1. Fischer identified that the open chain Penta hydroxyl aldehyde structure of glucose that he proposed did not completely explain its chemical behaviour.
2. Unlike simple aldehyde, glucose did not form crystalline hisuiphite compound with sodium bisuiphite. Glucose does not give Schifis test and pent.a acetate derivative of glucose was not oxidised by Tollen’s reagent. This behaviour could not be explained by open chain structure.
4. In order to explain these it was proposed that one of the hydroxyl group reacts with aldehyde group to form a cyclic structure (hemiacetal form). This also results in the conversation of the achiral aldehyde carbon into a chiral one leading the possibility of two isomers. These two isomers differ only in the configuration of C1 carbon. These isomers are called anomers.
5. The two anomeric forms of glucose are called – and β – forms. This cyclic structure of glucose is similar to pyran, a cyclic compound with 5 carbon and one oxygen atom, and hence is called pyranose form.
6. The specific rotation of pure α – and 3 – (D) glucose are 112° & 18.7° respectively. However, when pure form any one of these sugars dissolved in water, slow interconversion of – D glucose and β – D glucose via open chain form until equilibrium is established giving constant specific rotation + 53°. This phenomenon is called mutarotation.
