Question

In a sample of cis-1,2-dichlorocyclohexane at room temperature, the chlorines will: A. both be equatorial whenever the molecule is in the chair conformation. B. both be axial whenever the molecule is in the chair conformation. C. alternate between both equatorial and both axial whenever the molecule is in the chair conformation. D. both alternate between equatorial and axial but will never exist both axial or both equatorial at the same time.

Short answer

D

Step-by-step solution

Understand the Conformation

Cis-1,2-dichlorocyclohexane means both chlorine atoms are on the same side of the cyclohexane ring. In chair conformation, one will be axial and the other will be equatorial to minimize steric hindrance.

Eliminate Options A and B

Since one chlorine will be axial and the other equatorial, options A (both equatorial) and B (both axial) must be incorrect.

Consider Option C

Option C suggests that the molecule will alternate between both chlorines being equatorial and being axial. This is incorrect because it does not minimize steric hindrance.

Evaluate Option D

Option D states that both chlorines alternate between equatorial and axial but never at the same time. This is correct as it's the behavior expected to minimize steric hindrance in cis-1,2-dichlorocyclohexane's chair conformation.

Conclusion

The correct answer, based on minimizing steric hindrance, is Option D.

Key concepts

Chair Conformation

Chair conformation is a three-dimensional arrangement of the cyclohexane ring. It resembles a folding chair because of how the carbon atoms are positioned.
This structure is highly stable because it reduces angle strain and torsional strain.
Due to the staggered arrangement of its hydrogen atoms, there is less repulsion between them compared to other conformations like the boat form.
In the chair conformation, each carbon atom in the cyclohexane ring has two substituents placed in different positions: one axial and one equatorial.
The axial substituents are oriented perpendicular to the ring, while the equatorial substituents are roughly parallel to the plane of the ring.
This configuration plays a crucial role in the molecule’s stereochemistry and steric hindrance.
Understanding chair conformation is essential for grasping how substituents influence the stability and reactivity of cyclohexane derivatives.

Stereochemistry

Stereochemistry involves the study of the spatial arrangement of atoms in molecules and its impact on their chemical properties.
In cis-1,2-dichlorocyclohexane, the 'cis' prefix indicates that the two chlorine atoms are on the same side of the cyclohexane ring.
This specific spatial arrangement affects the molecule's overall reactivity and interactions.
The chair conformation helps in minimizing steric hindrance by distributing the bulkier groups more evenly in space.
When analyzing cis-1,2-dichlorocyclohexane, it's important to understand that one chlorine will be in an axial position and the other will be in an equatorial position.
By alternating between these positions, the molecule maintains a lower energy state, minimizing steric clashes and making the structure more stable.

Steric Hindrance

Steric hindrance occurs when the spatial arrangement of atoms prevents or slows down chemical reactions by crowding.
In cyclohexane derivatives, large substituents like chlorine can cause significant steric hindrance if placed too close to one another.
To minimize steric hindrance, substituents prefer to be arranged in positions that offer the most space, reducing repulsive forces.
For cis-1,2-dichlorocyclohexane, placing one chlorine atom in the axial position and the other in the equatorial position allows for reduced crowding and repulsion.
This arrangement leads to a more stable, lower-energy conformation.
Alternating positions of substituents in dynamic equilibrium helps further mitigate steric clashes, highlighting the importance of chair conformation in understanding molecular stability.

Axial and Equatorial Positions

Axial and equatorial positions refer to the specific orientations that substituents take on a cyclohexane ring in chair conformation.
Axial positions are perpendicular to the ring, extending above and below the plane, while equatorial positions are roughly parallel to the plane of the ring.
Substituents in equatorial positions are generally more stable due to reduced steric hindrance compared to axial positions.
For cis-1,2-dichlorocyclohexane, the chlorines must take on different positions—one axial, one equatorial—to minimize steric hindrance.
This arrangement ensures that the bulky chlorine atoms are not crowding each other too closely, leading to a lower energy state.
The dynamic flip of the chair conformation causes these positions to interchange, keeping one axial and one equatorial but never both axial or both equatorial at the same time.
This continuous alternation contributes to the complex behavior and stability of the molecule in chair conformation.