Aldoses vs Ketoses- Definition, 7 Major Differences, Examples

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Aldoses Definition

An aldose is a monosaccharide consisting of a carbon backbone and a carbonyl group at carbon-1, resulting in an aldehyde group.

  • The general formula of aldoses is the same as most carbohydrates, Cn(H2O)n. The carbon atoms in the carbon backbone are each bonded to a hydroxyl group.
  • All aldoses exhibit stereoisomerism as they have an asymmetrical carbon center. These compounds can exist in either L-form or D-form depending on the chirality of the asymmetric carbon.
  • Aldoses with alcohol groups on the right are termed D-aldoses, whereas the aldoses with alcohol on the left are termed L-aldoses.
  • Aldoses are polyhydroxy aldehydes that can also exist in a cyclic ring structure called hemiacetals. The cyclic structure can be seen in carbohydrates with more than 4 carbon atoms.
  • Aldoses are often referred to by names that indicate their stereoisomerism, as many biological systems can only use a particular enantiomer of a carbohydrate.
  • Aldoses can also tautomerize into ketoses through the dynamic process with an enol intermediate formation. The tautomerization is reversible, and the aldo-form is usually more stable than the enol-form.

Aldoses vs Ketoses

Ketoses Definition

A ketose is a monosaccharide consisting of a carbon backbone and a carbonyl group within the backbone.

  • The general formula for ketoses is the RCOR’ where the R is an alkyl group which can be the same or different from the other R’.
  • All monosaccharide ketoses are reducing sugars as they can be tautomerized into aldehyde, which then undergoes oxidation. However, ketoses that are bound to glycosides are nonreducing sugars.
  • The carbonyl group in ketoses is not present at the end of the chain, which results in a hemiketal cyclic ring structure in contrast to the hemiacetal ring structure in the aldehyde.
  • The carbon atoms in ketoses are asymmetrical, resulting in different forms of sugars due to the chirality of the asymmetrical carbon. The L- and D- forms of ketoses can be defined by the position of the hydroxyl group on the carbon backbone.
  • Ketoses can be differentiated from aldoses by Seliwanoff’s test. The test is based on the dehydration reaction, which is faster in ketoses, resulting in the faster test result.

7 Key Differences (Aldoses vs Ketoses)

Characteristics Aldoses Ketoses
Definition An aldose is a monosaccharide consisting of a carbon backbone and a carbonyl group at carbon-1, resulting in an aldehyde group. A ketose is a monosaccharide consisting of a carbon backbone and a carbonyl group within the backbone.
Functional group Aldoses have aldehyde as the functional group. Ketoses have ketone as the functional group.
Also called Aldoses are also called polyhydroxy aldehydes. Ketones are also called polyhydroxy ketones.
Seliwanoff’s Test Aldoses react slowly to Seliwanoff’s reagent and produce a light pink color. Ketoses react with Seliwanoff’s reagent quickly and produce a deep cherry-red color.
Tautomerization Aldoses can tautomerize into ketoses via enol intermediate formation. Ketose can only tautomerize into aldoses if the carbonyl group is present at the end of the hydrocarbon chain.
Cyclic structure Aldoses can exist in a cyclic form called hemiacetals. The cyclic ketoses are termed hemiketals.
Examples Glucose, ribose, arabinose, mannose are some of the examples of aldoses. Fructose, ribulose, dihydroxyacetone are some of the examples of ketoses.

Examples of Aldoses


  • Glucose is an aldose monosaccharide sugar with the molecular formula C6H12O6, primarily produced by plants and algae by photosynthesis.
  • Glucose is the most important source of energy in many organisms as it can be utilized by a large variety of organisms.
  • It can also be stored in the form of polymer in plants as starch and animals as glycogen. The polymers are then broken down into glucose units during metabolism.
  • The naturally occurring glucose exists in the D-glucose form, whereas the L-glucose can be produced synthetically for specific purposes.
  • The D-glucose is more important than L-glucose as the biological systems have mechanisms to utilize D-glucose. 
  • Glucose is an aldohexose as it has six carbon atoms and an aldehyde group. The glucose molecule can exist either in an open-chain (acyclic) or ring (cyclic) form.

Examples of Ketoses


  • Fructose is a simple hexose sugar commonly found in plants and is one of the three dietary monosaccharides along with glucose and galactose.
  • Fructose has a ketone functional group and the ring structure of fructose forms at the 2nd carbon position. The ring structure of fructose is a 5-carbon ring with an intramolecular hemiacetal structure.
  • It is the most water-soluble of all the sugars that, together with glucose, forms a disaccharide structure like sucrose.
  • Fructose derived from plant sources like sugar cane, maize, and beets is used to form high-fructose corn syrup with glucose as monosaccharides.
  • The hemiketal structure of fructose is stabilized by the internal hydrogen-bonding resulting in the crystalline form. The crystalline form is called D-fructopyranose.

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About Author

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Anupama Sapkota

Anupama Sapkota has a B.Sc. in Microbiology from St. Xavier’s College, Kathmandu, Nepal. She is particularly interested in studies regarding antibiotic resistance with a focus on drug discovery.

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