Question 8 of 13: Chair or Boat — Six Bridged/Caged Systems
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
National Exam 04-BS-12, Organic Chemistry — May 2016. 3 hours, closed-book examination;
one aid sheet (8.5×11", both sides) and a Casio or Sharp calculator permitted. Ten questions
constitute a complete exam paper (only the first 10 questions as they appear in the answer book are
marked, each of equal value) — the source paper in fact prints thirteen questions; all thirteen
are answered in full below.
Governing principle. An unconstrained cyclohexane ring always adopts the
chair — the only conformation with every bond staggered (zero torsional strain) and every
bond angle near the ideal tetrahedral 109.5° (zero angle strain). A ring is forced into the
higher-energy boat (or twist-boat) only when an additional bridge rigidly clamps
two ring atoms together at a distance/geometry the chair cannot accommodate. Each panel below is
judged by whether such a bridge is present.
1 — CHAIR. An unconstrained,
simply-substituted cyclohexane: no bridge restricts it, so it freely relaxes into the
fully-staggered, strain-free chair — the global energy minimum for any six-membered ring left
to its own devices.
2 — BOAT (forced). A one-atom oxygen bridge
(7-oxa-bicyclo[2.2.1]heptane-type) clamps two ring atoms (the bridgeheads) directly across the
ring. A chair's bridgehead-to-bridgehead (1,4) distance is far too long for a one-atom bridge to
span; the six-membered ring (traced the "long way" around, not through the bridge) is forced to pucker
into a boat to bring its bridgeheads close enough together.
3 — BOAT (forced), same bridging principle from a
different perspective. Another one-atom (oxa-)bridged bicyclic framework: the short bridge
again geometrically excludes the chair, for exactly the reasoning given for panel 2.
4 — BOAT (forced) — norbornane
(bicyclo[2.2.1]heptane). The single-carbon methylene bridge (C7) plays the identical
role as the oxygen bridge in panels 2/3: it is too short to let either six-membered "half" of the
bicycle reach a chair, so both are locked into boats. This is the single most commonly cited textbook
example of bridging-enforced boat conformation.
5 — CHAIR (all rings) — adamantane.
Adamantane's tricyclic cage is built entirely from fused (not short-bridged) cyclohexane
rings, arranged exactly as in the diamond lattice; every one of its six-membered rings independently
achieves a fully-staggered chair with no bridging strain at all — the classic
essentially-strain-free cage, contrasted deliberately with panels 2–4 and 6.
6 — BOAT (forced) — bicyclo[2.2.2]octane.
Three equivalent two-carbon bridges connect the same two bridgehead carbons. Because all three bridges
are rigid and roughly equal in length, none of the three six-membered rings that can be traced through
this cage is free to relax into a chair — each is over-constrained by the other two
bridges, and all three are forced into boats. This is the standard "second example" always paired with
norbornane to show the bridging principle generalises.
both are
treated here under the same "short bridge forces a boat" principle, since that (not their exact
relative regiochemistry) is what the question is testing. Schematic wireframes are used in place of
literal skeletal renderings for clarity of the 3-D shape being identified.
