The energies of some cyclohexane conformations relative to the chair (0) are: twist-boat 23, boat 30, half-chair 45 kJ/mol. The energy barrier for the chair-to-chair ring flip (the highest point on the lowest-energy path starting from a chair) is

A $23 kJ mo l^{- 1}$
B $30 kJ mo l^{- 1}$
C $45 kJ mo l^{- 1}$
D $22 kJ mo l^{- 1}$

Answer: C

The ring flip passes chair, half-chair, twist-boat, boat (between two twist-boats), twist-boat, half-chair, chair. The highest point is the half-chair at 45 kJ/mol above the chair, which is the barrier. The twist-boat and boat are only intermediate minimum and maximum on the way.

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The energies of some cyclohexane conformations relative to the chair (0) are: twist-boat 23, boat 30, half-chair 45 kJ/mol. The energy barrier for the chair-to-chair ring flip (the highest point on the lowest-energy path starting from a chair) is

Answer: C

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