Determination
of configuration of Geometrical Isomerism

Session Objectives

By the end of this
session, students will be able to:

• Define Geometrical Isomers

• Determine configuration of geometrical isomers

Geometrical isomers

• Found in alkenes and cyclic compounds

• In alkenes, there is restriction about double bonds

• When there are substituent groups attached to the double
bond, they can bond in different ways, resulting in trans (opposite side) and
cis (same side) isomers called geometrical isomers

• Have different physical and chemical properties

• Each isomer can be converted to another when enough energy
is supplied, e.g. by absorption of UV radiation or being heated to temperatures
around 300 ⁰C

• The conversion occurs because the π bond breaks when energy is absorbed, and
the two halves of the molecule can then rotate with respect to each other
before the π bond forms again

• When there is the same substituent attached to the double
bonded carbons, it is quite straightforward to designate trans or cis

• If there are more than one different groups or atoms
present, the situation becomes a bit more complicated for assigning cis and
trans

• To simplify this situation, the E/Z system is used for
naming geometrical isomers

• Z stands for German zusammen, which means the same side,
and

• E for German entgegen, meaning on the opposite side

E and Z system

• On each C atom of the double bond, we have to assign the
priority of the atoms bonded. The priority should be on the same basis as the
(R)/(S) system (i.e. on the basis of atomic number)

• If the two higher priority groups of the two C atoms are
on the same side of the double bond, it is called the (Z)-isomer

• If the two higher priority groups of the two C atoms are
on opposite sides of the double bond, it is called the (E)-isomer

• For example, 1-bromo-1,2-dichloroethene, atoms attached
are Br, Cl and H

• Br and Cl on C-1 and Cl and H on C-2

• Atomic number of substituents is Br > Cl > H

• E and Z system in cyclic compounds when two or more groups
are attached to a ring

• For example, 1-bromo-2-chlorocyclopentane, there are 2
chiral centers and four stereoisomers are possible

Determination of configuration of geometrical isomers

• Several methods are available to determine the
configuration

• Method can be selected depending upon nature of compound

• Use of multiple methods gives more reliable results

• Some of the methods are-

1) Method of cyclisation

• Wislicenus was the first to suggest this principle

• Intramolecular reactions are likely to occur the closer
together the reacting groups are in the molecule

• It appears for reactions in which rings are formed, but
does not hold for reactions in which double or triple bond is formed

Note:

• Cyclisation reactions to be performed carefully

• One isomer may be converted to other isomer which causes
unreliable results

• Here maleic acid cyclises readily, fumaric acid only after
prolonged heating

• And most probably the former is cis isomer and latter is
trans

2) Method of conversion into compounds of known
configuration

• By converting them into compounds for which configuration
is already known

• For example, two forms of crotonic acid known- one is
crotonic acid (m.p. 72 ⁰C) and other is isocrotonic acid (m.p. 15.5 ⁰C) 

• Now we have two trichlorocrotonic acids, (III) and (IV)
one of which can be hydrolysed to fumaric acid

• So one must be trans isomer and other cis isomer

• Both these trichlorocrotonic acids can also be reduced by
sodium amalgam and water or by zinc and acetic acid to crotonic acids

• From this crotonic is trans and isocrotonic is cis
isomer 

3) Method of
conversion into less symmetrical compounds

• Can be determined by converting into less symmetrical
compounds in which the number of geometrical isomers is increased

• Number of isomers formed will deduce the configuration

• For example, we have two
2,5-dimethylcyclopentane-1,1-dicarboxylic acids and on heating decarboxylated
to 2,5-dimethylcyclopentane-1-carboxylic acid

• Here cis form gives two isomers and trans form gives one

4) Method of optical activity

• Atleast one form may possess the requirements of optical
activity

• For example, in two hexahydrophthalic acids- cis form
possesses a plane of symmetry and is optically inactive and the other is active

5) Method of dipole moments

• This is applicable only for the groups attached to
olefinic carbon atoms have linear moments

• For example, cis-1,2-dichloroethylene has a dipole moment
1.85 D and trans isomer is zero

• If the dipole moment is not zero, then the difference of
dipole moment in cis and trans isomers will be too small to assign

• For example dipole moment of diethyl maleate is 2.54 D and
diethyl fumarate is 2.38 D

6) X-ray analysis method

• Best method to determine the configuration if possible

7) Spectroscopic
methods

A) Ultraviolet and
visible absorption spectra

• As per principle of UV, absorption in the compounds
containing conjugation is due to π- π* transitions

Longer the conjugation, longer the wavelength of absorption
and larger molar extinction coefficient

• Factor which can lower conjugation or overlap is steric
hindrance which can be observed in cis isomer

• For example, λmax 
and ε of cis stilbene are 278 nm and 9350 whereas trans stilbene are 294
nm and 24000

B) Infrared
absorption spectra

Here absorption brought about by =C-H bending is much more
intense than C=C stretching

C) NMR
Spectroscopy

Here cis and trans isomers have different coupling constants

D) Mass
Spectrometry

Here trans isomers give molecular ions of higher intensity
than cis isomers

Greater the steric effect, greater the difference in
intensity

8) Method of surface films

• Here isomers containing a terminal group capable of
dissolving in a solvent will form surface films

• Only trans form can form a close packed film- long chain
unsaturated fatty acids

9) Method of
formation of solid solutions

• Here the shape of trans form is similar to that
corresponding saturated compound whereas cis form is different

• For example, shape of fumaric and succinic acids are
similar, but of maleic acid is different

• Hence molecules which are of approximately same size and
shape tend to form solid solutions i.e., fumaric acid forms a solid solution with
fumaric acid

10) Method
based on generalizations of physical properties

• Melting point and intensity of absorption of cis isomer
are lower than those of trans

• Boiling point, solubility, heat of combustion, heat of
hydrogenation, density, refractive index, dipole moment and dissociation
constant of cis isomer are greater than trans

11) Method of stereoselective addition and elimination
reactions

• Stereoselective reaction is used when substrate produces
diastereoisomeric products in different amounts

• If one predominates the other, means reactions highly
stereoselective

• If both are in equal amounts, means reaction is weakly
stereoselective

• Same is applicable to stereospecific reaction

12) Addition reactions

A) Reduction

• Catalytic hydrogenation of alkenes and alkynes normally
gives the cis

• For example, catalytic hydrogenation (Pd) of
cis-2,3-diphenylbutene in acetic acid gives 98% of meso-2,3-diphenylbutane

• Trans gives racemic product

Summary

• Substituent groups attached to the double bond, they can
bond in different ways, resulting in Trans (opposite side) and cis (same side)
isomers called geometrical isomers

• Have different physical and chemical properties

• Changing the configuration of a molecule always means that
bonds are broken

• A different configuration is a different molecule

• Changing the conformation of a molecule means rotating
about bonds, but not breaking them

• Conformations of a molecule are readily interconvertible,
and are all the same molecule