Isomers are molecules that share the same molecular formula but possess different arrangements of atoms in space. These compounds are divided into two main groups: constitutional isomers (different connectivity) and stereoisomers (same connectivity but different spatial arrangement). Conformational isomers are a specific type of stereoisomerism resulting from the simple rotation of groups around a single bond. They represent the various shapes a single molecule can adopt without breaking any chemical bonds. These forms are often referred to as conformers or rotational isomers (rotamers).
The Mechanism of Formation
Conformational isomers arise because single bonds in organic molecules, specifically sigma (\(\sigma\)) bonds, are not rigid but are free to rotate. This allows the groups attached to the bonded atoms to spin relative to one another. The continuous rotation around the carbon-carbon bond creates an infinite number of three-dimensional arrangements.
This rotation is generally unrestricted at room temperature because the energy barrier to interconversion is quite low. For example, the energy barrier in ethane is only about 12 kJ/mol, which is easily overcome by the thermal energy of the molecules. Consequently, one conformer rapidly changes into another, and the molecule is constantly twisting and turning.
The rapid interconversion means conformers cannot typically be isolated or separated from each other. Unlike configurational isomers (which require breaking and reforming bonds to interconvert), conformers only require bond rotation. Although they exist in a dynamic equilibrium, the majority of molecules in a sample will be in the most stable conformation at any given moment.
The different conformers are not considered distinct, stable chemical compounds. Instead, they are momentary three-dimensional snapshots of the same molecule as it rotates around its single bonds.
Visualizing Molecular Shapes
To analyze the spatial relationship between groups and understand conformer stability, chemists use specialized drawing techniques. Standard skeletal drawings do not adequately convey the three-dimensional orientations resulting from bond rotation. The two primary visualization tools are the Sawhorse projection and the Newman projection.
The Sawhorse projection depicts the molecule from an oblique angle, looking at the carbon-carbon bond from the side. The C-C bond is drawn diagonally, and the groups attached to both the front and back carbons are clearly visible. This view helps illustrate the overall shape and angle of the carbon chain.
The Newman projection is preferred for conformational analysis because it simplifies the view by looking directly down the axis of the carbon-carbon bond. The carbon closest to the viewer is shown as a dot, while the carbon behind it is represented by a circle. Bonds attached to the front carbon radiate from the dot, and bonds on the back carbon emerge from the edge of the circle.
The Newman projection explicitly shows the dihedral angle—the angle between a bond on the front carbon and a bond on the back carbon. This end-on view is effective for visualizing the spatial proximity of groups and determining the amount of strain within the molecule.
Stability and Energy Differences
Conformational isomers possess different amounts of potential energy, which dictates their relative stability. The two most distinct arrangements are the staggered and eclipsed conformations, representing the lowest and highest energy states, respectively. In the staggered conformation, groups on the front carbon are positioned between the groups on the back carbon, maximizing the distance between them.
The staggered form is the most stable because the bonds are far apart, minimizing repulsive forces between electron clouds. Conversely, the eclipsed conformation is the least stable because groups on the front carbon directly line up with groups on the back carbon. This alignment forces adjacent bond electron pairs into close proximity, creating torsional strain, which raises the molecule’s energy.
The simplest example is ethane, where the eclipsed conformer is approximately 12 kJ/mol higher in energy than the staggered conformer. This energy difference creates a rotational barrier that must be overcome for the bond to rotate. Because this barrier is small, rotation is continuous at room temperature, but about 99% of ethane molecules exist in the more stable staggered form at any given moment.
In larger molecules like butane, the analysis is more complex, involving different types of staggered conformations and an additional kind of strain. Looking down the C2–C3 bond, the two methyl groups can adopt the anti staggered conformation, separated by a 180° dihedral angle. This anti conformation represents the lowest energy state, as there is no steric or torsional strain.
A second staggered conformation, called gauche, occurs when the two methyl groups are separated by a 60° dihedral angle. While gauche avoids torsional strain, it introduces steric strain—the repulsive interaction when bulky groups are forced too close together. This strain causes the gauche conformer to be about 3.8 kJ/mol higher in energy than the anti conformer.