3.6
Conformations of Ethane and Propane
In an organic molecule, free rotation about the carbon-carbon single bond results in energetically different conformers of the molecule. Due to this r…
A molecule of ethane, when viewed down the carbon-carbon bond, shows the C-H groups spaced at 60°dihedral angles. This is the staggered conformation of ethane.
The staggered form of ethane has the lowest energy. This is because the C-H bonds are furthest from each other, minimizing steric repulsion between the electrons in the bonds, and therefore, stabilizing the molecule.
Another factor that stabilizes the staggered conformation is the favorable interaction between the occupied, bonding molecular orbital and an unoccupied, antibonding molecular orbital.
Rotating the farther carbon by keeping the nearer carbon stationary generates an infinite number of conformations.
At 0° dihedral angles, the C-H bonds on the two carbon atoms are close and cover one another. This is the eclipsed conformation of ethane.
Due to an increased steric repulsion and absence of a stabilizing interaction, the energy of eclipsed ethane increases by 12 kJ/mol, and each eclipsing H-H interaction is assigned 4 kJ/mol.
The energy difference between eclipsed and staggered conformations is known as torsional strain or torsional barrier.
Rotating the molecule from 0º to 360º along the carbon-carbon bond generates several degenerate staggered and eclipsed states.
A sample of ethane gas, at room temperature, has approximately 99% of its molecules in the lowest energy staggered conformation.
The energy gained from molecular collisions is used to undergo internal rotation by overcoming the torsional barrier. The molecule, thus, moves into a different staggered form after passing through the high-energy eclipsed state.
The next hydrocarbon — propane — also has two major conformers: eclipsed and staggered.
The eclipsed conformer has a torsional strain of 14 kJ/mol. Each pair of eclipsing hydrogens contributes 4 kJ/mol, while the eclipsing CH3-H interaction contributes 6 kJ/mol.
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Q1: What is the staggered conformation of ethane?
The staggered conformation of ethane occurs when C-H groups are positioned at 60° dihedral angles, placing hydrogen atoms as far apart as possible. This arrangement minimizes steric repulsion between electron-rich bonds and is stabilized by favorable interactions between bonding and antibonding molecular orbitals, making it the lowest energy and most stable form of ethane.
Q2: Why is the eclipsed conformation of ethane less stable than the staggered form?
In the eclipsed conformation, C-H bonds align at 0° dihedral angles, causing increased steric repulsion and eliminating stabilizing orbital interactions. This results in 12 kJ/mol higher energy than the staggered form, with each H-H eclipsing interaction contributing 4 kJ/mol. This energy difference is called torsional strain or torsional barrier.
Q3: How does internal rotation affect ethane molecules at room temperature?
At room temperature, approximately 99% of ethane molecules exist in the lowest energy staggered conformation. Molecular collisions provide energy to overcome the torsional barrier, allowing molecules to rotate through high-energy eclipsed states and interconvert between different staggered conformations continuously throughout the sample.
Q4: What are the major conformers of propane?
Propane has two major conformers: staggered (low energy, stable) and eclipsed (high energy, unstable). The eclipsed conformer has 14 kJ/mol torsional strain. Each eclipsing H-H pair contributes 4 kJ/mol, while CH3-H eclipsing interactions contribute 6 kJ/mol, making propane's torsional barrier slightly higher than ethane's.
Q5: How does dihedral angle relate to ethane's conformational energy?
Dihedral angle measures the spatial relationship between C-H bonds on adjacent carbons. At 60° angles, ethane adopts the staggered, low-energy conformation. Rotating to 0° creates the eclipsed, high-energy form. Continuous rotation from 0° to 360° generates alternating staggered and eclipsed states with varying energy levels.
Q6: What role do molecular orbitals play in stabilizing staggered ethane?
In staggered ethane, the occupied bonding molecular orbital of one C-H bond interacts favorably with the unoccupied antibonding molecular orbital of another C-H bond. This orbital overlap provides additional stabilization beyond simple steric effects, contributing to the staggered conformation's lower energy and greater stability.
Q7: How does propane's torsional strain compare to ethane's?
Propane's eclipsed conformation has 14 kJ/mol torsional strain compared to ethane's 12 kJ/mol, making propane slightly less stable in its eclipsed form. The difference arises because propane contains CH3-H eclipsing interactions worth 6 kJ/mol each, in addition to H-H interactions, increasing overall strain significantly.