Reducing sugars: Benedict’s, Fehling’s and Tollens’ tests template

In base, β-D-glucose opens to the aldehyde; Cu²⁺ (Benedict’s or Fehling’s) is reduced to red Cu₂O and Ag⁺ (Tollens’) to a silver mirror while C1 is oxidised to gluconate. Sucrose gives no reaction.

Mechanism step by step

The hemiacetal opens to the aldehyde, which Cu²⁺ or Ag⁺ oxidises to the carboxylate; sucrose has no hemiacetal and does not react

1. Deprotonation – Hydroxide removes the anomeric OH proton
2. Ring opening – O⁻ reforms C=O and pushes out the ring oxygen
3. Protonation – Water protonates O5: the free aldehyde
4. Benedict’s / Fehling’s – Two Cu²⁺ (citrate or tartrate complex) take 2 e⁻: red Cu₂O
5. Tollens’ test – Two [Ag(NH₃)₂]⁺ take 2 e⁻: a silver mirror
6. Sucrose: negative – Both anomeric carbons are in the glycosidic bond, so no ring can open. Acid hydrolysis first makes it positive

Key points

• A reducing sugar has a free hemiacetal (or a ketose that isomerises in base): glucose, galactose, fructose, maltose and lactose are all positive.
• Benedict’s: RCHO + 2 Cu²⁺ + 5 OH⁻ → RCOO⁻ + Cu₂O + 3 H₂O. Tollens’: RCHO + 2 [Ag(NH₃)₂]⁺ + 3 OH⁻ → RCOO⁻ + 2 Ag + 4 NH₃ + 2 H₂O.
• Sucrose (Glc α1↔β2 Fru) is non-reducing; boiling with acid first hydrolyses it to glucose and fructose, and the test turns positive.
• Hot alkali also enolises and fragments sugars, so these equations are idealised; electron transfer to the metal is not drawn with curly arrows.

Drawn with 211 atoms, 13 arrows, 10 shapes – every one editable in MolDraw.

Sources

• Nelson & Cox, Lehninger Principles of Biochemistry, 8th ed., 2021, ch. 7 Textbook
• Berg, Tymoczko, Gatto & Stryer, Biochemistry, 9th ed., 2019 Textbook
• Benedict, Biochem. J. 1907, 2, 408 Original paper