Cyclopropane reacts with bromine (Br₂) under mild conditions without a catalyst, whereas cyclohexane does not. Which explanation best accounts for this difference?
ACyclopropane is more polar than cyclohexane, making it more susceptible to electrophilic attack.
BRing strain in cyclopropane weakens its C–C bonds and provides a thermodynamic driving force for ring-opening addition reactions.
CCyclopropane has fewer hydrogen atoms, reducing competition from substitution reactions.
DThe small ring size increases electron density on carbon, facilitating nucleophilic attack by Br₂.
Ring strain (~115 kJ/mol in cyclopropane) makes the C–C bonds weaker than normal C–C bonds (bent/banana bonds with poor orbital overlap). Breaking a ring bond to add Br₂ releases that stored strain energy, making the reaction thermodynamically favorable. Cyclohexane in the chair conformation is nearly strain-free, so no such driving force exists. The other options misattribute the reactivity to polarity, hydrogen count, or nucleophilicity — none of which explain the ring-opening selectivity.
Question 2 Multiple Choice
Why does the heat of combustion per CH₂ unit serve as a reliable measure of ring strain?
ALarger rings always release more total energy, directly indicating greater stored strain.
BRings with more CH₂ units are thermodynamically more stable and thus release less energy per unit.
CThe excess energy released per CH₂ compared to a strain-free reference (open-chain CH₂, ~658.6 kJ/mol) quantifies the extra stored strain energy.
DHeat of combustion is independent of ring size, so any deviation from the expected value flags measurement error.
In a perfectly strain-free ring, each CH₂ would contribute the same combustion energy as in an open-chain alkane. Any excess beyond that reference value represents strain energy converted to heat. Cyclopropane releases ~697 kJ/mol per CH₂ (excess of ~38 kJ/mol per CH₂); cyclohexane matches the reference almost exactly. This per-unit normalization is necessary because comparing total heats of combustion across different ring sizes would be confounded by ring size itself.
Question 3 True / False
Cyclohexane is strain-free in any of its ring conformations.
TTrue
FFalse
Answer: False
Cyclohexane is nearly strain-free only in the chair conformation, where bond angles (~111°) are close to tetrahedral and all adjacent C–H bonds are staggered. The boat conformation has flagpole C–H interactions and eclipsed bonds on the 'sides,' imposing significant torsional strain (~29 kJ/mol above the chair). The twist-boat is intermediate. Treating cyclohexane as universally strain-free is a common error — the conformational energy differences are real and large enough to matter in synthesis and biology.
Question 4 True / False
Cyclopentane adopts a non-planar envelope conformation rather than a flat structure, primarily to reduce torsional strain at the cost of slightly increased angle strain.
TTrue
FFalse
Answer: True
A planar cyclopentane would have bond angles of 108° (very close to ideal 109.5°, so angle strain is minimal) but all adjacent C–H bonds would be nearly eclipsed, creating significant torsional strain. Puckering into the envelope conformation lifts one carbon out of the plane, staggering most of the C–H bonds and reducing torsional strain. The geometric penalty on bond angles is small because 108° is already close to tetrahedral. This is why cyclopentane has only a small residual ring strain (~26 kJ/mol total).
Question 5 Short Answer
Why is cyclohexane considered nearly strain-free while cyclopentane still retains some residual ring strain? Address both angle strain and torsional strain in your answer.
Think about your answer, then reveal below.
Model answer: Cyclohexane in the chair conformation achieves bond angles of ~111° (very close to the ideal 109.5°) AND has all adjacent C–H bonds perfectly staggered — both angle strain and torsional strain are simultaneously minimized. Cyclopentane has near-ideal bond angles (~108°) so its angle strain is negligible, but its envelope conformation still leaves some C–H eclipsing interactions that cannot be fully eliminated, producing residual torsional strain. Cyclohexane's advantage is that its geometry allows both strain components to be minimized at once, which the smaller ring cannot achieve.
Ring strain has two sources: angle strain (deviation from 109.5°) and torsional strain (eclipsed C–H interactions). A ring can be strain-free only if both are simultaneously minimized. Cyclohexane's chair accomplishes this; cyclopentane's geometry does not allow complete staggering of all C–H bonds regardless of puckering. This two-component nature of strain is the key insight — focusing on only bond angles (angle strain) misses why cyclopentane is not strain-free.