Lactols

Lactols

General structure of lactols: on the left the shape of hemiacetals, on the right of hemiketals. The cyclic hemiacetal structure is marked in blue .

Lactols are according to IUPAC cyclic hemiacetals , wherein after IUPAC under hemiacetals and hemiketals be understood. Lactols are therefore intramolecular addition products of a hydroxyl group to an aldehyde or keto group .

They are especially important for carbohydrates ( saccharides ). These are monosaccharides such as aldoses and ketoses . Depending on the ring size, furanoses ( five rings ) or pyranoses ( six rings ) are formed.

Balance between open chain and ring shape

In aqueous solution, monosaccharides are in equilibrium between the open-chain and hemiacetal form, the cyclic form being almost always more stable. In the case of the fructose analogue 4-hydroxybutanal , 11.4% are in the open-chain and 88.6% in the furanose form. In the case of glucose , the equilibrium is 99.9974% on the side of the pyranose, while the analogue 5-hydroxypentanal is found in 6.1% in the open-chain and 93.9% in the hemiacetal form.

Formation of a lactol from 4-hydroxybutanal (analogous to fructose, above) and a lactol from 5-hydroxypentanal (analogous to glucose, below).

See also

• Lactones
• Lactams
• Lactime

Individual evidence

1. ↑ Entry on lactols . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.L03438 Version: 2.3.1.
2. ↑ ^{a b} H.D. Belitz, W. Grosch, P. Schieberle: Textbook of Food Chemistry , 2001, Springer-Verlag , ISBN 3540410961 .