The following reactions of D- glucose cannot be explained by its open chain structure.
1. D - glucose does not undergo certain characteristic reactions of aldehydes. For example glucose does not form NaHSO3 addition product, aldehyde-ammonia adduct, 2, 4, DNP derivative and does not respond to Schiff’s reagent test.
2. Glucose reacts with NH2OH to form an oxime but glucose pentaacetate does not. This implies that the aldehyde group is absent in glucose pentaacetate.
3. D (+) - Glucose exists in two stereoisomeric forms ie. α – glucose and β- glucose, α - D (+) - glucose is obtained when a concentrated aqueous or alcoholic solution is crystallised at 303K. It has a melting point of 419K and has a specific rotation of +111° in a freshly prepared aqueous solution. However when glucose is crystallised from water above 371 K β - D (+) glucose is obtained.
4. Both α - D glucose and β - D glucose undergo mutarotation in aqueous solution.