lowest asymmetric carbon is on the right side which is comparable to (+) glyceraldehyde, so (+) glucose is assigned D-configuration. Other asymmetric carbon atoms of glucose are not considered for this comparison. Also, the structure of glucose and glyceraldehyde is written in a way that most oxidised carbon (in this case –CHO)is at the top. CHO CH OH D–(+) – Glucose CHO CH OH D– (+) – Glyceraldehyde The structure (I) of glucose explained most of its properties but the following reactions and facts could not be explained by this structure.
. Despite having the aldehyde group, glucose does not give Schiff’s test and it does not form the hydrogensulphite addition product with NaHSO . . The pentaacetate of glucose does not react with hydroxylamine indicating the absence of free —CHO group.
. Glucose is found to exist in two different crystalline forms which are named as a and b . The a -form of glucose (m.p. K) is obtained by crystallisation from concentrated solution of glucose at K while the b -form (m.p.
K) is obtained by crystallisation from hot and saturated aqueous solution at K. This behaviour could not be explained by the open chain structure ( I ) for glucose. It was proposed that one of the —OH groups may add to the —CHO group and form a cyclic hemiacetal structure. It was found that glucose forms a six-membered ring in which —OH at C- is involved in ring formation.
This explains the absence of —CHO group and also existence of glucose in two forms as shown below. These two cyclic forms exist in equilibrium with open chain structure. The two cyclic hemiacetal forms of glucose differ only in the configuration of the hydroxyl group at C1, called anomeric carbon Cyclic Structure of Glucose (the aldehyde carbon before cyclisation). Such isomers, i.e., a -form and b -form, are called anomers .
The six membered cyclic structure of glucose is called pyranose structure ( a – or b –), in analogy with pyran. Pyran is a cyclic organic compound with