Fructose: The Laevorotatory Ketohexose and Its Anomers
Chemistry · Biomolecules · NEET
Fructose is a ketohexose. That means it has 6 carbons and a ketone group (C=O) at carbon-2, not an aldehyde. It is laevorotatory, so it turns plane-polarised light to the LEFT, and is written D-(–)-fructose. When it forms a ring, it makes a 5-membered furanose ring and exists as α and β anomers. Memory hook: "Fructose = Fruit sugar, Furanose ring, Five-membered, turns Left."
Fructose open chain has a ketone (C=O) at C-2 (ketohexose). The C-5 –OH adds to this C=O to make a five-membered furanose ring. C-2 becomes the anomeric carbon, giving α and β anomers.
Your doubts, answered
Is fructose an aldose or a ketose?
Fructose is a KETOSE. Its reactions show a ketone group (C=O) at carbon number 2. Glucose has an aldehyde group (–CHO) at C-1, so glucose is an aldose. Both have 6 carbons and the same formula C6H12O6, but the functional group is different. So fructose is a ketohexose and glucose is an aldohexose.
Why is fructose called laevorotatory?
Laevorotatory means it turns plane-polarised light to the LEFT (the (–) direction). Fructose does this, so it is written as D-(–)-fructose. Do not get confused: the letter D tells you the configuration (arrangement of –OH at the last chiral carbon, C-5), while the (–) sign tells you the direction it rotates light. A sugar can be D and still be laevorotatory. Glucose is D-(+)-glucose (rotates right).
Does fructose form a furanose or a pyranose ring?
Fructose forms a FURANOSE ring, which is a FIVE-membered ring (4 carbons + 1 oxygen). The ring forms when the –OH on C-5 adds across the C=O group at C-2. Glucose, in contrast, forms a six-membered PYRANOSE ring. Remember: Fructose → Furanose (both start with F, both are five-membered).
What are the anomers of fructose?
When fructose forms its ring, the ketone carbon (C-2) becomes a new chiral centre called the anomeric carbon. The new –OH there can point two ways. This gives two forms called anomers: α-D-fructofuranose and β-D-fructofuranose. They differ only in the position of the –OH at the anomeric carbon (C-2).
Is the anomeric carbon of fructose C-1 or C-2?
It is C-2. In fructose the carbonyl (C=O) is at C-2, so when the ring closes, C-2 becomes the anomeric carbon. In glucose the carbonyl is at C-1, so glucose's anomeric carbon is C-1. This matters in sucrose: the anomeric carbon of fructose (C-2) links to the anomeric carbon of glucose (C-1).
Where is fructose found and why is that a NEET clue?
Fructose is found in fruits, honey and vegetables. NEET often gives 'found in honey' as a clue. If a question lists clues like 'found in honey + keto sugar + has α/β anomers + laevorotatory', all four point to D-fructose (this was asked directly in NEET 2025).
⚠️ The NEET trap ✗ Fructose is dextrorotatory because it belongs to the D-series. ✓ Fructose belongs to the D-series (from its C-5 configuration) but it is LAEVOROTATORY, written D-(–)-fructose. The D/L label and the (+)/(–) rotation are two separate things. 🧠 D is CONFIGURATION, (+)/(–) is DIRECTION of rotation. They do not have to match.
Real NEET questions
2025
Sugar 'X': A. is found in honey B. is a keto sugar C. exists in α and β anomeric forms D. is laevorotatory. 'X' is:
A · Maltose
B · Sucrose
C · D-Glucose
D · D-Fructose ✓
Solution: Check each clue against D-fructose. (A) Honey is rich in glucose and fructose, so 'found in honey' fits. (B) Fructose has a ketone group at C-2, so it is a keto sugar (ketohexose). (C) Fructose cyclises to a five-membered furanose ring and exists as α-D-fructofuranose and β-D-fructofuranose (the α/β anomers). (D) Fructose rotates light to the left, so it is laevorotatory, D-(–)-fructose. All four clues match D-fructose. Maltose and sucrose are disaccharides, and glucose is an aldose that is dextrorotatory, so they fail one or more clues. 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 it, the anomeric carbon of α-D-glucose (C-1) is joined to the anomeric carbon of β-D-fructose (C-2) by a glycosidic linkage. On hydrolysis this linkage breaks and gives an equimolar mixture of α-D-glucose and β-D-fructose. This shows fructose exists in the β-anomeric furanose form inside sucrose. Answer: (A).
Solved Biomolecules NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Fructose has the molecular formula C6H12O6, the same as glucose. They are isomers: fructose is a ketohexose and glucose is an aldohexose.
How is fructose obtained?
Fructose is obtained along with glucose by the hydrolysis of the disaccharide sucrose. It also occurs naturally in fruits, honey and vegetables, and is used in pure form as a sweetener.
Is fructose a reducing sugar?
Yes, free fructose is a reducing sugar because it has a free ketonic group at the anomeric carbon. But inside sucrose the anomeric carbon is locked in the glycosidic bond, which is why sucrose itself is non-reducing.
How do you write fructose fully with configuration and rotation?
It is written D-(–)-fructose. 'D' shows the configuration at C-5, and '(–)' shows it is laevorotatory (turns light to the left).
What is the difference between the ring size of glucose and fructose?
Glucose forms a six-membered pyranose ring, while fructose forms a five-membered furanose ring. This is a common NEET trap.
