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However, aldoses are not locked into any one conformation: they can and do fluctuate between different forms. Aldoses can tautomerize to ketoses in a dynamic process with an enol intermediate (more specifically, an enediol). [1] This process is reversible, so aldoses and ketoses can be thought of as being in equilibrium with each other.
Seliwanoff’s test is a chemical test which distinguishes between aldose and ketose sugars. If the sugar contains a ketone group, it is a ketose. If a sugar contains an aldehyde group, it is an aldose. This test relies on the principle that, when heated, ketoses are more rapidly dehydrated than aldoses.
Ketoses and aldoses can be chemically differentiated through Seliwanoff's test, where the sample is heated with acid and resorcinol. [4] The test relies on the dehydration reaction which occurs more quickly in ketoses, so that while aldoses react slowly, producing a light pink color, ketoses react more quickly and strongly to produce a dark red color.
Ketoses must first tautomerize to aldoses before they can act as reducing sugars. The common dietary monosaccharides galactose , glucose and fructose are all reducing sugars. Disaccharides are formed from two monosaccharides and can be classified as either reducing or nonreducing.
The enediol is also an intermediate for the epimerization of an aldose or ketose. [1] [2] The reactions are usually base catalyzed, but can also take place under acid or neutral conditions. [1] A typical rearrangement reaction is that between the aldose glyceraldehyde and the ketose dihydroxyacetone in a chemical equilibrium.
Many sugars are ketones, known collectively as ketoses. The best known ketose is fructose; it mostly exists as a cyclic hemiketal, which masks the ketone functional group. Fatty acid synthesis proceeds via ketones. Acetoacetate is an intermediate in the Krebs cycle which releases energy from sugars and carbohydrates. [22]
The number of open chain stereoisomers for an aldose monosaccharide is larger by one than that of a ketose monosaccharide of the same length. Every ketose will have 2 (n−3) stereoisomers where n > 2 is the number of carbons. Every aldose will have 2 (n−2) stereoisomers where n > 2 is the number of carbons. These are also referred to as ...
When the carbonyl is in position 1, forming an formyl group (−CH=O), the sugar is called an aldohexose, a special case of aldose. Otherwise, if the carbonyl position is 2 or 3, the sugar is a derivative of a ketone, and is called a ketohexose, a special case of ketose; specifically, an n-ketohexose.