Reduction of Monosaccharides and Sugar Alcohols

Reduction of Monosaccharides

Reduction of Aldose and Ketose Sugars

Monosaccharides, such as aldoses and ketoses, can undergo reduction reactions to form sugar alcohols. This process involves the conversion of the carbonyl group (aldehyde or ketone) into an alcohol group, resulting in a polyhydroxy alcohol known as a sugar alcohol.

• Carbonyl Group Reduction: The carbonyl group is reduced to a hydroxyl group (-OH).

• Sugar Alcohol: A monosaccharide that has its carbonyl group converted into an additional -OH group.

• Reducing Agent: Commonly H2 (hydrogen gas) or NaBH4 (sodium borohydride).

• Catalyst: Often a metal catalyst such as Ni (nickel).

General Reaction:

• The carbonyl carbon gains a hydrogen atom and the carbonyl oxygen gains a hydrogen atom, forming an alcohol group.

Example Reaction:

Reduction of D-glucose (an aldose):

• Reactant: D-glucose

• Product: D-glucitol (also known as sorbitol)

Equation:

$\text{D-glucose} + \text{H}_2 \xrightarrow{\text{Ni catalyst}} \text{D-glucitol (sorbitol)}$

Reduction of Ketose Example:

Reduction of D-fructose (a ketose) produces two sugar alcohols due to the formation of a new chiral center:

• Products: D-glucitol and D-mannitol

$\text{D-fructose} + \text{H}_2 \xrightarrow{\text{Ni catalyst}} \text{D-glucitol} + \text{D-mannitol}$

Reduction of Monosaccharides Table

Reactant	Reducing Agent	Catalyst	Product (Sugar Alcohol)
D-glucose	H2 or NaBH4	Ni	D-glucitol (sorbitol)
D-mannose	H2 or NaBH4	Ni	D-mannitol
D-fructose	H2 or NaBH4	Ni	D-glucitol & D-mannitol

Common Naming of Sugar Alcohols

Naming Rules

Sugar alcohols are named similarly to their parent aldose or ketose sugars, with the ending changed from -ose to -itol.

• D-glucose → D-glucitol (sorbitol)

• D-mannose → D-mannitol

• D-galactose → D-galactitol

• D-ribose → D-ribitol

Example: Naming Sugar Alcohols

When D-ribose is reduced, the product is D-ribitol.

• Reactant: D-ribose

• Product: D-ribitol

$\text{D-ribose} + \text{H}_2 \xrightarrow{\text{Ni catalyst}} \text{D-ribitol}$

Practice Problems and Applications

• Example: The reduction of D-galactose produces D-galactitol.

• Application: Sugar alcohols are used as sweeteners in food products (e.g., sorbitol, mannitol).

Fischer Projection Practice

Drawing the Fischer projection for the reduction product of D-mannose (the C-2 epimer of glucose) yields D-mannitol.

• Reactant: D-mannose

• Product: D-mannitol

Key Point: The reduction of the carbonyl group in monosaccharides always produces a sugar alcohol, with the name ending in -itol.

Additional info: Sugar alcohols are important in biochemistry and nutrition, as they are less likely to cause dental caries and have lower caloric values than sugars.