Geometrical Isomerism and Conformations

Stereoisomers are molecules with the same molecular formula and connectivity of atoms but differing in their spatial arrangement. Geometrical isomers are a type of stereoisomer where the spatial arrangement of atoms or groups around a double bond or a ring in a molecule differs. 

1.0Conditions for Geometrical Isomerism

Condition 1: Restricted rotation must be present.

In a molecule with a double bond or a ring, the atoms or groups attached to the bonded atoms can be oriented differently in space. If rotation around the double bond or within the ring is restricted, the spatial arrangement of these atoms or groups becomes fixed, leading to the formation of geometric isomers.

For example, the difference in spatial orientation between cis-2-butene and trans-2-butene arises due to the restricted rotation around the double bond. In cis-2-butene, the rotation is restricted in such a way that both methyl groups end up on the same side, while in trans-2-butene, they end up on opposite sides.

The difference in spatial arrangement between geometric isomers can result in distinct physical and chemical properties. For instance, cis and trans isomers often have different boiling points, melting points, and reactivities. This is because their spatial orientations lead to different interactions with other molecules.

Condition 2: The two groups at each end of the restricted bond must be different.

When the two groups at each end of the double bond are different, it means that there's no internal symmetry within the molecule with respect to the double bond. As a result, the spatial arrangement of these different groups on either side of the double bond becomes significant.

Let's consider an example to understand this condition:

Let’s check GI in above example

Condition 3

Terminal valency should be present in the same plane (For Alkenes).

This condition ensures that the different groups attached to the double-bonded carbons can exhibit cis-trans isomerism based on their spatial arrangement.

When terminal valencies (the bonds formed by the atoms at the ends of the double bond) are present in the same plane, it means that the double bond lies flat in that plane. This orientation is crucial because it restricts the rotation around the double bond, allowing for the formation of geometric isomers.

For example, let's take the simplest alkene, ethene (CH₂=CH₂). In ethene, each carbon atom is bonded to two hydrogen atoms. Since both carbons have the same substituent groups (hydrogen), there is no geometrical isomerism observed.

However, if we consider a molecule like 2-butene (CH₃−CH(CH₃)=CH₂−CH₃), where different substituents (methyl and hydrogen groups) are attached to each carbon atom of the double bond, geometrical isomerism occurs. In this case, cis-2-butene and trans-2-butene are formed, where the methyl groups are arranged either on the same side (cis) or on opposite sides (trans) of the double bond.

Here is an overview-

Example	G.I.
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2.0Geometrical Isomerism in Different Compound

Geometrical isomerism in (C=N:)

Imine and Oxime

When unsymmetrical aldehydes or ketones are used to form imines, the resulting imines almost always display geometrical isomerism. This phenomenon occurs due to the different groups attached to the carbon and nitrogen atoms of the imine, which restricts rotation around the carbon-nitrogen double bond.

When an unsymmetrical ketone reacts to form an oxime, it produces two different oxime products due to the presence of different substituent groups. These products display geometrical isomerism because the restricted rotation around the carbon-nitrogen double bond results in two spatial arrangements of substituent groups: cis and trans isomers.

Geometrical isomerism in azo compounds (–N=N–)

Azo compounds containing the 𝑁=𝑁 group exhibit geometrical isomerism due to restricted rotation around the nitrogen-nitrogen double bond. This double bond restricts the movement of substituent groups attached to the nitrogen atoms, resulting in two possible spatial arrangements: cis and trans isomers. In cis isomers, substituent groups are on the same side of the double bond, while in trans isomers, they are on opposite sides. These geometric isomers often have different physical and chemical properties.

(i)   H–N=N–H(H₂N₂)

(ii)  Ph₂N₂ (Azobenzene)

Geometrical isomerism in cyclic compounds

Geometrical isomerism in cyclic compounds arises when different substituent groups attached to the cyclic structure prevent free rotation around a bond, resulting in distinct spatial arrangements. This restriction leads to the formation of Geometrical isomerism, where substituent groups are positioned either on the same side or on opposite sides of the ring. These isomers can exhibit different physical and chemical properties due to their unique spatial arrangements.

3.0Nomenclature of Geometrical Isomers

The nomenclature of geometrical isomers typically involves indicating the relative spatial arrangement of substituent groups around a double bond or within a cyclic structure. The most common systems for naming geometrical isomers are:

1. Cis-Trans System

This system is widely used for naming geometrical isomers of compounds with a carbon-carbon double bond or a cyclic structure. In this system, "cis" denotes that the similar substituent groups are on the same side of the double bond or ring, while "trans" indicates that they are on opposite sides. For example, cis-2-butene and trans-2-butene represent two different spatial arrangements of the methyl groups around the double bond.

2. E-Z System

The E-Z system is an IUPAC-approved nomenclature for naming geometrical isomers, particularly in complex molecules with multiple double bonds or substituents. In this system, the terms E (from the German word "entgegen," meaning opposite) and Z (from the German word "zusammen," meaning together) are used to describe the spatial arrangement of substituents based on their priority according to Cahn-Ingold-Prelog rules. 

The E configuration refers to the arrangement where the higher priority substituents are on opposite sides of the double bond, while the Z configuration indicates that they are on the same side. 

For example, (E)-1,2-dichloroethene and (Z)-1,2-dichloroethene represent two different spatial arrangements of the chlorine atoms around the double bond.

Syn-Anti Nomenclature

The syn-anti nomenclature is another system used to describe the spatial arrangement of substituent groups (H and OH) around a double bond.

In this system:

1. Syn: Indicates that the substituent groups are located on the same side of the double bond.
2. Anti: Indicates that the substituent groups are located on opposite sides of the double bond.

For Example- Acetaldoxime has two geometrical isomers:

1. Syn Acetaldoxime: The hydrogen (H) and hydroxyl (OH) group are on the same side of the molecule.
2. Anti Acetaldoxime: The hydrogen (H) and hydroxyl (OH) group are on opposite sides of the molecule.

These isomers differ in their spatial arrangement around the C=N double bond, influencing their physical and chemical properties.

4.0Physical Properties of Geometrical Isomerism

1. Density: The spatial arrangement of substituent groups can affect the density of geometrical isomers. Isomers with more compact arrangements may have higher densities compared to those with more extended structures.
2. Refractive Index: Geometrical isomers may have different refractive indices due to variations in their molecular structures, leading to differences in how they bend light as it passes through them.
3. Color and Odor: In some cases, geometrical isomers may exhibit differences in color or odor due to variations in their molecular structures, functional groups, or electronic configurations.

Here is a brief overview of other physical properties

5.0Methods of determination of configuration of geometrical isomers

Determining the configuration of geometrical isomers can be done through various methods, including:

1. X-ray Crystallography: Provides a direct visualization of molecular structure by analyzing X-ray diffraction patterns from crystallized samples.
2. Nuclear Magnetic Resonance (NMR) Spectroscopy: Uses the magnetic properties of nuclei to infer the spatial arrangement of atoms, with differences in coupling constants and chemical shifts helping distinguish cis from trans isomers.
3. Infrared (IR) Spectroscopy: Identifies different isomers by their unique IR absorption spectra, reflecting variations in bond angles and dipole moments.

6.0Conformational Isomerism

Conformational isomerism, also known as conformational isomerism or rotational isomerism, occurs due to the rotation around single bonds. This type of isomerism is dynamic, as the isomers can be interconverted just by rotating a single bond.

For Example:

Ethane (C₂H₆): Different spatial arrangements occur when one methyl group rotates relative to the other around the central C-C bond. The most notable conformations are the staggered conformation (where the H atoms on one carbon are positioned at maximum distance from the H atoms on the adjacent carbon, leading to less torsional strain) and the eclipsed conformation (where the H atoms on one carbon align directly with those on the adjacent carbon, increasing torsional strain).

Eclipsed Conformation 

In the eclipsed conformation, the hydrogen atoms on adjacent carbons are aligned directly with each other, leading to the minimum distance between these atoms. This close proximity results in maximum repulsion among the electron clouds of the C–H bonds. Consequently, this arrangement is the least stable due to the high torsional strain involved.

Staggered Conformation 

In contrast, the staggered conformation features the hydrogen atoms on adjacent carbons positioned as far apart as possible. This maximizes the distance between the C–H bonds, thereby minimizing repulsive forces among the electron clouds. This arrangement is the most stable due to the reduced torsional strain, making it energetically favorable.

Skew (Gauche) Conformations

Between the extremes of the eclipsed and staggered forms, there are infinite intermediate conformations known as skew or gauche conformations. These are not perfectly aligned like the eclipsed form nor perfectly offset like the staggered form. The stability of these conformations varies, typically being less stable than the staggered but more stable than the eclipsed, depending on the specific angles and the nature of the substituents involved.

7.0What is the difference between conformational and geometrical isomerism?