Seliwanoff's test
is a chemical test which distinguishes between aldose and ketose sugars sugars.. Ketoses are distinguished from aldoses via their ketone ketone//aldehyde functionality. If the sugar contains a ketone group, it is a ketose and if it contains an aldehyde group, it is an aldose. This test is based o n the fact that, when heated, ketoses are more rapidly dehydrated than aldoses. Seliwanoff’s test
Seliwanoff-Reaction
The reagents consist of resorcinol of resorcinol and concentrated sulphuric acid: acid:
The acid hydrolysis of polysaccharides and oligosaccharides yields simpler sugars. The dehydrated ketose then reacts with the resorcinol to produce a deep cherry red color. Aldoses may react slightly to produce a faint pink color.
Fructose and sucrose are two common sugars which give a positive test. S ucrose gives a positive test as it is a disaccharide consisting of fructose and glucose, Aldose
From Wikipedia, the free encyclopedia
Fischer projection of DD-glyceraldehyde
An aldose is a monosaccharide (a simple sugar) that contains only one aldehyde (-CH=O) group per molecule. The chemical formula takes the form Cn(H2O)n. The simplest possible aldose is the diose [1] glycolaldehyde, which only contains two carbon atoms. Because they have at least one asymmetric carbon centre, aldoses exhibit stereoisomerism. This means an aldose can exist in either a D form or L form of a Fischer projection. Biological systems tend to recognise D-aldoses more than L-aldoses. An aldose differs from a ketose in that it has a carbonyl group at the end of the carbon chain instead of in the middle. This allows ketoses and aldoses to be chemically differentiated through Seliwanoff's [2] [3] test. An aldose may isomerize to a ketose through the Lobry-de Bruyn-van Ekenstein transformation. Ketose
Fructose, an example of a ketose. The ketone group is the doublebonded oxygen. A ketose is a sugar containing one ketone group per molecule.
[1][2]
With 3 carbon atoms, dihydroxyacetone is the simplest of all ketoses and is the only one having no optical activity. Ketoses can isomerize into an aldose when the carbonyl group is located at the end of the molecule. Such ketoses are reducing sugars. B. Seliwanoff's Test for Ketone Hexoses Seliwanoff's test is used to distinguish between aldehyde and ketone hexoses (carbohydrates containing 6 carbon atoms). A ketone hexose, also called a ketohexose, will form a deep red color when reacted with Seliwanoff's reagent. An aldehyde hexose, also called a aldohexose, will show a light pink color that takes a longer time to develop when reacted with Seliwanoff's reagent. Place 10 drops of glucose, fructose, lactose, sucrose, starch, and an unknown in separate labeled test tubes. Add 2 mL of Seliwanoff's re agent to each sample. Place all of the test tubes in the boiling water and record the time. After 1 min, observe the colors of the solutions in each test t ube. The rapid formation of a deep red color indicates the pre sence of a ketohexose. Record your observations as either fast color change, slow color change, or no color change. Determine if each sample is a ketohexose or not.
Ketone Compound Groups
A ketone compound is any compound that contains a carbonyl group (CO) that is bonded to two other carbon atoms. These adjacent carbon atoms are known as the a -carbons. The resulting compound of the carbonyl group and the a-carbons thus takes the form: a-carbon1(CO)acarbon2. Structurally, the carbonyl group of a ketone exists in the internal carbon atom, such as found in the compound acetone (CH3-CO-CH3). Acetone, otherwise know as propanone, is the simplest known ketone.
Aldehyde Compound Groups
An aldehyde compound is any compound that contains a terminal carbonyl group (CO), or a carbonyl group at the end of the carbon chain. Aldehyde compounds are comprised of a carbon atom that is bonded to a hydrogen atom, and double bonded to an oxygen atom. The resulting compound found in the aldehyde group thus takes the form: O=CH-. The aldehyde compound group is also commonly referred to as the formyl or methanoyl group.
Why Glucose in an Aldose
The carbohydrate glucose (C6H12O6), often referred to by t he names "grape sugar," "blood sugar" or "corn sugar," is a simple sugar containing six atoms. These six atoms form a cyclic ring where each carbon is linked to an OH group, w ith the exception of one carbon atom that links to the outside of the ring. The carbon atom linking to the outside of the ring forms a carbonyl group at the end of the carbon chain, making it an aldehyde compound. Since the carbohydrate glucose contains an aldehyde compound, it is an aldose.
Fructose an Example of a Ketose
The carbohydrate fructose, as opposed to glucose, is an example of a ke tose. Although it contains the same molecular formula as glucose (C6H12O6), fructose has a different molecular structure. In the cyclic ring formed by fructose, the OH group of t he carbonyl group attaches to the second, internal carbon atom, bonding it to carbon atoms on either side. Since the carbonyl group for fructose is internal, the molecular structure of fructose contains a ketone compound, making fructose a ketose.
Sucrose
The sucrose molecule is a disaccharide, meaning it's made up of two smaller carbohydrate units called monosaccharides, where that word me ans "single sugar unit." The monosaccharides making up sucrose are called glucose and fructose. Fructose is fruit sugar, and glucose is a very common sugar that's found in most carbohydrates in nature, including starch and fiber. You ca n use carbohydrates, including sucrose, to provide for cellular energy needs. Ketoses
A ketose is a kind of monosaccharide. There are many different monosaccharides, or carbohydrate units, in nature. Ketoses are ketone-based sugars, meaning that the monosaccharide molecule contains a carbon atom with a double bond to oxygen, and two single bonds to other carbon atoms. In ketose, the carbon with the double bond to oxygen is particularly reactive; it's called the "anomeric carbon" and takes part in bonding reactions. Aldoses
Monosaccharides that aren't ketoses are called aldoses. These look very similar to ketoses; they also have a carbon atom with a double bond to oxygen. Instead of being bonded to two other carbon atoms, however, the carbon atom bonded to oxygen is bonded to one carbon and one hydrogen. As in ketoses, the carbon bonded to oxygen in an aldose is particularly reactive, and engages in bonding reactions with other monosaccharides. Fructose
In sucrose, fructose is a ketose, while glucose is an aldose. Despite this difference, however, your body uses fructose and glucose for many of the same purposes. When you eat sucrose, you take up both the fructose and glucose into the bloodstream, and they enter the cells from there. Your cells can burn either for immediate energy or make them into glycogen, a carbohydrate for storing ener gy, or into body fat.
Read more: http://www.livestrong.com/article/412792-is-sucrose-a-ketose/#ixzz1f3giDxUo
Xylose Xylose (cf. Greek ξυλος, xylos, "wood") is a sugar first isolated from wood, and name d for it. Xylose is classified as a monosaccharide of the aldopentose type, which means that it contains five carbon atoms and includes an aldehyde functional group. It is the precursor to hemicellulose, one of the main constituents of biomass. Like most sugars, it can adopt several structures depending on conditions. With its free carbonyl group, it is a reducing sugar.
