8. Which conformation of cyclohexane has a C3 axis of symmetry?
a. Boat
b. Twist boat
c. Chair
d. Envelope
The chair conformation of cyclohexane possesses a C3 axis of symmetry, making it the correct answer (option c) to this multiple-choice question commonly featured in CSIR NET Life Sciences exams on organic stereochemistry. This axis passes through the ring’s center, allowing 120-degree rotations that map the molecule onto itself.
Option Analysis
Boat Conformation lacks a C3 axis due to its symmetric “flagpole” hydrogens and eclipsed bonds, featuring instead a plane of symmetry but higher energy from steric strain.
Twist Boat Conformation is chiral with no symmetry elements like a C3 axis, as twisting breaks mirror planes and rotation axes, though it relieves some boat strain.
Chair Conformation exhibits D3d point group symmetry, including a C3 axis along the vertical through opposite carbon pairs, with staggered bonds eliminating angle and torsional strain for maximum stability.
Envelope Conformation (typically for five-membered rings but analogous here) distorts planarity without a C3 axis, causing high strain and no rotational symmetry.
Symmetry in Cyclohexane Chair
A C3 axis requires threefold rotational symmetry, present only in the chair where rotating 120° around the ring center aligns all axial/equatorial positions identically. Boat and twist-boat lack this due to flagpole interactions and asymmetry, while envelope’s pseudo-planar pucker prevents it. Visualizing via Newman projections confirms chair’s perfect staggering.
Exam Relevance for CSIR NET
This question tests conformational analysis and group theory basics in cyclohexane stereochemistry, key for molecular biology and biochemistry sections. Chair dominates (>99%) at room temperature via rapid ring flips, underscoring stability over higher-energy forms.
