Aldose vs Ketose: How to Classify Sugars by Functional Group
Chemistry · Biomolecules · NEET
An aldose is a sugar whose functional group is an aldehyde (-CHO). A ketose is a sugar whose functional group is a ketone (C=O in the middle). Glucose is an aldose; fructose is a ketose. Memory hook: "Aldose = Aldehyde at the top; Ketose = Keto in the middle."
Glucose (aldose) has the C=O as an aldehyde at the top carbon (C-1); fructose (ketose) has the C=O as a keto group in the middle (C-2). Both have formula C6H12O6, so they are isomers.
Your doubts, answered
Is glucose an aldose or a ketose?
Glucose is an ALDOSE. Its first carbon (C-1) has an aldehyde group (-CHO). Since glucose has 6 carbons and an aldehyde group, its full name is an aldohexose. Remember: glucose = aldose.
Is fructose an aldose or a ketose?
Fructose is a KETOSE. Its second carbon (C-2) has a keto group (C=O), not an aldehyde. Fructose has 6 carbons and a keto group, so it is a ketohexose. Remember: fructose = ketose. This is asked a lot in NEET.
What is the exact difference between aldose and ketose?
The difference is only the functional group. An aldose has an aldehyde group (-CHO) at the end carbon (C-1). A ketose has a ketone group (C=O) inside the chain (usually C-2). Both are monosaccharides, both can have the same number of carbons.
What do 'aldohexose' and 'ketohexose' mean?
These names combine two facts: the functional group + the number of carbons. 'Aldo' means aldehyde, 'keto' means ketone. 'Hexose' means 6 carbons (hex = 6). So aldohexose = aldehyde sugar with 6 carbons (glucose). Ketohexose = keto sugar with 6 carbons (fructose).
How do I quickly tell aldose from ketose in a structure?
Look at the top carbon of the Fischer projection. If C-1 is -CHO (H on one side, =O double bond), it is an aldose. If the =O is on C-2 (the second carbon, inside the chain), it is a ketose. The keto group is never at the very end.
Are both aldoses and ketoses monosaccharides?
Yes. Aldose and ketose is one way to classify monosaccharides (single sugar units). Another way is by carbon number: triose (3C), tetrose (4C), pentose (5C), hexose (6C). NEET often combines both, e.g. aldohexose or ketohexose.
⚠️ The NEET trap ✗ Fructose is an aldose because, like glucose, it also has the formula C6H12O6. ✓ Fructose is a KETOSE (ketohexose). Same molecular formula C6H12O6 as glucose does NOT mean same functional group. Glucose has -CHO (aldose); fructose has C=O keto group (ketose). They are isomers. 🧠 Same formula, different group. In NEET 2025, the clue 'is a keto sugar' pointed straight to fructose, not glucose.
Real NEET questions
NEET 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: The key clue is 'B. is a keto sugar' — this means X is a KETOSE, so it cannot be glucose (an aldose). Fructose is a ketohexose (keto group at C-2). It is found in honey (with glucose), forms α and β anomers when it cyclises, and is laevorotatory (rotates light to the left). All four clues fit D-Fructose. Answer: D.
NEET 2016 (Phase 2)
The correct corresponding order of names of the four ALDOSES with the given configurations (1)–(4) is:
A · L-erythrose, L-threose, L-erythrose, D-threose
B · D-threose, D-erythrose, L-threose, L-erythrose
C · L-erythrose, L-threose, D-erythrose, D-threose
D · D-erythrose, D-threose, L-erythrose, L-threose ✓
Solution: All four are aldoses (aldotetroses) — they carry an aldehyde -CHO at C-1. The D/L label is set by the lowest chiral carbon (C-3): -OH on the right = D, on the left = L. Erythrose has both -OH on the same side; threose has them on opposite sides. Reading the four structures gives D-erythrose, D-threose, L-erythrose, L-threose. Answer: D.
Solved Biomolecules NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
An aldose is a sugar that has an aldehyde group (-CHO) at its first carbon. Glucose is the most common example. It is one type of monosaccharide.
What is a ketose in simple words?
A ketose is a sugar that has a keto (ketone) group, a C=O in the middle of the chain, usually at the second carbon. Fructose is the classic example.
Give one example of each for NEET.
Aldose example: glucose (an aldohexose). Ketose example: fructose (a ketohexose). These two are the examples NEET expects you to know.
Why do NEET questions combine the functional group with the carbon number?
Because it names the sugar fully. 'Aldo/keto' tells the functional group and '-triose/-pentose/-hexose' tells the number of carbons. So glucose = aldohexose (aldehyde + 6 C) and fructose = ketohexose (keto + 6 C).
Can an aldose change into a ketose?
Yes, in basic (alkaline) conditions glucose and fructose can interconvert. This is why fructose also reduces Fehling's and Tollens' reagents even though it is a ketose — it isomerises to an aldose first. This links to reducing vs non-reducing sugars.
