Geometrical Isomerism

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In stereochemistry, most of the properties of organic compounds are linked with the molecule’s shape. Several chemical and biological reactions/properties, such as smell, taste, etc., of organic compounds are also affected by their molecular shape and structural arrangement. 

In alkenes, all of the atoms/substituents attached directly lie in a plane as rotation is not permitted due to the presence of pi-bonds. If these substituents attached to the alkene compound are substituted on the same side of the alkene or opposite sides of the alkene, then it results in compounds with different geometry/configurations. Such types of organic compounds are called geometrical isomers. In the geometrical isomers, if the methyl groups are present on the same side, then they are called the cis isomer, and if isomers have groups located on opposite sides, such types of isomers are called the trans-isomers. Trans isomers of compounds are usually more stable than cis isomers. 

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There are two types of stereoisomerism:

a) Conformational isomerism: Also termed as rotational isomers as they have free rotation of groups about C – C bond axis as they have different arrangements of atoms in space. This type of isomerism is found in alkanes, cycloalkanes, and their derivatives.

b) Configurational isomerism: They differ in the arrangement of atoms and have different properties. It is of two types geometrical isomerism and optical isomerism. These isomers do not have free rotation around the C-C single bond.

Geometrical isomerism: 

Geometrical isomers are stereoisomers that have a different arrangement of groups or atoms around double bonds.

Geometrical Isomerism is also known as Cis-trans isomerism. In the geometrical isomers, if the methyl groups are present on the same side, then they are called the cis isomer, and if isomers have groups located on opposite sides, such types of isomers are called the trans isomer. Cis isomers of compounds are less stable than trans-isomers.

• Cis isomers: If the functional groups (substituents) are on the same side of the plane.

• Trans isomers: If functional groups are on the opposing sides.

Geometric Isomers are stereoisomers which are the pair of molecules having the same formula but different functional groups. Geometric Isomerism occurs both in organic molecules and inorganic coordination complexes.

Let us now define geometrical isomerism for better understanding.  Geometric Isomerism is defined as two or more molecules that contain the same type of atoms and bonds but have different spatial arrangements of atoms. However, not all pairs of molecules have geometric isomers.

Geometric Isomers cannot move freely due to rigid structures like carbon-carbon, carbon-nitrogen, or nitrogen-nitrogen. Rigidity is due to a double bond. Some geometrical isomerism examples are : stilbene, C14H12, a cyclic compound, rigid due to the ring structure.

Geometrical isomerism is generally seen in alkenes ( >C=C<) and oximes ( >C=NOH). 

Necessary conditions for geometrical isomerism:

i) restricted rotation around the C – C double bond.

ii) Two atoms or groups attached to the molecule with the doubly bonded carbon atom must be different. However, it doesn’t matter whether the left side groups are the same or different on the right side. 

The distinction between cis and trans isomer

1. Dipole moment of polar groups have an additive effect in cis isomers and an opposing effect in trans isomers (tends to cancel each other). Thus, cis isomers have a higher dipole moment than corresponding trans-isomers. 

2. The structure is not symmetrical in cis-isomers and symmetrical in trans isomers; thus, a cis configuration has a lower melting point and trans isomer (due to higher lattice energy)hence a higher melting point.

3. Cis isomers are weakly held in the lattice, and trans – isomers are tightly held in the lattice, thus resulting in a higher melting point of cis type than trans-isomers.

4. Cis – isomers have a lower density in comparison with trans-isomers due to higher lattice energy.

5. Cis-isomer has a higher boiling point have comparatively low boiling point due to dipole interaction.

E – Z nomenclature of geometric isomers:

For naming geometrical isomerism, there is a system of nomenclature of geometrical isomerism developed by Ingold and Prelog called E-Z notation of Geometrical isomerism. According to this system of nomenclature, priority is assigned to the groups which are attached to double-bonded carbon atoms. If groups of similar priority lie on the similar side of a double bond, the isomer is designated as Z ( Zusammen, means together). If groups of similar priority lie on the opposite side of a double bond, the isomer is designated as E ( Entgegen, means the opposite).

Cis-Trans nomeclature:

Syn-Anti system of Nomenclature:

Physical properties of geometrical isomers:

• Cis-2-pentene while trans-2-pentene.

• The cis isomer is maleic acid, while the trans isomer is fumaric acid.

• Cis-1,2-dichloroethene while trans-1,2-dichloroethene.

• Cis is oleic acid, while trans is Elaidic acid.

Given here in below are a few examples of Geometrical isomers:

• Pt(NH3)2Cl2: species are arranged around Pt in the order Cl, Cl, NH3.

In another, it is arranged as NH3, Cl, NH3, Cl.

Geometrical isomerism in cyclic systems:

We know that geometrical isomerism can occur when there is a restricted rotation around a bond. Apart from restrictions around a double bond, there could be other possibilities also such that there is no possibility of rotation around any of the carbon-carbon bonds. Example cyclohexane. Suppose we replace two of the hydrogens in the cyclohexane molecule with two bromine atoms. 

In cycloalkane, the tetrahedral shape occurs around each carbon, and bromine can be attached in two different ways, either both above the ring or one can be above the ring and the other below.

Cyclic alkanes also show both cis and trans geometrical isomerism. If in cyclic alkanes the substituents attached to a ring system are on the same side of the ring, then cis isomers form, and if on the opposite side of the ring, then form trans isomerism occurs. The letters E and Z are not used in cyclic alkanes.

Conclusion 

Geometrical isomerism is a form of stereoisomerism also called configurational isomers. They can be in the form of cis and trans configuration. Geometrical isomerism also arises in cyclic compounds, i.e., from ring structures wherein rotation of bonds is greatly restricted. In cyclic compounds also both cis and trans geometrical isomers can occur. Since restricted rotation is possible in many compounds thus, it is sometimes difficult to recognize the geometrical isomers. The restricted movement occurs around the carbon-carbon double bond. So there are some essential conditions that include the restricted rotation around the carbon-carbon double bond, two different groups on the left-hand end of the bond, and two different groups on the right side of the compounds (these groups on both left and right sides can be the same or different that doesn’t matter).