Which sequence correctly lists cyclohexane conformations in increasing energy?

AChair < boat < twist-boat < half-chair
BTwist-boat < chair < boat < half-chair
CChair < half-chair < twist-boat < boat
DChair < twist-boat < boat < half-chair

Answer: D

The chair is the strain-free minimum. The twist-boat (about 23 kJ/mol above) relieves part of the flagpole and eclipsing strain of the boat (about 30 kJ/mol, a transition state between twist-boats), and the half-chair (about 45 kJ/mol) is the highest-energy point on the interconversion path.

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