Conformational Isomers

Core Concepts

In this article, we’ll talk about conformational isomers, show how they can change into each other, and explain why it happens easily.

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What are Conformational Isomers?

Isomerism refers to the existence of multiple compounds sharing the same molecular formula but adopting distinct structural arrangements. Among the various categories of isomers, one type known as conformational isomers, or conformers, stands out. Conformers are distinct in that they involve the rotational movement of sigma bonds, all while maintaining the same connectivity of atoms and the unchanged geometry of bonding. This unique characteristic allows conformational isomers to exist as different arrangements of the same molecule without any alteration in the overall structure.

Essentially, conformational isomers are different conformations of the same molecule, distinguished solely by the rotation of one or more sigma bonds. Conformational isomerism stems from the adaptability of molecular structure, which is not fixed but rather flexible due to sigma bonds. This flexibility enables molecules to swiftly interchange between multiple conformations, driven primarily by thermal energy.

Consequently, conformational isomers exist as a dynamic equilibrium of all possible conformations at a given temperature, rather than distinct entities.

Difference Between Conformational and Configurational Isomers

We can identify conformational isomers by following characteristics:

Although conformational isomers have the same chemical values, they possess different physical properties.
Stereoisomers that can be changed into one another by rotating the molecule around a single bond are known as conformational isomers.
The rotation of the molecule around a single bond may lead to multiple isomers in the case of conformational isomers.

For the configurational isomers we have this other characteristics:

Altering the orientation of a molecule around a single bond does not influence the configuration of isomers in configurational isomerism.
Configurational isomers actively preserve their individuality and cannot undergo interconversion through the rotation of a single bond.
The distinct arrangements of bonds and angles contribute to the varied chemical properties observed in configurational isomers.
Configurational isomers can be categorized into two types: optical isomers and geometrical isomers.

Conformational isomers of Ethane

Ethane, a simple compound comprising two carbon atoms and six hydrogen atoms, exhibits intriguing conformations resulting from rotation about its carbon-carbon bond.

Firstly we need to understand the angles in the different conformers of ethane. In the staggered conformation of a molecule the dihedral angle between the bonds at each of the carbon– carbon bonds is 180° and where atoms or groups bonded to carbons at each end of a carbon–carbon bond are as far apart as possible. Due to the distance in between electron pairs, in this conformation the ethane molecule assumes its lowest energy conformation.

In the eclipsed conformation, the atoms bonded to the carbons at each end of a carbon-carbon bond actively position themselves directly opposite each other. This direct opposition leads to a dihedral angle of 0° between the atoms.

The term “eclipsed conformation” gets its name from the fact that, when you look directly along the carbon-carbon bond, one atom visibly eclipses or overlaps the other atom. This close proximity creates a high degree of repulsive interaction, known as steric strain, between the atoms. The presence of steric strain in the eclipsed conformation contributes to the overall instability of this arrangement.

Conformational isomers of Cyclohexane

Cyclohexane, a cyclic hydrocarbon composed of six carbon atoms, exists in different conformations, each providing a distinct glimpse into the ever-changing nature of molecular structures.

Chair Conformation

One prominent conformation observed in cyclohexane is the chair conformation. Represented by a hexagonal ring, the chair conformation represents the most stable and lowest energy form. Within this conformation, carbon atoms act as vertices, connected by alternating single bonds, resulting in a three-dimensional shape resembling a chair.

The chair conformation of cyclohexane exhibits two distinct types of C-H bonds:

Axial C-H bonds: Oriented vertically either upward or downward with respect to the ring’s plane. There are six axial C-H bonds; three pointing upward and three downward.
Equatorial C-H bonds: Positioned parallel to the plane of the ring, forming a belt-like structure encircling the cyclohexane molecule.

Axial C-H bonds in red and Equatorial C-H bonds in blue

The image above also besides being a visual guide for Axial and Equatorial bonds also represents the Ring-flipping, which is the process of interconversion between two chair conformations of cyclohexane. During this process, the axial and equatorial positions interchange while the substituents maintain the same relative orientation.

Half-Chair Conformation

Cyclohexane distorts into a half-chair conformation, with one end in a chair shape and the other end twisted. This conformation is often observed during the transition state of ring-flipping.

Boat Conformation

The boat conformation of cyclohexane is another possible arrangement where the molecule adopts a boat-like shape. In this conformation, the bending of the carbon atoms on either end of the ring occurs, causing them to protrude out of the plane. Additionally, some of the hydrogen atoms are positioned in an eclipsed manner, resulting in steric strain.

Twist-Boat Conformation

The twist-boat conformation is a distorted form of the boat conformation. In this conformation, the molecule undergoes a twist, reducing the steric strain caused by the eclipsing of hydrogen atoms in the boat conformation.

By studying the conformers of cyclohexane, researchers gain valuable insights into the structural dynamics and energetics of this fascinating molecule. Understanding the relative stability of different conformations helps elucidate the principles governing molecular behavior and reactivity.