Aldoses and ketoses are two primary types of monosaccharides, which are simple sugars that play a crucial role in various biological processes. The classification of these sugars is based on the functional group they contain.
Aldoses are sugars that contain an aldehyde group (-CHO) at the end of their carbon chain, while ketoses have a ketone group (C=O) located within the carbon chain. These structural differences give aldoses and ketoses distinct chemical and physical properties.
They can further be classified based on the number of carbon atoms in their backbone, such as trioses, tetroses, pentoses, and hexoses, influencing their behavior and reactivity in different biochemical pathways. Understanding the classification of aldoses and ketoses is fundamental to the study of carbohydrate chemistry and their role in living organisms.
Carbohydrates are the most abundant class of organic compounds, and they are the products of photosynthesis, which is formed by the endothermic reductive condensation of carbon dioxide in the presence of the pigment chlorophyll and light energy. They are the primary source of energy for metabolism for animals and plants, as well as living organisms that rely on plants for food. Carbohydrates with relatively small sizes are classified as sugars, and carbohydrates are further classified as aldoses and ketoses based on the C=O functional group.
Ketose and aldose are monosaccharides that can be distinguished by the group in which they are found. An aldose is a monosaccharide with an aldehyde group in its carbon skeleton. They’re mostly found in plants. Ketose is a monosaccharide with a ketone group on its carbon skeleton. They can only isomerize to aldose in the presence of reducing sugar. They’re found in processed foods. Ribulose, fructose, and other sugars are examples of ketose. To distinguish between aldose and ketose, we can use Seliwanoff’s Test.
Carbohydrates must have an aldehyde group in order to be classified as aldose. Aldose is a monosaccharide with one aldehyde group per molecule.
Diose glycolaldehyde is the most basic aldose, with only two carbon atoms. Because aldoses contain at least one asymmetric carbon centre, aldoses with three or more atoms exhibit the phenomenon of stereoisomerism.
Xylose, ribose, allose, lyxose, threose, glyceraldehyde, glucose, idose, galactose, talose, mannose, altrose, and arabinose are examples of aldoses. All of these aldoses have one aldehyde group in common, but they differ in the number of carbon atoms in their carbon skeleton.
Aldoses with stereogenic centres can exist in either L- or D-forms, and their form is determined by the chirality of the penultimate carbon.
Ketose is a monosaccharide with the carbonyl function on the inner atoms of the carbon chain. Although dihydroxyacetone is not a sugar, it is a ketose analogue of glyceraldehyde. Commonly, a carbonyl group is attached at C-2, and the carbonyl function of ketoses can be reduced by sodium borohydride, yielding a mixture of epimeric products.
D-fructose is the sweetest of all-natural sugars, and its reduction yields a mixture of D-mannitol and D-glucitol (sorbitol), which are named after aldohexose and can also be obtained by the analogous reduction. Mannitol is also a naturally occurring carbohydrate. In general, ketoses are the distinct isomers of aldose monosaccharides. Because the chemistry of both classes is linked, the interconversion of aldoses and ketoses is simple in the presence of a base or acid catalyst.
The enediol tautomeric intermediate facilitates the corresponding epimerization and interconversion at the alpha sites of the carbon function.
Organic compound structures can be represented in a variety of ways. One of the most common methods is to employ Fischer projection formulas, which were developed by Nobel Prize-winning German chemist Emil Fischer.
Individual atoms are represented by their single-letter codes, and chemical bonds between them are represented by single, double, or triple dashes for single, double, and triple bonds, respectively, in these depictions of organic compounds drawn in a 2-dimensional field (on paper). These Fischer projection formulae are also commonly used to represent monosaccharides such as aldoses and ketoses.
The basic structure of an aldose is a carbon backbone with each carbon atom connected to its neighbouring carbon atom by a single bond. The aldehydic functional group is then attached to a carbon atom at each end of the backbone. The remaining carbon atoms are each bonded to one hydroxyl group (-OH) via a single bond.
The hydroxyl group attached to the carbon atom at one end of the backbone is referred to as a primary alcohol group, while the remaining hydroxyl groups are referred to as secondary alcohol groups. Bonding with hydrogen atoms fills the remaining valency of the carbon atoms.
The most common way to depict the chemical structures of ketoses, like aldoses, is to use Fischer projection formulae. The Fischer projection formula for Fructose, the most common ketose, with its functional group highlighted. Ketones, like aldoses, have a backbone composed of carbon atoms joined by a single covalent bond.
However, in ketoses, the carbonyl functional group is not attached to a carbon atom at either end of the chain.
As a result, the carbonyl carbon is linked to three different atoms: two carbon atoms (via two single bonds) and one oxygen atom (via a double bond). The remaining carbon atoms have hydroxyl groups attached to them, with the alcoholic groups at the backbone’s ends being primary alcohol groups.
A monosaccharide with an aldehydic functional group as its main functional group is known as an aldose, whereas a ketose has a ketonic functional group as its main functional group. They're both polyhydroxy alcohols, but one is an aldehyde and the other is a ketone.
The six-carbon ketohexose fructose is one of the most common ketoses found in nature. It is most commonly found in many naturally occurring fruits as well as some plant foods such as honey and some vegetables.
The primary distinction between an aldose and a ketose is the type of functional group found in each. A ketose is a ketone, whereas an aldose is a functional aldehyde.