Determination of configuration of Geometrical Isomerism

Determination of configuration of Geometrical Isomerism

Determination of configuration of Geometrical Isomerism

Determination of configuration of Geometrical Isomerism

Session Objectives

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

• Define Geometrical Isomers

• Determination of configuration of geometrical isomerism

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

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

• 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

Geometrical isomers

• 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

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

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

E and Z system

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

Determination of configuration of geometrical isomerism

• 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

Method of cyclisation

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 

Method of conversion into compounds of known configuration

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

Method of conversion into less symmetrical compounds

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

Ultraviolet and visible absorption spectra

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

Reduction

Determination of configuration of Geometrical Isomerism FAQs

1. What is the key difference between cis and trans isomers?

The key difference lies in the spatial arrangement of substituent groups around a double bond. In cis isomers, similar groups are on the same side, while in trans isomers, they are on opposite sides.

2. How do spectroscopic techniques help in identifying geometrical isomers?

Spectroscopic techniques like IR and NMR provide unique molecular fingerprints, allowing chemists to distinguish between isomers based on their structural differences.

3. Can geometrical isomerism affect the properties of pharmaceuticals?

Absolutely. Geometrical isomerism can influence the bioavailability and efficacy of pharmaceuticals. It’s a crucial consideration in drug development.

4. Are there any real-world examples of dynamic isomerism?

Yes, dynamic isomerism occurs when molecules undergo rapid interconversion between isomeric forms. One example is the isomerization of alkenes in solution.

5. How can I learn more about the applications of geometrical isomerism in industry?

For in-depth knowledge, consider enrolling in advanced chemistry courses or reading scientific journals. Understanding this topic can open doors to various career opportunities in chemistry-related fields.

Determination of configuration of Geometrical Isomerism 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

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