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1,3-Dioxane is a six-membered ring with oxygen at positions 1 and 3. It commonly adopts chair conformations. Axial substituent penalties differ sharply between C2 and C5 because the relevant 1,3 contacts involve different ring atoms.

In a chair conformation of cyclohexane, axial bonds point approximately parallel to the ring's threefold axis, alternating up and down around the ring. Equatorial bonds point outward around its perimeter, also alternating in their up/down relationship to the ring.

A ring flip exchanges axial and equatorial positions but preserves a substituent's up or down orientation. Thus axial does not mean up, and equatorial does not mean down. In methylcyclohexane the equatorial methyl conformation is favored because an axial methyl group has unfavorable close contacts with axial substituents on carbons three and five.^[ref]

The chair conformation is a low-energy nonplanar arrangement of the six-membered cyclohexane ring. Its carbon-carbon bonds are approximately staggered and its bond angles remain close to tetrahedral values, reducing torsional and angle strain.

Each ring carbon has one axial and one equatorial substituent position. A ring flip exchanges these positions while retaining whether each substituent points to the same or opposite side of the ring. Bulky substituents commonly prefer equatorial positions because this reduces unfavorable 1,3-diaxial interactions, making chair analysis useful for predicting conformational populations and reaction geometry.^[ref]

Cyclohexane is the cyclic alkane ${\mathrm{C}}_6{\mathrm{H}}_{12}$. Its preferred chair conformation allows near-tetrahedral bond angles and largely staggered neighboring bonds.

Ring flips exchange axial and equatorial positions. Consequently a substituent can retain its up/down orientation while changing the steric interactions that determine conformational preference and reaction accessibility.

Number the OH-bearing carbon C1 and the adjacent brominated carbon C2. In the trans bromohydrin, base generates an alkoxide. A chair conformation can place O at C1 and Br at C2 trans-diaxial, aligning the oxygen nucleophile for backside attack at C2. Intramolecular SN2 closure expels bromide and inverts C2: the retained C1–O bond and the newly formed C2–O bond are both on the original OH face, giving the epoxide shown. ring inversion can provide the reactive chair even if it is not the dominant ground-state conformer.

The cis bromohydrin cannot place OH and Br trans-diaxial: one is axial when the other is equatorial. In the chair with Br axial, the hydrogen on C1 can instead be antiperiplanar to C2–Br. An alkoxide-assisted semipinacol pathway moves the C1–H bonding pair to C2 while the oxygen lone pair forms C1=O and bromide departs. hydride migration gives the ketone without changing the carbon skeleton. The hydrogen is transferred to C2 rather than expelled as a free hydride. The original phenyl-bearing stereocentre is not involved in either bond-reorganization sequence. Configuration controls which properly aligned displacement or migration can occur; a flat ring drawing alone conceals that distinction.