one oxygen atom and five carbon atoms in the ring. The cyclic structure of glucose is more correctly represented by Haworth structure as given below. Fructose is an important ketohexose. It is obtained along with glucose by the hydrolysis of disaccharide, sucrose.
It is a natural monosaccharide found in fruits, honey and vegetables. In its pure form it is used as a sweetner. It is also an important ketohexose. Fructose also has the molecular formula C H O and on the basis of its reactions it was found to contain a ketonic functional group at carbon number and six carbons in straight chain as in the case of glucose.
It belongs to D-series and is a laevorotatory compound. It is appropriately written as D-(–)-fructose. Its open chain structure is as shown. It also exists in two cyclic forms which are obtained by the addition of —OH at C5 to the ( ) group.
The ring, thus formed is a five membered ring and is named as furanose with analogy to the compound furan. Furan is a five membered cyclic compound with one oxygen and four carbon atoms. Structure of Fructose The cyclic structures of two anomers of fructose are represented by Haworth structures as given. .
. . Fructose You have already read that disaccharides on hydrolysis with dilute acids or enzymes yield two molecules of either the same or different monosaccharides. The two monosaccharides are joined together by an oxide linkage formed by the loss of a water molecule.
Such a linkage between two monosaccharide units through oxygen atom is called glycosidic linkage. In disaccharides, if the reducing groups of monosaccharides i.e., aldehydic or ketonic groups are bonded, these are non-reducing sugars, e.g., sucrose. On the other hand, sugars in which these functional groups are free, are called reducing sugars, for example, maltose and lactose. (i) Sucrose : One of the common disaccharides is sucrose which on hydrolysis gives equimolar mixture of D-(+)-glucose and D-(-) fructose.
. . Disaccharides These two monosaccharides are held together by a glycosidic linkage between C1 of a -D-glucose and C2 of