1
Question
Elucidate the structure of fructose.
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Solution
Structure of fructose:
(i) Elemental analysis and molecular weight determination show that the molecular formula of fructose is C6H12O6.
(ii) Complete reduction of fructose with oncentrated hydriodic acid in the presence of red phosphorus gives n-hexane as the major product. This indicates that six carbon atoms in the fructose molecule form a consecutive unbranched chain.
FructoseC6H12O6HI/P→ReductionCH3−CH2−CH2−CH2−CH2−CH3n−Hexane
(iii) It dissolves in water to give a neutral solution. This indicates that the fructose molecule does not contain a carboxyl group.
(iv) It reacts with acetic anhydride in the presence of pyridine to form a pentaacetate. This reaction indicates the presence of five hydroxyl groups in a fructose molecule.
(v) It reacts with hydroxylamine to form a monoxime or adds only one mole of HCN to give a cyanohydrin.
This reaction indicate the presence of either an aldehyde −C−H||O (or) a ketone group −C−||O.
(vi) It is not oxidised by bromine water indicating the absence of aldehydic group.
(vii) Oxidation of fructose with concentrated nitric acid yields a mixture of glycollic acid and tartaric acid. Since this oxidation occurs with the rupture of the carbon chain the carbonyl group must be present as a ketone group.
(viii) When it is treated with HCN, it forms an addition product which upon hydrolysis and subsequent reduction with HI and Red P gives 2-methyl hexanoic acid. This indicates that the ketone group is adjacent to one of terminal carbon atoms.
(ix) From the above evidences, we conclude that fructose is a pentahydroxy hexanone(a ketohexose) and can be represented by the following structure.
(i) Elemental analysis and molecular weight determination show that the molecular formula of fructose is C6H12O6.
(ii) Complete reduction of fructose with oncentrated hydriodic acid in the presence of red phosphorus gives n-hexane as the major product. This indicates that six carbon atoms in the fructose molecule form a consecutive unbranched chain.
FructoseC6H12O6HI/P→ReductionCH3−CH2−CH2−CH2−CH2−CH3n−Hexane
(iii) It dissolves in water to give a neutral solution. This indicates that the fructose molecule does not contain a carboxyl group.
(iv) It reacts with acetic anhydride in the presence of pyridine to form a pentaacetate. This reaction indicates the presence of five hydroxyl groups in a fructose molecule.
(v) It reacts with hydroxylamine to form a monoxime or adds only one mole of HCN to give a cyanohydrin.
This reaction indicate the presence of either an aldehyde −C−H||O (or) a ketone group −C−||O.
(vi) It is not oxidised by bromine water indicating the absence of aldehydic group.
(vii) Oxidation of fructose with concentrated nitric acid yields a mixture of glycollic acid and tartaric acid. Since this oxidation occurs with the rupture of the carbon chain the carbonyl group must be present as a ketone group.
(viii) When it is treated with HCN, it forms an addition product which upon hydrolysis and subsequent reduction with HI and Red P gives 2-methyl hexanoic acid. This indicates that the ketone group is adjacent to one of terminal carbon atoms.
(ix) From the above evidences, we conclude that fructose is a pentahydroxy hexanone(a ketohexose) and can be represented by the following structure.
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