A monosubstituted cyclohexane with a tert-butyl group at C1 undergoes ring flip. After the flip, the tert-butyl group is now in the axial position. Why is this new conformer far less stable than the original equatorial conformer?
AThe axial tert-butyl group eclipses the adjacent C–H bonds, creating torsional strain
BThe axial tert-butyl group points toward the axial hydrogens on C3 and C5, causing severe 1,3-diaxial steric interactions
CThe axial position forces the tert-butyl group into a gauche interaction with the ring carbons, raising angle strain
DThe axial position prevents ring flip from occurring in the reverse direction, trapping the molecule in a high-energy state
1,3-diaxial interactions arise because axial substituents on alternating carbons (C1, C3, C5) all point in the same direction. An axial substituent at C1 is spatially close to the axial hydrogens at C3 and C5. A tert-butyl group is extremely bulky, making these interactions catastrophically destabilizing — on the order of 22 kJ/mol preference for equatorial. This is why tert-butyl effectively locks the ring in the chair with tert-butyl equatorial. The interactions are steric (spatial clashing), not related to eclipsing (option A, a torsional effect) or angle strain (option C).
Question 2 Multiple Choice
Why is cyclopropane significantly more strained than cyclopentane, even though both form rings that require carbons to adopt non-ideal geometries?
ACyclopropane has more carbons, so more total bonds must deviate from the ideal angle
BCyclopropane's ring has C–C–C angles of approximately 60°, deviating dramatically from the tetrahedral ideal of 109.5°, creating severe angle strain
CCyclopropane cannot undergo ring flip, trapping it in a single high-energy conformation
DCyclopropane experiences more torsional strain because all six C–H bonds are eclipsed simultaneously
Angle strain increases with deviation from the 109.5° tetrahedral ideal. Cyclopropane has internal angles of 60° — a 49.5° deviation — creating enormous strain. Cyclopentane has internal angles of ~108°, only ~1.5° from ideal, so it is nearly strain-free. Both also have torsional strain, but angle strain is the dominant source for cyclopropane. Cyclohexane solves both problems with the chair conformation: bond angles of ~109.5° and fully staggered C–H bonds, essentially eliminating both types of strain.
Question 3 True / False
In the chair conformation of cyclohexane, axial substituents on adjacent (neighboring) carbons point in the same direction — both up or both down.
TTrue
FFalse
Answer: False
Axial positions alternate around the chair: if the axial position on C1 points up, the axial position on C2 points down, C3 up, C4 down, and so on. This alternating pattern is a geometric consequence of the chair geometry. Adjacent carbons have opposite axial orientations. It is carbons two positions apart (C1 and C3, or C2 and C4) whose axial bonds point in the same direction — which is exactly why axial substituents at C1 and C3 are spatially close and create 1,3-diaxial interactions.
Question 4 True / False
The flat hexagonal structure commonly drawn for cyclohexane accurately represents its three-dimensional geometry.
TTrue
FFalse
Answer: False
The flat hexagonal drawing is purely conventional shorthand for connectivity — it does not represent the actual shape. A planar cyclohexane would have 120° internal angles (forcing sp² geometry), all adjacent C–H bonds eclipsed, and severe torsional strain. The actual structure is a puckered chair with ~109.5° bond angles and fully staggered bonds. This is one of the most common misconceptions: the 2D hexagon communicates that six carbons are bonded in a ring, not that the ring is flat. Even textbooks that draw it flat understand the three-dimensional reality is quite different.
Question 5 Short Answer
Explain why an axial substituent on cyclohexane is less stable than the same substituent in an equatorial position, specifically identifying the structural feature responsible.
Think about your answer, then reveal below.
Model answer: In the chair conformation, axial substituents project straight up or down, parallel to the ring's axis. This places them spatially close to the axial hydrogens on the carbons two positions away (C3 and C5 for a substituent at C1). These are called 1,3-diaxial interactions — steric clashes between the axial substituent and the axial H atoms on alternating carbons pointing in the same direction. Equatorial substituents project outward and away from the ring, avoiding these clashes entirely. The energy cost of the axial position increases with substituent size: a methyl group costs ~7.6 kJ/mol, while a bulky tert-butyl costs ~22 kJ/mol because its greater spatial extent makes the diaxial clashes much more severe.
The 1,3-diaxial interaction is the cyclohexane analogue of the gauche interaction in butane — both arise from two groups being forced into close spatial proximity by the geometry of the carbon framework. The key structural insight is the alternating pattern of axial bonds: C1 axial up, C2 axial down, C3 axial up — meaning C1 and C3 axial bonds point in the same direction and their substituents are roughly 2.5 Å apart, well within van der Waals contact for groups larger than hydrogen.