Alditols

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General structural formula of alditols (n ≥ 1)

Alditols (formerly: Aldite ) are non-cyclic polyols that have a hydroxyl group attached to each of their carbon atoms . They are organic compounds that are mostly derived as reduction products from carbohydrates (sugars). They are also known as sugar alcohols .

structure

The alditols include, for example, mannitol (mannitol), isomalt , lactitol , sorbitol (sorbitol or glucitol) and xylitol (xylitol), threitol , erythritol and arabitol .

The designation of the alditols follows the following convention: The ending -ose (e.g. glucose) of the original sugar is replaced by the ending -it (glucitol) or -itol (glucitol). Since some alditols can be formally derived from both aldoses and ketoses , it happens that an alditol has several names, for example glucitol and sorbitol refer to the same substance.

Glycerine is formally the simplest alditol, it is derived from glyceraldehyde . The simplest chiral alditol is threitol , which is derived from threose , a carbohydrate with four carbon atoms. From erythrose , also a carbohydrate having four carbon atoms, the passes erythritol from which, in contrast to threitol optically inactive is ( meso compound ).

properties

The taste of alditols is similar to that of sucrose, but not the same. They taste sweet, but rarely reach the relative sweetness of sucrose , are not cariogenic and show a laxative effect when consumed more than 20 to 30 g per day. Some alditols are used as sugar substitutes in diet foods because they do not raise blood sugar levels and do not require insulin to break down. That is why they are suitable for diabetics .

Alditols and sucrose also differ in solubility , pH and temperature stability , melting point and boiling point . These factors can be decisive for the use of alditols as a food ingredient.

synthesis

Alditols can be made from aldoses or ketoses by reducing the carbonyl group . This is achieved industrially by catalytic hydrogenation over a nickel catalyst . In this way, D - sorbitol is synthesized from D - glucose . Sodium borohydride (sodium boronate, NaBH 4 ) is suitable as a reducing agent on a laboratory scale .

Since the stereocenters are not changed when reducing aldoses, exactly one alditol is always formed from an aldose . However, the same alditol can arise from two different aldoses:

Reduction of D -glucose (1) and L -ulose (3) to D -sorbitol (2)

The L- gulose shown in the above scheme in the Fischer projection rotated by 180 ° also yields D -sorbitol when reduced.

The reduction of ketoses, on the other hand, generates a new stereocenter in both possible configurations, so that a mixture of two different ( epimeric ) alditols is formed from one ketosis :

Reduction of D- fructose (2) to D- mannitol (3) and D- sorbitol (1)

Since their separation causes considerable processing costs, some alditols are also produced by biocatalytic conversion with living microorganisms.

use

Alditols are mainly used in the food industry, where they are used as sweeteners. Above all, sorbitol has a wide range of applications, from use as a sugar substitute and uses in the cosmetics industry and as a sweetener for toothpaste to humectants for tobacco, building material additives and uses in the chemical industry, e.g. for the production of nitrosorbitol , surfactants , polyols for polyurethanes is sufficient. Xylitol has similar areas of application as sorbitol; it is ideal for the food industry, as studies have shown anticariogenic effects.

literature

  • Peter Nuhn : Natural Products Chemistry - Microbial, Plant and Animal Natural Products, 4th, revised. Edition. Hirzel Verlag, Stuttgart 2006, ISBN 3-7776-1363-0 , pp. 130-132.