Geometrical Isomerism

Geometrical isomers are stereoisomers which differ in spatial arrangement of atoms or groups attached to double bonds or rings in a molecule differs. The main criteria for geometrical isomerism is the restriction in rotation around C-C single bond.

Explanation: In alkenes, (C=C) bond is made of σ–and π–bonds. A π–bond is made by the sideways overlap of the unhybridised π–orbitals of two carbon atoms above and below the plane of two carbon atoms. If one of the atoms is rotated through 90°, orbitals will no longer overlap and π–bond would break, which requires 25kJ/mol of energy. Hence, the rotation around (C=C) bond is not free but restricted. Due to this restricted rotation, the relative position of atoms or groups attached to the C atoms of the double bond gets fixed, which results in the formation of two distinct forms called geometrical isomers.

Types of Geometrical Isomerism

There are three types of geometrical isomerism on the basis of groups attached to double bond or the site of restricted rotation. They are discussed below.

1. Cis-Trans Isomerism
2. E–Z Isomerism
3. Syn–Anti Isomerism

Cis-Trans Isomerism

Cis and trans came from Latin, where cis means on this side of and trans means on the other side of the functional group. If same groups are on the same side then the configuration is cis and if the same groups are on the other side then it is trans. For example, cis-2-butene and trans-2-butene represent two different spatial arrangements of the methyl groups around the double bond.

Physical Properties of cis-trans Isomers
Physical Properties	Comparison	Remarks
Dipole Moment	Cis > Trans	Cis isomers has resultant dipole moment while trans isomers has cancel out.
Boiling Point	Cis > Trans	Molecules having higher dipole moment have higher boiling   point due to larger intermolecular force of attraction.
Solubility in water	Cis > Trans	More polar molecules  are more soluble  in water.
Melting Point	Trans > Cis	More symmetric isomers have higher melting points due to better packing in crystalline lattice & trans isomers are more symmetric than cis isomers.
Stability	Trans > Cis	The molecule having more vander wall strain are less stable. In cis isomer the bulky group are closer they have larger van der waal strain.

E–Z Isomerism

The letter 'E' from the German word entgegen meaning opposite and the leter '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 (bigger) substituents are on opposite sides of the double bond, while the Z configuration indicates that they are on the same side.

 

For example, (Z)1-Chloro 2-methyl butene-1 and (E)1-Chloro 2-methyl butene-1 represent two different spatial arrangements of chlorine atom and ethyl group around the double bond.

Syn-Anti Nomenclature

The cis-trans isomerism in some cases (such as oximes, diazoates, and azo) containing one or more carbon to nitrogen or nitrogen to nitrogen double bonds is designated by syn/anti-isomerism. Syn-anti nomenclature is based upon two substituents in an acyclic molecule. In stereoisomeric oxime, the configuration of oximes is usually denoted by prefixes syn and anti instead of cis and trans.

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

For example, Acetaldoxime has two geometrical isomers:
Syn Acetaldoxime: The hydrogen (H) and hydroxyl (OH) group are on the same side of the molecule.
Anti Acetaldoxime: The hydrogen (H) and hydroxyl (OH) group are on opposite sides of the molecule.

Geometrical Isomers in Cycloalkanes

Cycloalkanes show Geometrical isomers due to restricted rotation about single bond. Only those cycloalkanes show geometrical isomers in which at least two different carbons have two different groups.

Number of Geometrical Isomers in Polyenes

R₁–CH=CH–CH=CH ... CH=CH–R₂

If R₁ is not equal to R₂,
the number of geometrical isomers = 2ⁿ
where, n = number of double bonds

Example: CH₃–CH=CH–CH=CH–CH=CH–CH₂–CH₃
Here, n= 3
The number of geometrical isomers = 2³ = 8

If R₁ is equal to R₂,
the number of geometrical isomers = 2^{(n − 1)} + 2^{(p − 1)}
where, p = n/2 when n is even
p = (n + 1)/2 when n is odd

Example: CH₃–CH=CH–CH=CH–CH=CH–CH₃
Here, n= 3
so, p = (3 + 1)/2 = 2
The number of geometrical isomers = 2² + 2¹= 4 + 2 = 6

Also read Isomerism | Optical Isomerism | Conformational Isomerism