= Solution
Use C1 for the carbonyl <carbon>, C2 for the adjacent <carbon>, and C3 for the beta <carbon> of the reactive <enone> fragment $\mathrm{R{-}C1(=O){-}C2H{=}C3H{-}R'}$. \b[<water> adds conjugately at C3, giving a beta-hydroxy ketone.] The <oxygen> <lone pair> of neutral <water> attacks C3; move the C2=C3 pi pair toward C1–C2 and the carbonyl pi pair onto <oxygen>. This produces the charge-separated <enolate>–oxonium intermediate
$$
\mathrm{R{-}C1(O^-){=}C2H{-}C3H(OH_2^+){-}R'}.
$$
A second <water> molecule can relay a <proton> from the newly attached oxonium <oxygen> to the <enolate> system. Using the carbon-centered <resonance structure>, reform C1=O and protonate C2 while the donating O–H bond returns its pair to the added <oxygen>. Equivalently, <oxygen> protonation first gives an <enol>, followed by water-mediated <keto-enol tautomerism>. Either path yields a neutral beta-hydroxy ketone and regenerates the relay <water>. There is no need to introduce externally supplied H$^+$ or OH$^-$; solvent molecules perform the <proton> transfers. The uncatalyzed charge separation has a large barrier, so this mechanism describes a slow formal pathway, not a claim of rapid hydration in pure <water>.
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2025/paper-2-mechanisms.png]
{title=Reactive-fragment mechanisms for conjugate hydration, steroid reduction and finasteride trapping}
Back to article page
