![]() ![]() The carbohydrates are a major source of metabolic energy, both for plants and for animals that depend on plants for food. n CO 2 + n H 2O + energy C nH 2nO n + n O 2Īs noted here, the formulas of many carbohydrates can be written as carbon hydrates, C n(H 2O) n, hence their name. They originate as products of photosynthesis, an endothermic reductive condensation of carbon dioxide requiring light energy and the pigment chlorophyll. As I’ve mentioned earlier, carbohydrates, like many biomolecules, have been known for quite some time and have acquired their own naming system that got stuck with us.Carbohydrates are the most abundant class of organic compounds found in living organisms. Why not stick to the good ‘ol R and S? Because of the traditional naming convention that existed before the Cahn-Ingold-Prelog system. We use the letters D and L for this purpose. One piece of stereochemistry that we do specify is the configuration of the last chiral carbon in the chain. ![]() You’ll see them all the time in the biological pathways (especially, glucose), so it’ll make your life easier if you can recognize those. Such common carbohydrates are glucose, mannose, galactose, and fructose. It might be a good idea, though, to commit a few common ones to memory. ![]() Each of those molecules has its own unique name! Most instructors, however, won’t require you to actually memorize all of the structures. This means that glucose has a grand total of 16 stereoisomers! Number of stereoisomers = 2 n, where “n” is the number of chiral centers. So, for a molecule like glucose, you have 4 chiral carbons. However, as you go down the line to more complex carbohydrates, you get more and more stereocenters. The simplest one, glyceraldehyde, only has one. Stereochemistry of CarbohydratesĬarbohydrates have multiple stereocenters. Then, say how many carbons you have in the molecule and add -ose to signify the carbohydrate. The nomenclature of ketoses follows the same principles as for aldoses: you start by saying keto- to point out the functional group. Most common ketoses have a ketone functional group on the second carbon in the chain. As ketoses contain a ketone functional group, we obviously cannot have it at the beginning of the chain. And we finish by adding the -ose ending to specify that it’s a carbohydrate we’re dealing with. We then say how many carbons are there in the molecule. As you can guess, aldo- goes together with an aldehyde, and keto- with the ketone-containing carbohydrates.Īs you can see from these examples, we start the name by saying that the molecule is an aldehyde using the aldo- prefix. We specify this in the name by adding aldo– or keto– prefix to the carbohydrate name. However, sugars will only have one aldehyde OR one ketone functional group. Carbohydrates generally have multiple alcohol functional groups, so we never focus on those. Sugars, or carbohydrates, have two major functional groups: an aldehyde or a ketone (both are collectively called carbonyls), and an alcohol functional group. Naming the Major Functional Group in a Carbohydrate So, when I say that we’re dealing with a hexose, that doesn’t mean much except for the fact that the molecule contains 6 carbons. This list includes glucose, galactose, fructose, mannose, etc. For instance, there are 24 different hexoses (12 of which exist in nature). This type of a name, however, doesn’t tell us the exact nature of the molecule. For instance, the glucose is an example of a hexose because it has six carbons in the molecule. For instance, a triose is a carbohydrate with 3 carbons, while hexose is a carbohydrate with 6 carbons in the molecule. We use the greek numerals to call the number, aka tri-, tetra-, penta-, hexa-, and add the ending -ose to denote that it’s a carbohydrate. ![]()
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