reducing sugar

Meaning: A reducing sugar is a carbohydrate that acts as a reducing agent, so the sugar itself is oxidized. In the ring form its anomeric carbon is a hemiacetal or hemiketal with a free OH. The ring opens and closes in solution, exposing a free aldehyde, which is oxidized to a carboxylic acid (an aldonic acid). Ketoses such as fructose also test positive, because under basic conditions they tautomerize through an enediol to aldoses. When the anomeric OH is replaced by OR in a glycosidic bond, the carbon becomes an acetal or ketal. It cannot open, so that carbon cannot be oxidized.

Equation: RCHO + 2Cu²⁺ + 5OH⁻ → RCOO⁻ + Cu₂O + 3H₂O. RCHO is the open-chain sugar, Cu²⁺ is the blue oxidizing agent, and Cu₂O is the red-brown precipitate. This is a qualitative test, so no SI units apply. With Tollens' reagent, Ag⁺ is reduced to a silver mirror.

On the MCAT: You are shown a disaccharide or glycoside and asked whether it gives a positive Benedict's or Tollens' test. Maltose (glucose α-1,4 glucose) and lactose keep one free anomeric carbon, so both are reducing. Sucrose joins glucose C1 to fructose C2, so both anomeric carbons are tied up and it is non-reducing.

Common confusion: Having a glycosidic bond does not make a disaccharide non-reducing. It is reducing if at least one anomeric carbon is still free. Ketoses are not excluded, and the word "reducing" describes the sugar's role: it is oxidized while the reagent is reduced.