Carbohydrates Glucose Fructose Structure Tests Tollens

Carbohydrates are compounds of carbon, hydrogen and oxygen with the general formula Cₓ(H₂O)ᵧ — literally "hydrates of carbon". Glucose and fructose, both C₆H₁₂O₆, are the most important monosaccharides: glucose is an aldohexose (an aldehyde group at carbon 1) while fructose is a ketohexose (a ketone group at carbon 2), and this one functional-group difference changes their chemistry.

Glucose is the body's main fuel and the building block of starch and cellulose; fructose sweetens fruits and honey. Tollens' and Fehling's tests reveal whether a sugar is "reducing" — essential for JEE, NEET and board exams.

Key Concepts

• Fischer projection: glucose has −CHO at C1 with −OH groups on C2, C3, C4, C5 in a specific left–right pattern; fructose has C=O at C2.
• Cyclic forms: in solution glucose cyclises to glucopyranose (a six-membered hemiacetal ring); the two forms differing at C1 are called α and β anomers.
• Reducing sugar: any sugar with a free aldehyde (or α-hydroxy ketone) group that can reduce Tollens' or Fehling's reagent.
• Units: molar mass of glucose and fructose is 180 g mol⁻¹.

Tollens' reagent ([Ag(NH₃)₂]⁺) is reduced by aldehydes to a silver mirror. Fehling's reagent (alkaline Cu²⁺ with Rochelle salt) gives a brick-red Cu₂O precipitate. Fructose still tests positive because in the alkaline medium it isomerises to glucose and mannose (Lobry de Bruyn–van Ekenstein transformation).

Formulas

The oxidation reactions behind the two tests are:

• Tollens': R−CHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → R−COO⁻ + 2Ag↓ + 4NH₃ + 2H₂O. Silver ions are reduced to a silver mirror; valid in ammoniacal medium.
• Fehling's: R−CHO + 2Cu²⁺ + 5OH⁻ → R−COO⁻ + Cu₂O↓ + 3H₂O. The blue solution gives a brick-red Cu₂O precipitate on heating; valid in strongly alkaline medium.
• Key assumption: one mole of aldose reduces two moles of Ag⁺ (Tollens') or two moles of Cu²⁺ to one mole of Cu₂O (Fehling's). These tests work only for reducing sugars; sucrose, with no free anomeric carbon, is non-reducing and gives negative results.

Worked Example

What mass of brick-red Cu₂O precipitate forms when 0.90 g of glucose is heated with excess Fehling's solution? Step 1: moles of glucose = 0.90 ÷ 180 = 0.0050 mol. Step 2: from the equation, 1 mol of glucose produces 1 mol of Cu₂O. Step 3: moles of Cu₂O = 0.0050 mol. Step 4: molar mass of Cu₂O = (2 × 63.5) + 16 = 143 g mol⁻¹. Step 5: mass = 0.0050 × 143 = 0.715 g. So 0.90 g of glucose yields 0.715 g of Cu₂O precipitate.

Common Misconceptions

• "Fructose is a ketone, so it cannot reduce Tollens' or Fehling's." Wrong — in the alkaline test medium fructose isomerises to glucose and mannose, so it tests positive.
• "Glucose exists only as an open chain." No — in aqueous solution over 99% of glucose is in cyclic hemiacetal (pyranose) form; the open-chain aldehyde form is present only in traces.
• "All disaccharides are reducing sugars." False — sucrose is non-reducing because the anomeric carbons of both glucose and fructose are locked in the glycosidic bond.

Real-World Applications

• Medical diagnostics: Benedict's test (a variant of Fehling's) was historically used to detect glucose in urine for diabetes screening.
• Food industry: reducing-sugar tests check honey purity and monitor the browning (Maillard) reactions in baking.
• Biochemistry: glucose is the universal cellular fuel, and its cyclic structure is the basis of starch, cellulose and glycogen.

Glucose (aldose) and fructose (ketose) share the formula C₆H₁₂O₆ but differ in structure and cyclic forms. Tollens' and Fehling's tests detect the aldehyde group — and fructose's positive result shows alkaline conditions can rearrange the sugar itself.