Reducing Sugars

Figure 113.

Furanoses are five-membered rings and pyranoses are six-membered rings. These are relatively stable cyclic compounds because they are relatively free of ring strain. These are similar to the molecules seen in organic chemistry, such as tetrahydrofuran (a fivemembered carbon and oxygen ring that is fully saturated) and tetrahydropyran (a sixmembered carbon and oxygen ring that is fully saturated).

Fructose is considered a furanose and glucose is considered a pyranose because of the number of carbons in the chain and the fact that they are not saturated. In addition, Dglucose is referred to as an aldose or aldohexose, while fructose is called a ketose or ketohexose. To be an aldose, the sugar needs to have an aldehyde group on it; to be a ketose, the sugar needs to have a ketone group.

REDUCING SUGARS

Reducing sugars are those sugars that will reduce copper from copper (II) to copper (I)—effectively reducing the oxidation number of the copper ion from +2 to +1. They must contain an aldehyde group that itself is oxidized to carboxylic acid. So, when it comes to sugars, the sugar needs to be an aldehyde sugar that opens to make a reactive aldehyde. This also means that the sugar must be hemiacetal, which rules out glycosides that cannot be hemiacetals (these are acetals). Ketoses can also be reducing sugars because they can isomerize to become aldoses through the enediol intermediate.