Question

Most stable conformation of cyclohexane is (A) Chair (B) Boat (C) Twist boat (D) Half chair

Short answer

The most stable conformation of cyclohexane is (A) Chair, as it has the lowest amount of steric and torsional strain compared to the other conformations.

Step-by-step solution

Understand cyclohexane conformations

Cyclohexane, a cyclic molecule with six carbon atoms, can adopt various conformations (3-dimensional molecular shapes) due to the rotation of single bonds. These conformations include: 1. Chair conformation: In chair conformation, each carbon atom is connected to two other carbon atoms at approximately 109.5° bond angle. This conformation has alternating axial and equatorial bonds and is the most stable form due to minimal steric strain and minimal torsional strain. 2. Boat conformation: In the boat conformation, the six carbon atoms are positioned in such a way that it resembles a boat. However, this conformation is energetically unfavorable due to higher steric strain and torsional strain. 3. Twist boat conformation: This conformation is an intermediate form between chair and boat conformations. It has slightly less steric strain than the boat conformation, but it still has higher energy than the chair conformation. 4. Half chair conformation: This conformation is also an intermediate form, with one end of the molecule being flat like in the boat conformation, and the other end bent like in the chair conformation. It is relatively unstable due to high torsional strain and steric strain.

Identify the most stable conformation

The most stable conformation of cyclohexane is the one with the lowest energy and the least amount of steric and torsional strain. From the descriptions provided in step 1, it is evident that the chair conformation has the lowest amount of steric and torsional strain, making it the most stable conformation. Therefore, the most stable conformation of cyclohexane is: (A) Chair

Key concepts

Chair Conformation

The chair conformation is the most stable and common conformation of cyclohexane. But why is that? Well, imagine each carbon atom in cyclohexane as a point where three lines (or bonds) meet each forming a bond angle of about 109.5°, which is very close to the ideal tetrahedral angle.
The molecules lay in such a way that they alternate between up and down positions, which resembles a chair shape if you look sideways. This specific arrangement results in minimal steric strain and minimal torsional strain, making it the most favored conformation. One reason the chair form is preferred is because it allows for atoms in the molecule to be as far apart as possible, which reduces any repulsive interactions between them.

• Axial bonds: These bonds are oriented parallel to an imaginary vertical axis through the center of the molecule.
• Equatorial bonds: These bonds are oriented around the equator of the cyclohexane ring, extending outward in a more horizontal plane.

Both axial and equatorial positions alternate around the ring, thereby minimizing strain.

Steric Strain

Steric strain in cyclohexane conformations arises when atoms or groups of atoms are forced closer to each other than is energetically favorable. In the context of cyclohexane, this happens when groups are in proximity that increases repulsion between electron clouds, leading to higher energy.
In the chair conformation, steric strain is minimal because the hydrogen atoms and any substituents attached to the cyclohexane ring are positioned to minimize these close contacts. Unlike the boat or half-chair conformations, the chair configuration keeps substituents well-separated, thus maintaining a low-energy state.

• Chair conformation: Minimal steric strain because substituents are spaced out.
• Boat conformation: Higher steric strain due to 'flagpole' hydrogens coming too close together.

This is another reason the chair conformation is preferred and is the most stable conformation.

Torsional Strain

Torsional strain occurs in a molecule when atoms separated by three bonds have angles that deviate from their optimal staggered arrangement. This kind of strain is elevated in several cyclohexane conformations because of eclipsed interactions between hydrogen atoms.
In the chair conformation, however, torsional strain is almost entirely eliminated because the atoms are in a staggered formation, thereby maximizing the distance and minimizing the repulsion between adjacent hydrogen atoms.

• Chair conformation: Offers a staggered arrangement leading to minimal torsional strain.
• Boat conformation: Introduces eclipsed interactions, thus more torsional strain.

Consequently, minimizing both steric and torsional strain makes the chair conformation the most energetically favorable arrangement for cyclohexane.