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.
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)
|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.
References and Sources
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