What defines a reducing sugar?

A reducing sugar is defined as a sugar that has the ability to donate electrons or reduce other molecules, a property that is often attributed to the presence of free aldehyde or ketone groups. These functional groups can participate in redox reactions, particularly when they are in their open-chain (or straight-chain) form.

In the context of carbohydrates, reducing sugars are characterized by the presence of an accessible aldehyde group, which can reduce other substances such as copper(II) ions in Benedict's solution or Fehling's solution, resulting in a color change. While most monosaccharides (like glucose and fructose) are reducing sugars due to their aldehyde or ketone groups, some disaccharides can also be reducing sugars if they contain a free aldehyde or ketone group in their structure.

In contrast, sugars that cannot form a cyclic structure would not predominantly exist in a form that allows for these redox reactions. Similarly, sugars that do not contain hydroxyl groups would not fit the definition of typical carbohydrates, as hydroxyl groups are essential components of sugar structures. Lastly, a sugar that is not metabolizable does not indicate anything about its ability to act as a reducing agent, as metabolic pathways do not directly correlate with redox