6. Configuration* of Glucose:
Since the above structure possesses four asymmetric carbon atoms (shown by asterisks), it an exist in 24 = 16 optically active forms, i.e., eight pairs of enantiomers. All these are known and correspond to the D-and L-forms of glucose, mannose, galactose, allose, glucose, idose and talose.
Naturally occurring glucose is the dextrorotatory glucose (+), one of the 16-stereoisomers.
Notations D- and L- for denoting configuration were given by Rosanoff; according to this convention, any compound whose bottom asymmetric carbon atoms has the configuration similar to the configuration of dextrorotatory glyceraldehyde (drawn above, i.e. the bottom carbon atom has -OH to the left and H to the right is given L-configuration. Remember that the symbols D-and L- have no relation with the specific rotation value, i.e., with (+) or (-) value. For example, the natural (-) fructose belongs to D-series, i.e., it is D(-)-fructose).
7. Objections to open-chain structure of glucose:
Even though the open chain structure of (+) glucose explains most of its reactions, it fails to explain the following facts about it.
i) Glucose does not restore Schiff's reagent colour.
ii) Glucose does not form a bisulphite and aldehyde-ammonia compound.
iii) Glucose forms two isomeric pent-acetates neither of which reacts with carbonyl reagents.
iv) The existence of the two isomeric glucose and the change in specific rotation (mutarotation) is not explained by an open-chain formula.
v) Glucose reacts with methanol in presence of dry HCl gas to form two isomeric glucosides.
Since glucose is less soluble in ethanol, it separates out on cooling the reaction mixture. Commercially, it is obtained by the hydrolysis of starch which is available from relatively inexpensive source such as maize, potatoes and rice.
Constitution of Glucose:
1. Molecular Formula: By the usual analytical methods, the molecular formula glucose is found to be C6H12O6.
2. Straight Chain of six carbon atoms:
i) Reduction of glucose with conc. HI and phosphorus gives 2-iodohexane and n-hexane. This indicates that six carbon atoms in glucose are present in a straight chain.
ii) Glucose when oxidized with bromine water gives gluconic acid which when reduced with excess of HI gives n-hexanoic acid, CH3.(CH2)4.COOH confirming the presence of a straight chain of six carbon atoms in glucose.
3. Presence of five hydroxyl groups: When treated with acetic anhydride, glucose forms pent-acetate indicating the presence of 5 -OH groups and since glucose is a stable compound, the five -OH groups must be attached to 5 different carbon atoms.
4. Presence of an aldehydic groups:
i) Glucose forms a cyanohydrin with hydrogen cyanide and a mono-oxime with hydroxylamine suggesting the presence of a carbonyl group.
ii) Glucose reduces Fehling's solution and Tollen's reagent indicating that the carbonyl group is aldehydic in nature.
iii) The presence of aldehydic gruop in glucose is confirmed by its oxidation to gluconic acid having the same number of carbon atoms.
Now since aldehydic group is monovalent, it must be present on the end of the chain.
5. Open chain structure: On the basis of the above points, glucose may be assigned following prt structure orientation shown in the a anomer has the -OH trans to the -CH2OH group and the b anomer has the -OH cis to the -CH2OH group.