12 Difference Between Reducing Sugar And Non Reducing Sugar With Examples

What is Reducing Sugar?

A reducing agent is a compound (like sugar) or an element (like calcium) that loses an electron to another chemical type in a redox chemical reaction. Reducing sugars, like glucose and lactose, have free ketone or aldehyde functional groups, which enable the formation of a hemiacetal, a carbon connected to two oxygen atoms: an alcohol (OH) and an ether (OR). 

Monosaccharides which contain an aldehyde group are known as aldoses, and those with a ketone group are known as ketoses. The aldehyde can be oxidized via a redox reaction in which another compound is reduced. Thus, aldoses are reducing sugars. Sugars with ketone groups in their open chain form are capable of isomerizing via a series of tautomeric shifts to produce an aldehyde group in solution. Therefore, ketones like fructose are considered reducing sugars but it is the isomer containing an aldehyde group which is reducing since ketones cannot be oxidized without decomposition of the sugar. This type of isomerization is catalyzed by the base present in solutions which test for the presence of reducing sugars

The aldehyde functional group allows the sugar to act as a reducing agent, for example, in the Tollens’ test or Benedict’s test. The cyclic hemiacetal forms of aldoses can open to reveal an aldehyde, and certain ketoses can undergo tautomerization to become aldoses. However, acetals, including those found in polysaccharide linkages, cannot easily become free aldehydes.

Characteristics of Reducing Sugars

• A reducing sugar has a free aldehyde (-CHO) or ketone (-C=O) group that can act as a reducing agent.
• They give a dark-red color (brick color) when they react with Benedict’s solution.
• They have a sweet taste.
• Molecular weight of reducing sugars is relatively low.
• They give a positive reaction towards the Fehling’s test.
• They have the ability to reduce cupric ions of Benedict’s or Fehling solution to cuprous ions.
• They give positive result with Tollen’s test.
• Presence or absence of reducing sugars can be identified by carrying out different chemical tests.
• Most of the reducing sugars are monosaccharides.
• The reducing forms for reducing sugars are in the hemiketal or the hemiacetal.

What Is Non-Reducing Sugar?

Non-reducing agents don’t have free ketone or aldehyde groups, and therefore contain an acetal instead of a hemiacetal. An acetal has two O-R groups, one –R group and a –H atom attached to the same carbon. (The key difference between an acetal and a hemiactal is that in a hemiacetal, an –OH group replaces one of the –OR acetal groups.) A sugar without a hemiacetal is non-reducing because it does not behave as a reducing agent toward oxidizing metal salts. Sucrose is one example of a non-reducing sugar.

Characteristics Of Non-reducing Sugar

• A non-reducing sugar does not have a free aldehyde or ketone and therefore it cannot act as a reducing agent.
• They do not give a red color but remains green in color when it reacts with Benedict’s solution.
• They have a less sweet taste.
• The molecular weight of reducing sugars is relatively high when compared to that of reducing sugars.
• They give a negative reaction towards the Fehling’s test.
• They lack ability to reduce cupric ions of Benedict’s solution to coprous ions.
• They give negative result with Tollen’s test.
• The presence or absence of non-reducing sugars cannot be identified by different tests.
• Most of non-reducing sugars are polysaccharides whereas others are disaccharides.
• The non-reducing sugar form is in the acetal or the ketal form.

Difference Between Reducing Sugar And Non Reducing Sugar In Table Form

Chemical Tests For Reducing And Non-reducing Sugars

Different tests can work out whether a sugar is reducing or non-reducing, by detecting the presence of free aldehyde or ketone groups. The Benedict’s test heats a mixture of Benedict’s reagent (a deep-blue alkaline solution) and sugar. If a reducing sugar is present, the reagent changes color: from green to dark red or rusty-brown, depending on the quantity and type of sugar. If you add a non-reducing sugar, like sucrose, the reagent remains blue.

The Fehling test involves two reagents: a clear blue solution containing copper sulphate and a colorless solution containing sodium tartrate. You add both solutions to the sugar and place the entire mixture in boiling water. If the sugar is reducing, a brick red precipitate forms. If you add sucrose or another non-reducing sugar, the mixture stays clear blue.