alkenes is in the field of
pharmaceuticals. The biological activity of many drugs is highly dependent on their
three-dimensional structure.
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17. REFERENCES
1) Organic Chemistry, S.M. Mukherji, S.P. Singh, R.P. Kapoor and R. Dass, 2nd Edition,
(2010). Published by New Age International Publication, 219-236.
2) Stereochemistry: Conformation and Mechanism, P.S. Kalsi, 1st Edition, (1990).
Published by New Age International Publication, 71-78.
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