Fructose is a ketohexose (C=O at C2) — ketones normally do NOT reduce Tollens'/Fehling's reagent. Yet fructose gives positive results. Why?

In the basic medium of these tests, fructose undergoes Lobry de Bruyn-Alberda van Ekenstein transformation: fructose ⇌ enediol intermediate ⇌ glucose + mannose. The aldoses (glucose, mannose) then reduce the reagent.

Seliwanoff's test distinguishes ketoses from aldoses: HCl + resorcinol — ketoses give cherry-red color FASTER (within 1-2 minutes) than aldoses (which need prolonged heating). Fructose → dehydration → hydroxymethylfurfural → red complex with resorcinol.

Osazone formation: both glucose and fructose give the SAME osazone with phenylhydrazine (glucosazone = fructosazone) because C1 and C2 are both involved in osazone formation — the difference between glucose and fructose is only at C1/C2.