D-Arabinose is predominantly found in the cyclic form, which is in equilibrium with a minute amount — Physical Chemistry — Thermodynamics Chemistry Question

D^-Arabinose is predominantly found in the cyclic form, which is in equilibrium with a minute amount of the open form. Treatment of D-arabinose with hydrogen cyanide (HCN) yields a diastereomeric mixture of cyanohydrins A and B. Hydrolysis of cyanohydrins A and B yields their corresponding carboxylic acids, which lactonize on heating and form the 5-membered lactones C and D (the planar structures of the lactones are shown below), respectively. The lactones C and D can be reduced with sodium amalgam (or sodium borohydride) to yield D-mannose and D-glucose, respectively. This chain-extension procedure is known as Kiliani^-Fischer synthesis.

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