Structure Determination of D-Glucose
Structure Determination of Open Chain D-Glucose
The structure of glucose has been derived from a the following facts and conclusions which are given below-1. Elemental analysis and molecular weight determination show that the molecular formula of glucose is C6H12O6.
2. Complete reduction of glucose with concentrated hydriodic acid in the presence of red phosphorous produces n-hexane as the major product. This indicates that the six carbon atoms in the glucose molecule are in a straight chain.
3. Glucose readily dissolves in water to give a neutral solution. This indicates that the glucose molecule does not contain a carboxyl group.
4. Glucose reacts with hydroxylamine to form a monoxime, or adds only one mole of hydrogen cyanide to give a cyanohydrin. These reactions indicate the presence of either an aldehyde (-CHO) or a ketone -C=O) group, but not both.
5. Mild oxidation of glucose with bromine water gives gluconic acid, a monocarboxylic acid with molecular formula C6H12O7. This indicates the presence of an aldehyde group since only the aldehyde group can be oxidised to an acid by gaining one oxygen atom without losing any hydrogen atoms. Since the six carbon atoms in glucose form a straight chain, the aldehyde group, must occupy one end of this chain.
6. Further oxidation of gluconic acid with nitric acid gives glucaric acid, a dicarboxylic acid with molecular formula C6H10O8. This indicates the presence of a primary alcohol group, since oxidation occurs with the loss of two hydrogens and gain of one oxyger atom.
-CH2OH → -COOH
7. Glucose reduces an ammonical solution of silver oxide (Tollen's reagent) to metallic silver, or a basic solution of cupric ion (Fehling's solution) to red cuprous oxide. These reactions further confirm the presence of a terminal aldehyde group.
8. Glucose reacts with acetic anhydride in the presence of pyridine to form a penta acetate. This reaction indicates the presence of five hydroxyl groups in a glucose molecule.
As the glucose molecule is stable so all the hydroxyl group are present on separate carbon atoms.
From the above evidence we conclude that glucose is a pentahydroxyhexanal (an aldohexose), and can be represented by the following structure.
