Ring forms of glucose and fructose

Facts the open chain cannot explain: glucose gives no Schiff's test and no hydrogensulphite addition product with NaHSO₃, and its pentaacetate does not react with hydroxylamine, so no free –CHO is present.

Glucose also crystallises in two forms. The α-form (m.p. 419 K) comes from a concentrated solution at 303 K; the β-form (m.p. 423 K) from a hot saturated aqueous solution at 371 K.

Explanation: the –OH on C5 adds to the –CHO group, closing a six-membered ring as a cyclic hemiacetal. With no free –CHO, the aldehyde tests fail.

Ring closure makes C1 a new asymmetric centre, so there are two cyclic forms that differ only in the –OH on C1. C1 is the anomeric carbon and the two forms, α and β, are anomers. Both cyclic forms stay in equilibrium with the open chain.

A six-membered ring of five carbons and one oxygen is named pyranose, after pyran. The Haworth formula draws it as a flat hexagon with groups above and below.

Fructose is a ketohexose, C₆H₁₂O₆, with its keto group at C2 and six carbons in an unbranched chain. It comes with glucose from hydrolysis of sucrose and is found in fruits, honey and vegetables; pure, it is used as a sweetener.

In fructose the C5 –OH adds to the C2 keto group, closing a five-membered ring of four carbons and one oxygen: a furanose, after furan. It too has α and β anomers.