Q.(a) Describe the conformations of cyclohexanol. Comment on their stability.

(a) Like cyclohexane, cyclohexanol exists mainly as chair conformers; the equatorial-OH chair is favoured over the axial-OH chair because it minimises 1,3-diaxial strain, with boat/twist-boat forms being much less stable and negligibly populated. (b) Three classic derivatisations of salicylic acid and of formic acid.

(a) Conformations of cyclohexanol

Cyclohexanol, like cyclohexane itself, is not planar; the ring puckers to relieve angle and torsional strain, adopting mainly the chair conformation, in which all ring bonds are perfectly staggered (zero torsional strain) and bond angles are close to the tetrahedral $109.5^\circ$ (minimal angle strain) — this is the most stable conformation available to the ring.

In the chair form, each ring carbon bears one axial and one equatorial substituent. For cyclohexanol, the $-OH$ group can therefore occupy either an axial or an equatorial position:

• Equatorial-OH chair: the $-OH$ group points outward, roughly in the 'equatorial belt' of the ring, and experiences minimal steric interference from the rest of the ring.
• Axial-OH chair: the $-OH$ group points perpendicular to the mean plane of the ring and suffers 1,3-diaxial interactions — steric repulsion with the axial hydrogens on the two carbons three bonds away (C3 and C5 relative to C1) — which raises its energy.

Because $-OH$ is a relatively small group, the energy difference between the two chair forms is modest, but the equatorial-OH chair is still the more stable and therefore the predominant conformer at equilibrium. The two chair forms rapidly interconvert at room temperature by ring-flipping, which simultaneously converts every axial substituent to equatorial and vice versa; the position of this equilibrium favours the conformer with $-OH$ equatorial.

The alternative boat and twist-boat conformations are significantly higher in energy — the boat form suffers eclipsing (torsional) strain along its 'side' bonds plus transannular ('flagpole') steric clash between the two flagpole hydrogens, and although the twist-boat relieves some of this strain relative to the pure boat, both remain considerably less stable than either chair form and are only negligibly populated at equilibrium.

(b) Conversions

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