The eclipsed conformer of ethane has the maximum energy, making it the least stable conformation.
Ethane (C₂H₆) is the simplest alkane containing a carbon-carbon single bond, around which rotation can occur. This rotation leads to different spatial arrangements of the hydrogen atoms, known as conformers. The two primary conformers of ethane are the staggered and eclipsed forms, each possessing a distinct energy level.
Understanding Ethane Conformations
The various conformations of ethane can be visualized by looking down the C-C bond, often using a Newman projection.
- Staggered Conformation: In the staggered conformation, the hydrogen atoms on the front carbon are positioned directly between the hydrogen atoms on the back carbon. This arrangement maximizes the distance between the bonding electron pairs, minimizing repulsion.
- Eclipsed Conformation: In the eclipsed conformation, the hydrogen atoms on the front carbon are directly aligned with the hydrogen atoms on the back carbon. This alignment brings the bonding electron pairs into closer proximity, leading to increased repulsion.
Why the Eclipsed Conformer Has Higher Energy
The eclipsed conformation is the least stable and therefore possesses the maximum energy. This energy difference primarily arises from a phenomenon called torsional strain.
The energy of the eclipsed conformation is approximately 3 kcal/mol higher than that of the more stable staggered conformation. This energy difference represents the rotational barrier that must be overcome for the molecule to rotate from one staggered conformation through an eclipsed one to another staggered conformation.
Factors Contributing to Higher Energy
Several factors contribute to the higher energy of the eclipsed conformer:
- Torsional Strain: This is the primary reason for the energy difference in ethane. It arises from the repulsive interactions between the bonding electron pairs on adjacent atoms when they are in an eclipsed orientation. The electrons in the C-H bonds on one carbon repel the electrons in the C-H bonds on the adjacent carbon when they are directly aligned.
- Steric Hindrance (Minor in Ethane): While more significant in molecules with larger substituents, steric hindrance refers to the repulsion between the electron clouds of non-bonded atoms or groups when they are forced into close proximity. In ethane, the hydrogen atoms are small, so this contribution is minimal compared to torsional strain. However, in the eclipsed form, the hydrogens are at their closest approach, leading to a slight increase in steric repulsion.
Comparing Staggered and Eclipsed Conformations
The table below summarizes the key differences between the two primary conformers of ethane:
| Feature | Staggered Conformation | Eclipsed Conformation |
|---|---|---|
| Relative Energy | Lowest (most stable) | Highest (least stable) |
| H-H Dihedral Angle | 60°, 180°, 300° (hydrogens are staggered) | 0°, 120°, 240°, 360° (hydrogens are directly aligned) |
| Torsional Strain | Minimal | Maximum (due to eclipsing C-H bonds) |
| Steric Strain | Minimal (hydrogens are farthest apart) | Present (hydrogens are closest, though minor for ethane) |
| Energy Difference | Reference point (0 kcal/mol relative) | ~3 kcal/mol higher than staggered |
| Relative Abundance | More abundant at room temperature | Less abundant (transition state or short-lived intermediate) |
| Stability | More stable | Less stable |
| Representation (Newman) | Hydrogens on the back carbon visible between those on the front | Hydrogens on the back carbon hidden behind those on the front |
Energy Profile of Ethane Rotation
As ethane rotates around its carbon-carbon bond, its potential energy changes. This can be depicted in an energy profile diagram:
- Starting from a staggered conformation (energy minimum).
- Rotation by 60° leads to an eclipsed conformation (energy maximum).
- Further rotation by 60° (total 120°) leads to another staggered conformation (energy minimum).
- This pattern repeats every 60° of rotation.
The energy barrier of approximately 3 kcal/mol is small enough that at room temperature, ethane molecules are constantly rotating between these conformations. However, at any given instant, a higher percentage of molecules will be found in the more stable staggered conformation.
Implications in Chemistry
The concept of conformational analysis, pioneered by chemists like Derek Barton and Odd Hassel, is fundamental in organic chemistry. Understanding the relative energies and stabilities of different conformers is crucial for:
- Predicting Molecular Structure: Molecules tend to adopt conformations that minimize their potential energy.
- Understanding Reactivity: Reaction rates and pathways can be influenced by the conformation of reactants.
- Designing Drugs: The three-dimensional shape of a molecule (its conformation) plays a vital role in its ability to bind to biological targets.
For more information on conformational analysis, you can explore resources like LibreTexts Chemistry.
