Determination of configuration of Geometrical Isomerism. PPT

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Lecture No. 12

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

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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

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Geometrical isomers
• 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

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Geometrical isomers
• 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

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E and Z system
• A) 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)

• B) 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

• C) 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

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E and Z system
• 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

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E and Z system
• E and Z system in cyclic compounds when two or more groups are

attached to a ring

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E and Z system

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

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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

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Determination of configuration of
geometrical isomers

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

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Determination of configuration of
geometrical isomers

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

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Determination of configuration of
geometrical isomers

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Determination of configuration of
geometrical isomers

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

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Determination of configuration of
geometrical isomers

H H H H

H3C CH3 H3C H

H H H H
CO2H CO2H

H H H CH3

CO2H CO2H

cis-form trans-form

-CO -CO2
2

H H H H H H

H H
3C CH3 H 3C H

3C CH3

H H H H
CO H CO

2H H
CO 2H

2H

H H H H
H CH3

H CO H
2H

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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

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Thank You
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