Carbohydrates are an essential part of our diets, and they come in a variety of forms. Monosaccharides, derived from the Greek word “mono,” meaning one, are the simplest group of carbohydrates, and they are more commonly referred to as simple sugars. Monosaccharides have the general formula Cn (H2O)n, and they cannot be further hydrolyzed.

Monosaccharides, or simple sugars, are the building blocks of carbohydrates and serve as the foundation for all other sugar molecules. There is a wide range of different monosaccharides, but they can all be divided into distinct categories based on their functional groups and the number of carbon atoms.


Classification on the basis of functional Groups

1. Aldoses

An important factor in determining the type of monosaccharide is the functional group present. When this group is an aldehyde, these monosaccharides are known as aldoses and have specific properties and characteristics.


  • .glyceraldehyde
  • glucose
  • 2 . Ketoses

    Ketoses, or molecules that contain the keto functional group, are an important component of organic chemistry. These molecules have a wide range of potential applications and are used in many industries from medicine to food production.


  • dihydroxyacetone
  • fructose
  • Classification on the basis of the number of carbons

    Monosaccharides are classified into different categories based on their carbon atom count. These include:

  • trioses (3C),
  • tetroses (4C),
  • pentoses (5C),
  • hexoses (6C),
  • and heptoses (7C).
  • To differentiate between different types of monosaccharides in an easier way, functional groups and terms like hexose and triose, etc are used while naming them. Let’s take an example: glucose is an aldohexose while fructose is a ketohexose.

    Classification examples

  • Monosaccharides            Aldose             Ketose
  • Trioses (C3H6O3)          Glyceraldehyde      Dihydroxyacetone
  • Tetroses (C4H8O4)        Erythrose                Erythrulose
  • Pentoses (C5H10O5)     Ribose                    Ribulose
  • Hexoses (C6H12O6)      Glucose                  Fructose
  • Heptoses (C7H14O7)     Glucoheptose        Sedoheptulose


    Stereoisomerism is a crucial concept in understanding the complexity of the chemical structure of monosaccharides. It is important to understand stereoisomers and their significance for a comprehensive understanding of biochemistry. Stereoisomers are molecules that have the same structural formulae but differ in their spatial configuration.

    A compound’s isomers are strongly influenced by the number of asymmetric carbon atoms (n) it contains. Asymmetric carbons, in turn, are defined as carbons that are attached to four different atoms or groups. The number of isomers of a molecule can be found by a formula of 2 to the power of n, where n represents the number of asymmetric carbon.


    Glucose (4 asymmetric Carbon) = 2 to the power 4 = 16 isomers



    The simplest form of sugar, glyceraldehyde (triose) is a monosaccharide made up of three carbon atoms. Interestingly, 2 of those carbon atoms have an asymmetric structure, meaning that this sugar exists in two different forms – stereoisomers – which can be mirror images of each other.

    Glyceraldehyde (triose) is an important compound in the world of carbohydrates, as it serves as a reference molecule to represent the structure of other monosaccharides.


    D-and L-isomers

    Isomers are molecules with the same chemical formula but different structures, which can be incredibly helpful in understanding the properties of a compound. One type of isomer is the D and L isomer; these two molecules are mirror images of each other.

    The role of spatial orientation of hydrogen (H) and hydroxy groups (OH) on the carbon atom adjacent to the terminal primary alcohol carbon is essential in determining whether sugar is a D- or L-isomer.

  • OH Group right side = D-series
  • OH Group left side = L-series
  • Monosaccharides found in mammalian tissues are mostly of a specific type – D-configuration. The enzyme machinery of cells is key to understanding this phenomenon as it metabolizes the D-series of monosaccharides.


    Optical activity of sugars

    Optical activity is an interesting and fascinating characteristic of certain compounds. It is known as the phenomenon in which light passing through a substance is rotated to either the left or right, depending on the type of molecules the substance contains.

    This phenomenon was first discovered in 1815 by scientist Jean-Baptiste Biot, who noticed that when light passed through a solution of a certain type of organic compound called an optical isomer, it would be rotated to either the left or right.

    The terms dextrorotatory (d+ ) and levorotatory (l- ) are used to describe compounds that respectively rotate the plane of polarized light to the right or to the left.

    When discussing optical isomers, they may be designated as D(+), D(- ), L(+), and L(- ) based on their structural relation with glyceraldehyde.


    Racemic mixture

    Have you ever wondered what happens when both d and l-isomers are present in equal concentrations? This phenomenon, known as a racemic mixture or dl mixture, can be surprisingly fascinating.

    A racemic mixture does not exhibit any optical activity because the dextro-and levorotatory activities balance each other out.

    When it comes to medical terminology, there can be a lot of confusion. The term dextrose is often used in the healthcare field, but what exactly does it mean? Dextrose is actually just another name for glucose in solution. This is due to glucose’s unique property known as dextrorotatory nature.



    Epimers refer to two monosaccharides that differ from each other in their configuration around a single specific carbon atom that isn’t the anomeric carbon.


  • glucose and galactose
  • Glucose and mannose


    Enantiomers are an intriguing type of stereoisomers that can be quite fascinating to study. They are mirror images of each other.



    Reactions of monosaccharides


    Tautomerization or enolization

    Tautomerization is an important yet often overlooked process that occurs in the world of biochemistry. It involves a hydrogen atom being shifted from one carbon atom to another, producing enediols. This process is even more relevant when sugars possess an anomeric carbon atom because they undergo tautomerization in alkaline solutions.

    The Lobry de Bruyn-von Ekenstein transformation is an important reaction in organic chemistry. It involves the isomerization of glucose when kept in an alkaline solution for several hours. This reaction produces two new compounds – D-fructose and D-mannose.


    Reducing properties

    In general, sugars are divided into two categories: reducing or nonreducing. These groups are determined by the free aldehyde or keto group of anomeric carbon present in each sugar molecule.

    There are a number of different tests that can be used in the lab to detect the presence of reducing sugars. Benedict’s test, Fehling’s test, and Barfoed’s test – are just some of the common techniques employed by scientists worldwide.



    Oxidation reactions are a key part of organic chemistry, with many practical applications in the real world. Depending on the type of oxidizing agent used, it can be possible to selectively oxidize either an aldehyde group or an alcohol group, or even both groups simultaneously.


  • Oxidation of the aldehyde group = gluconic acid.
  • Oxidation of the terminal alcohol group = glucuronic acid.


    Monosaccharides are often reduced to their corresponding alcohols to produce products used for various purposes. One way of reducing carbohydrates is by treating them with sodium amalgam, a type of reducing agent.


  • D-Glucose → D-Sorbitol
  • D-Galactose → D-Dulcitol
  • D-Mannose → D-Mannitol
  • D-Fructose → D-Mannitol + D-Sorbitol
  • D-Ribose → D-Ribitol


    When treated with concentrated sulfuric acid, these simple sugars undergo a process known as dehydration which results in the elimination of three water molecules.


  • hexoses = hydroxymethyl furfural
  • pentoses = furfural

    What is Bial’s test?

    Pentoses, or five-carbon sugars, react with hydrochloric acid (HCl) to form furfural derivatives, which can then be reacted with orcinol to form a green-colored complex. This type of reaction is widely used in the field of biochemistry and has been utilized in various fields from chemical testing to diagnosing essential pentosuria by detecting xylose in the urine.


    What are Glycosides?

    Glycosides are a fascinating type of molecule found in nature that has some remarkable properties. They are formed when the hemiacetal or hemiketal hydroxyl group of a carbohydrate reacts with the hydroxyl group of another carbohydrate or noncarbohydrate. This reaction creates a new compound.

    Glycosidic bonds are formed between two molecules. And if there is a noncarbohydrate moiety present then it is referred to as an aglycone.


  • Glucovanillin
  • Cardiac glycosides
  • Streptomycin
  • Ouabain
  • Phlorhizin
  • Derivatives of monosaccharides

    Monosaccharides are the simplest and most basic form of carbohydrate that serves as the building blocks for many other forms of sugar found in nature. Interestingly, there are several derivatives of monosaccharides, some of which play an important role in our bodies. some of the derivatives are:

  • Sugar acids
  • Sugar alcohols
  • Alditols
  • Amino sugars
  • Deoxysugars
  • L-Ascorbic acid (vitamin C)
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    Tommy E. Junkins

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