7.7 More Carbohydrates
Glucose is an aldose, meaning its open chain is an aldehyde. There are also ketoses, such as fructose:
Two forms of fructose.
Fructose is a 2-ketose, meaning the carbonyl group is at the C2 position. Therefore, in the cyclic form, the carbon attached to two oxygen atoms is C2, not C1.
Fructose has five different tautomers that freely interconvert in solution. The disaccharide of glucose and fructose is sucrose, or table sugar. In sucrose, the glucose is in its α- form, while the fructose is in a furanonse form. They are connected via the glucose's C1 and the fructose's C2:
Sucrose.
Lactose is a disaccharide of glucose and galactose, joined via a β(1→4) linkage. Galactose is an aldose like glucose. Since the glucose unit can be either alpha or beta, there are alpha and beta tautomers of lactose.
Lactose.
Glucose, fructose, and galactose are all hexoses, because their molecules have 6 carbon atoms, but there are also pentoses with 5 carbon atoms, tetroses with 4, and trioses with 3 carbons: two enantiomers of glyceraldehyde plus the achiral dihydroxyacetone HOCH2COCH2OH. Each of these occur as aldoses and ketoses, and the keto group is not limited to C2. There is a mind bogglingly large array of possible sugars. Not to worry, we will only be learning a few of them:
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D-erythrose. |
D-ribose. |
D-deoxyribose. |