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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.

Phenomenon of mutarotation

Haworth  formulas indicate arrangement of carbon atoms in space in ring forms. Pyranose rings exhibit two configurations, the chair form and the boat form, the former being more stable and predominant than the latter. D-glucose is shown in pyranose form and ketose sugar (D-fructose) in furanose form . A few important sugar are also shown in Haworth projections.

Pyranose form of monocharrides

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