Reducing vs Non-Reducing Sugars: Fehling's and Tollens' Test
Chemistry · Biomolecules · NEET
A reducing sugar has a free -CHO or free keto group (a free anomeric -OH) that can give electrons away. So it turns Fehling's solution red and makes a silver mirror in Tollens' test. All monosaccharides (glucose, fructose) and most disaccharides (maltose, lactose) are reducing; sucrose is NON-reducing because its two "free" groups are locked inside the glycosidic bond. Memory hook: "FREE group = passes the test; LOCKED group (sucrose) = fails."
A reducing sugar keeps a free anomeric (-CHO/keto) group and turns Fehling's red and Tollens' into a silver mirror. Sucrose locks both anomeric carbons in its glycosidic bond, so it fails both tests and is non-reducing.
Your doubts, answered
Which sugars are reducing and which is non-reducing?
Reducing: all monosaccharides (glucose, fructose, galactose, ribose) AND the disaccharides maltose and lactose. Non-reducing: sucrose. NCERT rule: a sugar is reducing if it can reduce Fehling's solution and Tollens' reagent. Sucrose is the classic NEET exception because both its reducing groups are used up in the glycosidic bond.
Why is sucrose non-reducing but maltose and lactose are reducing?
In sucrose the glycosidic linkage joins C1 (anomeric carbon) of glucose to C2 (anomeric carbon) of fructose. BOTH reducing groups are locked, so no free -CHO or keto group is left. In maltose and lactose only ONE anomeric carbon is used in the linkage; the other unit still has a free anomeric -OH that can open to a free aldehyde. That free group makes them reducing.
Fructose is a ketone, so how does it pass Tollens' and Fehling's tests?
Tollens' and Fehling's are usually for aldehydes, but they are run in an alkaline (basic) medium. In base, fructose (a ketose) undergoes rearrangement (via an enediol) into an aldose. So it gives a free -CHO and reduces the reagents. That is why NCERT says 'all monosaccharides whether aldose or ketose are reducing sugars.'
What is the difference between the Fehling's test and the Tollens' test?
Fehling's uses Cu2+ (deep blue). A reducing sugar reduces Cu2+ to give a brick-red precipitate of Cu2O (cuprous oxide). Tollens' uses ammoniacal silver nitrate [Ag(NH3)2]+. A reducing sugar reduces Ag+ to metallic silver, giving a shiny silver mirror on the tube. Both are positive for reducing sugars and negative for sucrose.
What is Fehling's solution A and solution B made of?
Fehling's A is aqueous copper(II) sulphate (CuSO4). Fehling's B is an alkaline solution of sodium potassium tartrate (Rochelle's salt). They are mixed just before use to form a deep-blue cupric tartrate complex. This exact composition was asked directly in NEET 2024.
What does 'free anomeric carbon' mean and why does it decide reducing power?
When a sugar cyclises, C1 (in glucose) or C2 (in fructose) becomes the anomeric carbon and carries a hemiacetal -OH. This ring can open back to a free -CHO/keto group. If that anomeric -OH is FREE, the sugar can open, react, and reduce the reagent. If it is tied up in a glycosidic bond (like both anomeric carbons in sucrose), the ring cannot open, so the sugar is non-reducing.
⚠️ The NEET trap ✗ Picking glucose, maltose or lactose as the 'non-reducing' sugar because they look like ordinary sugars. ✓ Sucrose is the only common non-reducing sugar. Glucose (free -CHO), maltose and lactose (one free anomeric carbon each) are all reducing. 🧠 Only SUCROSE is non-reducing among the common sugars. Both its anomeric carbons are locked in the glycosidic bond, so no free group is left to reduce Fehling's or Tollens'.
Real NEET questions
2016
Which one given below is a non-reducing sugar?
A · Maltose
B · Lactose
C · Glucose
D · Sucrose ✓
Solution: In sucrose the glycosidic linkage joins the anomeric carbon of glucose (C1) to the anomeric carbon of fructose (C2), so NO free hemiacetal/hemiketal (-OH on the anomeric C) group is left. With no free -CHO or keto group, sucrose cannot reduce Fehling's or Tollens', so it is non-reducing. Maltose and lactose each keep one free anomeric carbon, and glucose has a free -CHO, so all three are reducing. Answer: (D).
2024
Fehling's solution 'A' is:
A · alkaline CuSO4
B · alkaline solution of sodium potassium tartrate (Rochelle's salt)
C · aqueous sodium citrate
D · aqueous CuSO4 ✓
Solution: Fehling's reagent is made fresh by mixing two parts. Solution 'A' is aqueous copper(II) sulphate (CuSO4), and solution 'B' is an alkaline solution of sodium potassium tartrate (Rochelle's salt). On mixing they form a deep-blue cupric tartrate complex. A reducing sugar reduces Cu2+ to red Cu2O. So Fehling's A = aqueous CuSO4. Answer: (D).
2020
Sucrose on hydrolysis gives:
A · α-D-Glucose + β-D-Fructose ✓
B · α-D-Fructose + β-D-Fructose
C · β-D-Glucose + α-D-Fructose
D · α-D-Glucose + β-D-Glucose
Solution: Sucrose is a non-reducing disaccharide in which the anomeric carbons of α-D-glucose and β-D-fructose are joined by an α,β-glycosidic linkage. On hydrolysis this linkage breaks to give an equimolar mixture of α-D-glucose and β-D-fructose (this mixture is 'invert sugar', which is laevorotatory). Answer: (A).
Solved Biomolecules NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. Glucose has a free aldehyde (-CHO) group, so it reduces Fehling's solution (red Cu2O) and gives a silver mirror with Tollens' reagent. All monosaccharides are reducing sugars.
Is fructose a reducing sugar even though it is a ketone?
Yes. In the alkaline medium of the tests, fructose rearranges into an aldose form and then reduces the reagents. NCERT states all monosaccharides, aldose or ketose, are reducing sugars.
Are maltose and lactose reducing sugars?
Yes, both. Each keeps one free anomeric carbon (a free -OH that can open to a -CHO), so they reduce Fehling's and Tollens'. Only sucrose, among the common disaccharides, is non-reducing.
What color change shows a positive Fehling's test?
The deep-blue Cu2+ solution changes to a brick-red precipitate of cuprous oxide (Cu2O). This confirms a reducing sugar. Tollens' instead gives a shiny silver mirror.
Why does this matter for NEET?
NEET asks it directly and repeatedly: naming the non-reducing sugar (sucrose), Fehling's A/B composition, and sucrose hydrolysis products all appeared as one-mark questions in 2016, 2020 and 2024. Learn the free-vs-locked anomeric carbon rule and you can answer all of them.
