How is a reducing sugar defined in terms of its structure?

A reducing sugar is specifically defined by its ability to act as a reducing agent, which is largely determined by the presence of a hemiacetal or aldehyde group on its anomeric carbon. This structural feature allows the sugar to participate in redox reactions, where it can donate electrons to other molecules, thus reducing them.

In more detail, a reducing sugar must have a free aldehyde group or a hemiacetal structure, which is formed when the molecule cyclizes. The anomeric carbon is the carbon atom that is newly formed in the cyclic form of the sugar, and if this carbon retains the ability to open up to form an aldehyde or remains part of a hemiacetal, it contributes to the reducing capabilities of the sugar.

This is why monosaccharides like glucose (which has a free aldehyde group) and certain disaccharides that can form free aldehydes or hemeiacetals are classified as reducing sugars. These sugars can reduce metal ions, which is typically tested using chemical reactions in laboratory settings, further confirming their classification as reducing sugars.

The other structural features mentioned in the other choices, while relevant to sugar chemistry, do not define a reducing sugar. For example, the presence of ketone