Aldose vs Ketose: Structural Isomeric Monosaccharides

Aldoses and ketoses and monosaccharides (simple sugar molecules) with differences in the type of functional groups. Aldoses are monosaccharide molecules containing the aldehyde functional group. They have a general formula Cn(H2O)and occur as both D (dextrorotatory) and L (levorotatory) type optical isomers. Ketoses on the other hand are similar monosaccharides with the ketone functional group. They also have a general formula as Cn(H2O)and occur as D-L optical isomers.

Monosaccharides that are polyhydroxy aldehydes are termed ‘aldoses’ whereas polyhydroxy ketones are termed ‘ketoses’.

Aldose vs Ketose difference image

Difference between Aldose and Ketose sugars

Aldose Sugars Ketose Sugars
Aldehyde functional group containing simple sugar molecules Ketone functional group containing simple sugar molecules
Generally found in plant extracts Generally found in processed foods
Turn light pink upon reaction with Seliwanoff's reagents Turn red upon reaction with Seliwanoff's reagents
Aldoses isomerize to ketoses via LB-AVE transformation Ketoses isomerize to aldoses in the presence of basic environment and Grignard reagent
For example, glucose, ribulose, galactose, glyceraldehyde, etc For example, fructose, ribose, erythrulose, xylulose, etc
Aldoses contain more stereo genic centers Aldohexose (Glucose) contains 4 chiral centers ketoses contain lesser stereo genic centers Ketohexose (Fructose) contains 3 chiral centers
Aldoses have more stereoisomers
Aldohexose (Glucose) contains
24 = 16 stereoisomers
Ketoses have less stereoisomers
Ketohexose (Fructose) contains
23 = 8 stereoisomers

What are Aldose sugars?

Aldoses are sugar molecules (monosaccharides) with a carbon backbone cyclic and open-chain structure. Most importantly, aldoses have an aldehyde functional group in open-chain form.

Examples of aldoses are;

Examples of aldose sugars

The general formula for aldoses is Cn(H2O)n and they start from triose (n=3) structures. There are no aldose sugars containing formaldehyde (n=1) or glycolaldehyde (n=2), so they are not generally considered carbohydrates.

Aldoses are differentiated from ketoses via Seliwanoff’s test as they turn into pale pink color. They generally isomerize into ketose forms by Lobry-de Bruyn-Alberda van Ekenstein (LB-AVE) transformation. Aldose sugars are very important biological molecules and a monomer of table sugar (sucrose).

What are Ketose sugars?

Ketoses are sugar molecules (monosaccharides) with a carbon backbone having both cyclic and open chained structures.

Examples of ketoses are;

Examples of ketose sugars

Their general formula is also Cn(H2O)n. The ketose structures start from the parent molecule, ketotriose (dihydroxyacetone). Further molecules like ketopentoses and ketohexoses, etc are prepared by simply inserting some stereogenic centers (H-C-OH) beneath carbonyl carbon.

Ketose compounds have a pivotal role in the metabolism of biological molecules and they isomerize into aldose forms via enediol isomerism with a base. Furthermore, Ketose and aldose sugars combine as monomers to make disaccharides like sucrose, etc.

Seliwanoff’s Test reaction

In 1887, a Russian scientist named ‘Theodor Seliwanoff’ devised a test to distinguish between aldoses and ketoses. Seliwanoff’s test is the chemical test to distinguish between aldose and ketose groups.

The reagents of this test are resorcinol and concentrated hydrochloric acid. Ketose sugars like fructose dehydrate faster than aldose sugars like glucose. This becomes the reason why ketose sugars give deep red color with Seliwanoff’s test while aldose sugars give faint pink color.

Aldoses and Ketoses: Key differences

The main and the foremost difference between aldoses and ketoses are the identity functional groups. Aldehydic functional groups in open-chain form and a hydrogen atom present on anomeric carbon in cyclic form refers a sugar compound to be aldose type. Whereas ketoses contain a ketonic functional group and no hydrogen atoms on anomeric carbon in cyclic forms.

Difference table for Aldose vs Ketose Structural Isomeric Monosaccharides

Concepts Berg

How do you distinguish an aldose and a ketose in cyclic form?

The differentiation between aldose and ketose sugars is quite easy in open-chain forms due to their functional group and structural variation. In cyclic forms, a hydrogen atom present on anomeric carbon will show the presence of a cyclic aldose type compound. On the other hand, the presence of hydroxyl groups and carbon atoms on anomeric carbon shall reflect cyclic ketose compounds.

Which test is used to distinguish between aldoses and ketoses?

Aldoses and ketoses are differentiated by Seliwanoff’s test. Ketoses being more prone to getting dehydrated upon heating, turn cherry red upon addition of Selianwoff reagents. Aldoses, on the other hand, being less prone to this type of dehydration as compared to ketoses, turn to light pink color with the same reagents.

What are aldoses examples?

Glyceraldehyde, glucose, galactose, ribose, erythrose, and threose, etc are some aldoses.

What are ketoses examples?

Fructose, ribulose, erythrulose, octulose, dihydroxyacetone, xylulose, psicose, tagatose, and sorbose, etc are all ketoses.

Why is fructose considered a reducing sugar?

A reducing agent reduces others. Fructose, being a ketose sugar is capable of reducing other species and getting oxidized in the presence of oxidizing and basic environments. So fructose is considered a reducing sugar.

What is the structural difference between Glucose and Fructose?

Glucose and Fructose are generally both carbonyl functional groups containing, six-membered, open structures, and both have the same molecular formula, yet are different compounds i.e. they are structural isomers (Functional group isomers). Glucose is an aldose sugar with the aldehydic functional group and a hydrogen atom on anomeric carbon. Fructose on the other hand is a ketose sugar with a ketonic functional group and no hydrogen over anomeric carbon in cyclic form.

What is the stability of aldehyde and ketone?

Ketones are generally considered to be more stable than aldehydes because the electron-donating effect of methyl groups (due to hyperconjugation) renders electrophilic carbonyl carbon less effective. Aldehydes, however, have more space and electrophilicity for carbonyl carbons to be prone to more attacks by nucleophiles. In short, ketones [R-(C=O)-R] are more stable than aldehydes [H-(C=O)-H] or [H-(C=O)-R] due to more methyl groups attached to the electrophilic carbon.

Is it possible to convert aldehyde to ketone?

Aldehydes do not directly get converted to ketones. Although an indirect way of first converting aldehydes into alcohols by reaction with the Grignard reagent and then oxidizing via PCC (Pyridinium chlorochromate) can lead aldehydes to be ketones.

Why is sucrose called ‘invert sugar’?

Sucrose is a dextrorotatory sugar (+66.6°) which upon hydrolysis turns out to be a mixture of dextrorotatory glucose (+52.5°) and levorotatory fructose (-90.4°). Now that the levorotation is much more than dextrorotation [-90.4° > +52.5°] in the hydrolyzed mixture of sucrose, the solution becomes levorotatory in nature. So in fact, sucrose being a dextrorotatory sugar inverts to levorotatory nature after being hydrolyzed. Hence, sucrose is called an inverted sugar.

Reference book(s)

  • Organic Chemistry (4th edition), Francis A. Carey, University of Virginia

Reference link(s)

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