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Difference between Ketose and Aldose is that Aldose are monosaccharides that contain aldehyde groups at the end of the carbon chain whilst Ketoses are monosaccharides that contain the ketone groups in the carbon chain. Ketose and aldose are both monosaccharides that differ only based on the carbonyl group present in them. Sugars can be monosaccharides, disaccharides, or polysaccharides. Monosaccharides are defined as sugars that are made up of single repetitive units. Disaccharides are defined as sugars that are made up of two subunits whilst polysaccharides are defined as sugars that are made up of two or more subunits.
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Key terms: Aldose, Ketose, Aldehydes, Ketones, Carboxylic Groups, Sugars, Carbon Compounds, Monosaccharides
Aldose
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Aldoses are a type of monosaccharides that consists of an aldehyde group on its carbon chain. Generally, there is a presence of one aldehyde group per monosaccharide molecule. The aldehyde group denoted by (-CH=O) is a reactive chemical group of compounds.
The simplest aldose is a molecule that consists of two carbon atoms, where one carbon atom has bonded with the aldehyde which is triose: glyceraldehyde. The general chemical formula to represent an aldose is given by Cn(H2O)n.
Glyceraldehyde
Aldoses usually consist of asymmetric carbon centers where four different types of systems are bonded to the considered carbon in the center. Hence, an aldose that has a minimum of three carbon atoms is considered to have one stereogenic center that exhibits optical activity.
Aldoses can further be divided into two groups based on the nature of their chiral activity. These groups are:
- D-aldose
- L-aldose.
When the structure of a compound is drawn in an open form it is known as the ‘Fischer Projection’. When classifying the aldose monosaccharide into D-aldose or L-aldose, we consider the chirality of the second furthest carbon atom from the position of the aldehyde group. If the alcohol group is on the right-hand side of the Fischer projection then we categorize it as D-aldose and if it is on the left-hand side it is categorized as L-aldose. Generally, natural biological receptors are more sensitive towards D-aldoses than L-aldoses.
Aldoses and ketoses are differentiated and recognised through Seliwanoff’s test. In this test, the sample is heated with acid and resorcinol. The test is dependent on the process of dehydration. When aldoses are subjected to Seliwanoff’s test, they react slowly and produce a light pink color.
Some of the common aldoses are triose consisting of glyceraldehyde, tetroses consisting of erythrose and threose, pentoses consisting of ribose, arabinose, xylose, and lyxose, and hexoses that consists of glucose.
Examples of Aldose found in Nature
Glucose: It is one of the most important types of sugar that is found in our blood and constitutes our body's primary source of energy. It is present in everyday food items like rice, bread, potatoes (or other tuber-like foods), fruits, milk, etc.
Galactose: It is rarely ever found in its free form. It is a major component of the type of sugar found in milk (lactose). It is also used as a source of energy by our bodies in the absence of glucose.
Ribose: Unlike Glucose and Galactose which consists of 6 carbon atoms, this aldose consists of only 5 carbon atoms in its backbone, but is also one of the most important parts of living organisms. It plays an important role in the formation of the backbones of polymers of nucleic acids (DNA and RNA), which are the most basic and extremely important components of living systems.
Arabinose: this sugar is a major component of the cell walls of almost all the plants we see growing on the land.
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Ketose
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Ketoses are a type of monosaccharide where it consists of a ketone group in the carbon chain. Generally, there is one ketone group per sugar molecule. The ketone group denoted by (-C=O) is a reactive chemical group of compounds. As in aldoses, ketoses also consist of several stereogenic centres with the carbon chain. The simplest form of ketose is made up of three carbon atoms, attached to the ketone group which is dihydroxyacetone. This organic compound does not exhibit any optical activity.
The general chemical formula to represent an aldose is given by Cn(H2nOn)
Ketoses are categorized as reducing sugars. They can also tautomerize into aldose. This is achieved with the help of an enediol intermediate. The final aldehyde group can further be oxidized too. Examples of this reaction are Tollen’s test or Benedict’s test. Ketoses isomerize into aldoses through Lobry-de-Bruyn-Van Ekenstein transformation.
Examples of ketoses include Trioses consisting of dihydroxyacetone, Tetroses consisting of erythrulose, Pentoses consisting of ribulose, xylulose, Hexoses consisting of fructose, psicose, sorbose, tagatose, Heptoses consisting of sedoheptulose, and Octoses consisting of D-manno-octulose.
Ketoses in general are reducing sugars but there are exceptions. For example, ketoses that are bound to glycosides are categorized into non-reducing sugars as in the case of fructose moiety of sucrose.
Examples of Ketose found in Nature
Fructose: This is a six-carbon-containing monosaccharide that is most commonly found in fruits, as well as some plant foods like honey and some vegetables. It is the sweetest of all the sugars.
Dihydroxyacetone (DHA): The simplest form of ketose. It is used actively as a cosmetic ingredient, being a major component of sunless tanning products.
Erythrulose: This monosaccharide is also commonly used as a component of tanning products and is the four-carbon containing ketose.
Tagatose: This ketose is formed when milk is heated. It is also found in many dental products. It has also been marketed as an artificial sweetener.
Differences Between Aldose and Ketose
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Key differences between Aldose and Ketose are:
| Aldose | Ketose |
|---|---|
| Aldose is a type of monosaccharide that consists of aldehyde in its carbon chain | Ketose is a type of monosaccharide that consists of a ketone group on its carbon chain |
| It is found to be present naturally in plants | It is present in processed foods |
| Aldose isomerization into ketone | Ketone isomerised into aldose in the presence of a reducing sugar |
| When aldose is subjected to Seliwanoff’s test, it reacts slowly and produces a light pink color | When ketose is subjected to Seliwanoff’s test, it reacts slowly and produces a deep cherry red |
| Example: Glucose, ribose, and galactose | Example: Fructose, erythrulose, and ribulose |
Things to Remember
- Both aldose and ketose are monosaccharides which differ due to the presence of different carboxylic groups
- Aldose is a type of monosaccharide that consists of aldehyde in its carbon skeleton.
- Ketose is a type of monosaccharide that consists of a ketone group on its carbon chain
- Aldose isomerizes to ketose and ketose isomerizes to aldose in the presence of a reducing sugar
- Both are chemically differentiated and identified when subjected to Seliwanoff’s test, where aldose reacts slowly and produces a light pink colour and ketose reacts faster and produces a cherry red colour
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| Class 12 Chemistry Related Concepts | ||
|---|---|---|
| Gattermann – Koch Reaction | Williamson Ether Synthesis | Electrophilic Aromatic Substitution |
| Ester Hydrolysis | Reimer Tiemann Reaction | Gabriel phthalimide synthesis |
Sample Questions
Ques. How are aldose different from ketose? (2 marks)
Ans. Ketose and aldoses are monosaccharides that are differentiated based on the carboxylic group which they contain. Aldoses are defined as those monosaccharides that have an aldehyde group on their carbon chain. These are primarily found in plants. On the other hand, ketoses are monosaccharides that have a ketone group on its carbon chain. They are used in processing food. Some of the examples of ketose are fructose and ribulose.
Ques. How are aldoses named? (3 marks)
Ans. Aldoses are differentiated by the number of carbons in the main carbon chain. The minimum carbons present in the backbone needed to form a molecule are considered a carbohydrate and the carbohydrates with three carbons are called trioses. There is only one aldo-triose which is called glyceraldehyde that has one chiral stereo-centre with two enantiomers that are the L-glyceraldehyde and D-glyceraldehyde. Some of the common aldoses are as follows:
- Triose which has 3 carbons - glyceraldehyde
- Tetroses which have 4 carbons- erythrose
- Pentoses have 5 carbons- xylose, ribose
- Hexoses have 6 carbons - glucose
The most widely discussed category of aldoses is the hexoses which have 6 carbons. They are aldohexoses that have common names like D-(+) Allose and D-(+)-Alltrose.
Ques. What is the stereochemistry of aldose? (2 marks)
Ans. Aldoses are referred to by names specific to the stereoisomer of the compound. They are important in Biochemistry. Aldoses do not just have a single conformation but they can fluctuate between different forms. Aldoses can donate a proton or electron i.e. undergo the process of tautomerization to form ketoses with an enol or enediol intermediate.
This reaction is reversible and ketoses are a more stable form than the enol forms. This allows stereo-isomerisation. Cyclic aldoses show Haworth projections and the open-chain forms of aldoses have Fischer projections which give stereochemical information about the structures.
Ques. What is the chemical nature of ketoses? (2 marks)
Ans. Ketose is a monosaccharide that has one ketone group per molecule and the simplest form of this group is the dihydroxyacetone. This contains only 3 carbon atoms and thus do not show any optical activity properties.
One must note that all the ketoses that are monosaccharides belong to the group of reducing sugars. This is because they can tautomerize into aldoses via an enediol or enol intermediate which results in an aldehyde that can be oxidised. For example, in Benedict's test and Tollens' test. Other ketoses that are bound to glycosides are non-reducing sugars. An example is the fructose moiety of sucrose.
Ques. What test can be done to differentiate the ketose and aldose? (2 marks)
Ans. Ketones and aldoses can be differentiated or distinguished chemically by performing Seliwanoff's test in which a sample is heated with acid and resorcinol. The mechanism of the reaction is mainly based on dehydration reaction which occurs quicker in the ketoses than the aldoses. Aldoses react in a slow manner resulting in a light pink colour whereas; the ketoses which react quickly will produce a dark red colour. Ketoses can isomerise to aldoses by the Lobry-de Bruyn-van Ekenstein process.
Ques. Which is the sugar that is most commonly found in fruits (1 mark)
1. Sucrose
2. Ribose
3. Lactose
4. Fructose
Ans. 4. Fructose is the sugar that is very commonly found in fruits. It is also known to be the sweetest sugar molecule.
Ques. Which sugar is found to be an integral part of the nucleic acid? (1 mark)
Ans. Ribose is the monosaccharide that essentially forms the building blocks of the base pairs that is found in nucleic acid.
Ques. A ketose is able to chemically convert to an aldose. Which of the following characteristics of ketose supports this reaction? (1 mark)
1. Oxidation
2. High boiling point
3. Complex sugar
4. Reducing sugar
Ans. 4. Reducing sugar plays an important part during the isomerization of ketose to aldose without which the conversion does not occur.
Ques. Which characteristic is unique to aldose sugar molecules? (1 mark)
1. Stereoisomerism
2. Reducing Agent
3. Ionization
4. Electron Capture
Ans. 2. Reducing agent. The aldose sugar, containing aldehyde acts as a reducing agent which is not possible with ketones.
Ques. Which naturally occurring a-amino acid is achiral? (1 mark)
1. glycine
2. glutamine
3. Neucine
4. Serine
Ans. 1. Glycine. The only naturally occurring achiral alpha amino acid which has no substitute on the alpha carbon is glycine.
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