Given the planar trisubstituted cyclohexane. Fill in the missing substituents (with H or Cl) to complete the two possible cyclohexane chair conformations. Then determine the more stable conformer. You might find it helpful to make a model of the cyclohexane to help visualize the chair conformations.
The Correct Answer and Explanation is:
To solve this problem, we must correctly place the chlorine (Cl) and hydrogen (H) atoms in the chair conformations of a trisubstituted cyclohexane and determine which chair conformation is more stable.
Step-by-Step Analysis:
- Identify substituents from the planar structure:
The planar structure shows a cyclohexane with three chlorine atoms at carbon positions 1, 3, and 5. - Assign positions in Chair 1:
- Start with one Cl at an equatorial position on carbon 1 (for stability).
- Next Cl at carbon 3 must then be placed axial, due to the alternating pattern of axial/equatorial.
- The third Cl at carbon 5 will then be equatorial.
Fill the remaining positions with hydrogens.
- Chair Flip to Chair 2:
- All axial positions become equatorial and vice versa.
- So Cl at carbon 1 goes to axial.
- Cl at carbon 3 goes to equatorial.
- Cl at carbon 5 goes to axial.
- Evaluate Stability:
In general, equatorial positions are more stable for bulky groups like chlorine due to reduced 1,3-diaxial interactions. So, we count the number of Cl atoms in equatorial positions for each chair:- Chair 1: Cl at C1 (equatorial), C3 (axial), C5 (equatorial) → 2 equatorial Cl atoms
- Chair 2: Cl at C1 (axial), C3 (equatorial), C5 (axial) → 1 equatorial Cl atom
Final Answer:
✅ Chair 1 is the more stable conformer.
Explanation
Cyclohexane exists in a dynamic equilibrium between two chair conformations, which differ in the spatial orientation of substituents—either axial (pointing up/down, perpendicular to the ring) or equatorial (pointing outward, more stable for bulky groups). In this case, we examine a trisubstituted cyclohexane with chlorine atoms at positions 1, 3, and 5.
Chlorine is bulkier than hydrogen, so it prefers the equatorial position to reduce steric hindrance and unfavorable 1,3-diaxial interactions. When constructing Chair 1, placing the Cl on C1 in the equatorial position automatically determines the axial/equatorial orientation of the other substituents due to the fixed pattern around the cyclohexane ring. After filling in all groups, Chair 1 has two Cl atoms in equatorial positions and one in axial, making it relatively stable.
A chair flip inverts all axial/equatorial positions. After the flip (Chair 2), only one Cl is in the equatorial position, while two are in the less stable axial positions. This increases steric strain and makes Chair 2 less favorable.
Thus, the conformation with the most chlorine atoms in equatorial positions (Chair 1) is the more stable conformer. This principle is key in conformational analysis and helps predict molecular behavior in solution.
