Given the planar trisubstituted cyclohexane below, fill in all boxes (with H, F,Cl or Br) to complete the two possible cyclohexane chair conformations.
Which is the more stable conformer?
a)Chair1
b) Chair2
The correct answer and explanation is:
The stability of the two chair conformations of trisubstituted cyclohexane depends on steric hindrance and 1,3-diaxial interactions. The more stable chair conformation will have the bulkier groups in the equatorial positions to minimize steric strain.
Step 1: Identifying Substituents
From the image, the given substituents on the planar structure are H, F, Cl, and Br. Since Br is the largest, followed by Cl, then F, and finally H (the smallest), we need to place them in the chair conformation appropriately.
Step 2: Drawing the Two Chair Conformations
- Chair 1: The substituents are positioned based on the axial/equatorial flips. One possibility has Br in an axial position, which is unfavorable due to its large size.
- Chair 2: The substituents are flipped, placing Br in an equatorial position, reducing steric strain.
Step 3: Determining Stability
- Large groups prefer equatorial positions to reduce 1,3-diaxial interactions (steric strain with axial hydrogens).
- Bromine (Br) is the largest halogen, so it should ideally be equatorial for maximum stability.
- Fluorine (F) is the smallest and can tolerate axial positioning better than Cl or Br.
- Chlorine (Cl) is intermediate in size but is still better suited in an equatorial position if possible.
Conclusion: More Stable Conformer
- The more stable conformer will have Br and Cl in equatorial positions, which occurs in Chair 2.
Thus, the answer is:
(b) Chair 2 is the more stable conformer.