3.7
Conformations of Butane
Unlike ethane and propane that have only two major conformations, butane has more than two conformers. The staggered form of butane in which the bulky…
In butane, rotations about the central carbon-carbon bond generate different conformations of the molecule. The conformers, when viewed along the C2–C3 bond, show the various Newman projections.
Energy changes that result from the C2–C3 bond rotations generate several staggered and eclipsed conformations of butane, with varying dihedral angles between the two methyl groups.
The staggered form of butane — with a dihedral angle of 180° — has the lowest energy and is the most stable form. It is called the anti conformation.
The increased stability of anti butane is due to the reduced steric strain, owing to the large spatial separation between the two methyl groups.
In the other two staggered forms, the methyl groups are separated by dihedral angles of ± 60°. These are called gauche conformations.
Since the two bulky methyl groups are relatively closer, their electron clouds strongly repel each other. This steric interaction, called a gauche interaction, increases the energy of the gauche conformer by 3.8 kJ/mol. Hence, gauche butane is less stable than anti butane.
The two gauche conformations are degenerate and mirror images of each other.
Among the three eclipsed conformers, the totally eclipsed form has a dihedral angle of 0° between the two methyl groups.
Because the methyl groups are very close in this conformation, their hydrogens experience strong van der Waals repulsions.
The resulting steric strain, along with the torsional strain, makes the totally eclipsed form of butane the least stable conformer with a high energy cost of 19 kJ/mol.
The other two eclipsed forms with dihedral angles of 120° and 240° have similar energies, and one form is a reflection of the other form.
Due to the reduced steric strain in these forms, the energy of the eclipsed conformer decreases to 16 kJ/mol.
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Q1: What is the most stable conformation of butane?
The anti conformation of butane is the most stable form, occurring at a dihedral angle of 180° between the two methyl groups. In this staggered arrangement, the bulky methyl groups are positioned on opposite sides of the molecule, maximizing spatial separation and minimizing steric repulsion. This configuration has the lowest energy and greatest stability due to reduced electron cloud overlap.
Q2: Why are gauche conformations of butane less stable than the anti form?
Gauche conformations occur at dihedral angles of ±60°, positioning the two methyl groups much closer together than in the anti form. This proximity causes strong electron cloud repulsion between the bulky methyl groups, creating steric interactions called gauche interactions. These unfavorable interactions increase the energy of gauche butane by 3.8 kJ/mol compared to the anti conformer.
Q3: How do Newman projections help visualize butane conformations?
Newman projections provide a direct view along the C2–C3 bond of butane, showing the spatial arrangement of atoms and groups attached to the two central carbons. This perspective clearly displays the dihedral angles between substituents and helps identify whether a conformation is staggered or eclipsed. Newman projections are essential for understanding conformational differences and predicting relative stability.
Q4: What makes the totally eclipsed conformation of butane the least stable?
The totally eclipsed form of butane has a dihedral angle of 0°, placing the two methyl groups directly aligned. In this conformation, the hydrogen atoms on the methyl groups experience strong van der Waals repulsions, combined with torsional strain. These factors result in the highest energy cost of 19 kJ/mol, making it the least stable conformer of butane.
Q5: How do the two gauche conformations of butane compare to each other?
The two gauche conformations of butane are degenerate, meaning they have identical energy and are mirror images of one another. Both occur at dihedral angles of ±60° and experience the same 3.8 kJ/mol energy increase from gauche interactions. Their equivalence reflects the symmetry of the butane molecule and the identical steric environment in each gauche arrangement.
Q6: What is the energy difference between eclipsed conformations in butane?
The totally eclipsed conformation of butane has an energy cost of 19 kJ/mol, while the other two eclipsed forms each cost 16 kJ/mol. The two non-totally eclipsed conformations are degenerate and feature two CH3-H eclipses and one H-H eclipse. The 3 kJ/mol difference reflects reduced steric strain when methyl groups are not directly opposed.
Q7: How does butane differ from ethane and propane in terms of conformational complexity?
Unlike ethane and propane, which have only two major conformations, butane exhibits more than two distinct conformers due to its two methyl groups on the central carbons. These methyl groups create variable steric interactions at different dihedral angles, generating multiple staggered and eclipsed forms with different energy levels. This increased complexity makes butane a better model for understanding conformational analysis in larger alkanes.