Discussion
Most stable conformation of 5-hydroxy-1, 3-dioxane is :
- A.
- B.
- C.
- D.
Answer: A
The most stable conformation is (A), the chair with the OH group axial.
In 5-hydroxy-1,3-dioxane the axial OH points toward the two ring oxygens on the same face. It can form an intramolecular hydrogen bond with them, and there are no 1,3-diaxial C-H interactions to oppose it, because the axial positions at C-1 and C-3 are ring oxygens with lone pairs rather than hydrogens. This outweighs the usual preference for equatorial substituents, especially in non-polar solvents.
Chair (B) has the OH equatorial, so it loses the hydrogen bond. The boat-like forms (C) and (D) suffer torsional and flagpole strain and are higher in energy than either chair.
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