Fructose is another commonly known monosaccharide having the same molecular formula as glucose. It is levorotatory and a ketohexose. It is present abundantly in fruits and hence it is also called as fruit sugar.
Fructose is obtained from sucrose by heating with dilute H 2 SO4 or with the enzyme invertase
Fructose is separated by crystallisation. The mixture having equal amount of glucose and fructose is termed as invert sugar.
Fructose is prepared commercially by hydrolysis of Inulin (a polysaccharide) in an acidic medium.
Fructose is the sweetest of all known sugars. It is readily soluble in water. Fresh solution of fructose has a specific rotation -133º which changes to – 92 º at equilibrium due to mutarotation. Similar to glucose the structure of fructose is deduced from the following facts.
1. Elemental analysis and molecular weight determination of fructose show that it has the molecular formula C 6 H12 O6
2. Fructose on reduction with HI and red phosphorus gives a mixture of n – hexane (major product) and 2 – iodohexane (minor product). This reaction indicates that the six carbon atoms in fructose are in a straight chain.
3. Fructose reacts with NH2OH and HCN . It shows the presence of a carbonyl groups in the fructose.
4. Fructose reacts with acetic anhydride in the presence of pyridine to form penta acetate. This reaction indicates the presence of five hydroxyl groups in a fructose molecule.
5. Fructose is not oxidized by bromine water. This rules out the possibility of absence of an aldehyde (-CHO) group.
6. Partial reduction of fructose with sodium amalgam and water produces mixtures of sorbitol and mannitol which are epimers at the second carbon. New asymmetric carbon is formed at C-2. This confirms the presence of a keto group.
7. On oxidation with nitric acid, it gives glycollic acid and tartaric acids which contain smaller number of carbon atoms than in fructose.
This shows that a keto group is present in C-2. It also shows that 1O alcoholic groups are present at C- 1 and C- 6. Based on these evidences, the following structure is proposed for fructose (Figure 14-7)
Like glucose, fructose also forms cyclic structure. Unlike glucose it forms a five membered ring similar to furan. Hence it is called furanose form. When fructose is a component of a saccharide as in sucrose, it usually occurs in furanose form.