Monosaccharides show an interesting in aqueous solution. The aldehyde group at C- 1 and hydroxyl group at C-5, come closer and this proximity facilitates hemiacetal formation. When water is added, a hydrate of aldehyde is formed and if alocohol is added the hemiacetal is formed. We have so far considered open chain fischer projections of monosaccharides. According to these projections, D-glucose exists in two isomers : α-D-glucose which are similar in composition. The α and β isomers of D-glucosein aqueous solution undergo optical rotation, reaching an equilibrium state in time. This is called mutarotation. It has been suggested that α and β isomers are not open chain structures, but are ring forms generated by reaction of the OH group at C-5, and with CHO group at C-1. Haworth in 1929 proposed that ring form (hemiacetals) of sugars containing an asymmetric carbon atom can exist in  α and β anomers.