= Solution
<protonation> of the secondary OH permits <water> departure. The neighbouring phenylthio <sulfur> can assist this departure by forming a bond to the adjacent <alcohol> <carbon>, generating a <thiiranium ion> spanning that <carbon> and the original <sulfur>-bearing <cyclohexane> <carbon>. Label the latter Q and the former L. \b[The temporary Q–S–L bridge transfers the phenylthio attachment from Q to L when its Q–S bond subsequently breaks.] <sulfur> attack opposite the departing OH accounts for the new C–S <stereochemistry> without requiring a freely rotating <carbocation>.
For the protected terminal <alcohol>, no free tethered OH is available to trap the bridge. Removal of a beta <hydrogen> from a <cyclohexane> <carbon> adjacent to Q forms the ring <alkene> while Q–S breaks. <sulfur> remains bonded to L, giving the allylic phenyl <thioether>. <acid (chemistry)> is regenerated by the <deprotonation>, so the <catalyst> need not be consumed.
For the free terminal <alcohol>, its <oxygen> attacks Q intramolecularly, opening Q–S instead. Count the new <ether> ring: \b[O–CH2–CH(Me)–L–Q], a five-membered ring sharing Q with the <cyclohexane>. <deprotonation> gives the spirocyclic <ether>. This is capture of the same neighbouring-group-stabilized intermediate rather than migration of the <methyl group>. The pre-existing methyl-bearing <carbon> retains its configuration; its wedge/dash appearance changes when the product is redrawn in a different ring projection. The two substrates differ in whether a suitably placed <oxygen> <nucleophile> is available for intramolecular trapping.
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2022/paper-1-sulfur-pathways.png]
{title=Original local atom maps of sulfur participation, the equivalent paths of the bridged thietanium ion, and the five-membered intermediate in thiirane sulfur exchange}
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