What Are Sugars (concept and Chemical Classification)

The sugars They are the simplest form of carbohydrates. They are made up of oxygen atoms (EITHER)carbon (C) and hydrogen (H) and conform to the chemical formula CnH2nEITHERn. They are also called simple monosaccharides.

In nature, there are different types of sugars according to their chemical structure. The best known are the glucose and the fructosebut the variety is wide and includes the lactose (found in milk), among others.

The table sugarcall saccharoseIt contains glucose and fructose.


On the left, we see table sugar, which contains glucose and fructose; on the right, we find milk, which contains lactose.

sugars are important because they are the base or the skeleton of the carbohydrates more complex.

Chemical classification of sugars

The chemical classification of sugars depends on three factors:

  1. According to location of oxygen atom in the molecule.
  2. According to number of carbons which has the core structure.
  3. According to orientation of the molecules of hydroxyl (-OH) of the penultimate carbon with respect to the central chain.

Example of the factors considered when classifying a sugar.


The three factors used to classify sugars using D-lyxose as an example.

Before explaining in more detail the classification according to these three factors, let’s see an example with glucose.

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The enantiomers of glucose.

  • The glucose is a aldose because it has an aldehyde group in it C1.
  • The glucose is a hexose because it has 6 carbons.
  • The D-glucose have the C5 oriented to the rightthe L-glucose have the C5 oriented toward the left.

Now we will explain in more detail how this classification is arrived at.

Based on the location of the oxygen atom in the molecule

Considering the Location of the atom oxygen with respect to carbon nº1 (C1) in the molecule, we find:

  • Aldoses: contain a group aldehyde in the carbon C1. That is, a carbon associated with a double-bonded oxygen. (=O)a single bond hydrogen (-H) and another single bond carbon (-C).
  • Ketoses: contain a group ketone in the carbon Ctwo. That is, a carbon associated with a double-bonded oxygen. (=O)and two other single bond carbons (-C).

Let’s look at the example with glucose and fructose:


Classification of sugars according to the functional group from which it derives. On the left, an aldose (derivative of the aldehyde), on the right, a ketose (derivative of the ketone).

The glucose has the double bond oxygen at carbon #1 (C1)while the fructose has the double-bonded oxygen at carbon #2 (Ctwo).

Depending on the number of carbons in the central structure

In function of the number of carbons that it contains in the central structure, we can identify the following sugars:

Sugars derived from aldehyde or aldosesaccording to number of carbonsare the following:

  • 3 carbons: glyceraldehyde.
  • 4 carbons: erythrose and treose.
  • 5 carbons: ribose, arabinose (included in the diet of diabetics), xylose (used in diagnostic tests) and lyxose (found in some bacteria).
  • 6 carbons: allose, altrose, glucose, mannose (present in the membrane of white blood cells), gulose, idose, galactose (precursor of breast milk) and talose.
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Sugars derived from ketone or ketosesaccording to number of carbonsare the following:

  • 3 carbons: dihydroxyacetone (present in cane sugar).


Cane sugar contains dihydroxyacetone.

  • 4 carbons: erythrulose (found in strawberries).


Strawberries contain erythrulose.

  • 5 carbons: ribulose (involved in carbon fixation in plants) and xylulose.
  • 6 carbons: psychose, fructose (present in honey), sorbose and tagatose (sweetener).


Honey has fructose.

According to the orientation of the hydroxyl molecules of the penultimate carbon

Attending to the orientation of the group hydroxyl (-OH) from the penultimate carbon, the sugars could be subdivided as follows:

  • D- or dextro-rotatory: when the hydroxyl (-OH) penultimate carbon (C) is located towards right.
  • L- or levo-rotary: when the hydroxyl (-OH) penultimate carbon (C) is located towards left.

The oriented molecules D Y L They are known as isomers. Sugars contain a mixture of both isomers but in living beings it is normal to find the d-shape of the sugars.

For more information you can read Carbohydrates.