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Reducing sugar
 
 
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A reducing sugar is any sugar that, in basic solution, forms some aldehyde
Aldehyde

An aldehyde is an organic compound containing a terminal carbonyl group. This functional group, which consists of a carbon atom bonded to a hydrogen atom and double bond to an oxygen atom , is called the aldehyde group....
 or ketone
Ketone

In organic chemistry, a ketone is a type of organic compound which contains a carbonyl group bonded to two other carbon atoms in the form:Neither of the substituents R1 and R2 may be equal to hydrogen ....
. This allows the sugar to act as a reducing agent
Reducing agent

A reducing agent is the element or compound in a redox reaction that reduces another Chemical species. In doing so, it becomes oxidized, and is therefore the electron donor in the redox....
, for example in the Maillard reaction
Maillard reaction

The Maillard reaction is a chemical reaction between an amino acid and a reducing sugar, usually requiring heat. It is vitally important in the preparation or presentation of many types of food, and, like caramelization, it is a form of non-enzymatic browning....
 and Benedict's reaction
Benedict's reagent

Benedict's reagent is a chemical reagent named after an American chemist, Stanley Rossiter Benedict.Benedict's reagent is used as a test for the presence of all monosaccharides, and generally also reducing sugars....
.

Reducing sugars include glucose
Glucose

Glucose , a monosaccharide also known as grape sugar, blood sugar, or corn sugar, is a very important carbohydrate in biology....
, fructose
Fructose

Fructose is a simple Reducing sugar sugar found in many foods and is one of the three important dietary monosaccharides along with glucose and galactose....
, glyceraldehyde
Glyceraldehyde

Glyceraldehyde is a triose monosaccharide with chemical formula Carbon3Hydrogen6Oxygen3. It is the simplest of all common aldoses....
, lactose
Lactose

Lactose is a sugar that is found most notably in milk. Lactose makes up around 2?8% of milk . The name comes from the Latin word for milk, plus the -ose ending used to name sugars....
, arabinose
Arabinose

Arabinose is an aldopentose — a monosaccharide containing five carbon atoms, and including an aldehyde functional group. It has chemical formula Carbon5Hydrogen10Oxygen5 and a molar mass of 150.13 g/mol....
 and maltose
Maltose

Maltose, or malt sugar, is a disaccharide formed from two units of glucose joined with an a linkage. It is the second member of an important biochemical series of glucose chains....
. All monosaccharides which contain ketone
Ketone

In organic chemistry, a ketone is a type of organic compound which contains a carbonyl group bonded to two other carbon atoms in the form:Neither of the substituents R1 and R2 may be equal to hydrogen ....
 groups are known as ketoses, and those which contain aldehyde
Aldehyde

An aldehyde is an organic compound containing a terminal carbonyl group. This functional group, which consists of a carbon atom bonded to a hydrogen atom and double bond to an oxygen atom , is called the aldehyde group....
 groups are known as aldoses. Significantly, sucrose
Sucrose

Sucrose is a disaccharide of glucose and fructose, with the molecular formula C12H22O11. Its systematic name is a-D-glucopyranosyl- -?-D-fructofuranoside ....
, and trehalose
Trehalose

Trehalose, also known as mycose, is a natural alpha-linked disaccharide formed by an a, a-1, 1-glucoside bond between two a-glucose units....
 are not reducing sugars.

A reducing sugar occurs when its anomer
Anomer

In sugar chemistry, an anomer is a special type of epimer. It is a stereoisomer of a saccharide that differs only in its configuration at the hemiacetal carbon, also called the anomeric carbon....
ic carbon (the carbon which is linked to two oxygen atoms) is in the free form. Since sugars occur in a chain as well as a ring structure, it is possible to have an equilibrium between these two forms. When the hemi-acetal or ketal hydroxyl group is free, i.e.






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A reducing sugar is any sugar that, in basic solution, forms some aldehyde
Aldehyde

An aldehyde is an organic compound containing a terminal carbonyl group. This functional group, which consists of a carbon atom bonded to a hydrogen atom and double bond to an oxygen atom , is called the aldehyde group....
 or ketone
Ketone

In organic chemistry, a ketone is a type of organic compound which contains a carbonyl group bonded to two other carbon atoms in the form:Neither of the substituents R1 and R2 may be equal to hydrogen ....
. This allows the sugar to act as a reducing agent
Reducing agent

A reducing agent is the element or compound in a redox reaction that reduces another Chemical species. In doing so, it becomes oxidized, and is therefore the electron donor in the redox....
, for example in the Maillard reaction
Maillard reaction

The Maillard reaction is a chemical reaction between an amino acid and a reducing sugar, usually requiring heat. It is vitally important in the preparation or presentation of many types of food, and, like caramelization, it is a form of non-enzymatic browning....
 and Benedict's reaction
Benedict's reagent

Benedict's reagent is a chemical reagent named after an American chemist, Stanley Rossiter Benedict.Benedict's reagent is used as a test for the presence of all monosaccharides, and generally also reducing sugars....
.

Reducing sugars include glucose
Glucose

Glucose , a monosaccharide also known as grape sugar, blood sugar, or corn sugar, is a very important carbohydrate in biology....
, fructose
Fructose

Fructose is a simple Reducing sugar sugar found in many foods and is one of the three important dietary monosaccharides along with glucose and galactose....
, glyceraldehyde
Glyceraldehyde

Glyceraldehyde is a triose monosaccharide with chemical formula Carbon3Hydrogen6Oxygen3. It is the simplest of all common aldoses....
, lactose
Lactose

Lactose is a sugar that is found most notably in milk. Lactose makes up around 2?8% of milk . The name comes from the Latin word for milk, plus the -ose ending used to name sugars....
, arabinose
Arabinose

Arabinose is an aldopentose — a monosaccharide containing five carbon atoms, and including an aldehyde functional group. It has chemical formula Carbon5Hydrogen10Oxygen5 and a molar mass of 150.13 g/mol....
 and maltose
Maltose

Maltose, or malt sugar, is a disaccharide formed from two units of glucose joined with an a linkage. It is the second member of an important biochemical series of glucose chains....
. All monosaccharides which contain ketone
Ketone

In organic chemistry, a ketone is a type of organic compound which contains a carbonyl group bonded to two other carbon atoms in the form:Neither of the substituents R1 and R2 may be equal to hydrogen ....
 groups are known as ketoses, and those which contain aldehyde
Aldehyde

An aldehyde is an organic compound containing a terminal carbonyl group. This functional group, which consists of a carbon atom bonded to a hydrogen atom and double bond to an oxygen atom , is called the aldehyde group....
 groups are known as aldoses. Significantly, sucrose
Sucrose

Sucrose is a disaccharide of glucose and fructose, with the molecular formula C12H22O11. Its systematic name is a-D-glucopyranosyl- -?-D-fructofuranoside ....
, and trehalose
Trehalose

Trehalose, also known as mycose, is a natural alpha-linked disaccharide formed by an a, a-1, 1-glucoside bond between two a-glucose units....
 are not reducing sugars.

A reducing sugar occurs when its anomer
Anomer

In sugar chemistry, an anomer is a special type of epimer. It is a stereoisomer of a saccharide that differs only in its configuration at the hemiacetal carbon, also called the anomeric carbon....
ic carbon (the carbon which is linked to two oxygen atoms) is in the free form. Since sugars occur in a chain as well as a ring structure, it is possible to have an equilibrium between these two forms. When the hemi-acetal or ketal hydroxyl group is free, i.e. it is not locked in an acetal or ketal linkage, the aldehyde (or keto-) form (i.e. the chain-form) is present. The aldehyde can be oxidized to a carboxyl group via a redox reaction. The chemical that causes this oxidation becomes reduced. Thus, a reducing sugar is one that reduces certain chemicals. Even though a ketone
Ketone

In organic chemistry, a ketone is a type of organic compound which contains a carbonyl group bonded to two other carbon atoms in the form:Neither of the substituents R1 and R2 may be equal to hydrogen ....
 cannot be oxidized directly, a keto sugar can sometimes be converted to an aldehyde via a series of tautomeric shifts to migrate the carbonyl
Carbonyl

In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double bond to an oxygen atom : C=O.The term carbonyl can also refer to carbon monoxide as a ligand in an inorganic or organometallic complex ; in this situation, carbon is triple-bonded to oxygen : C=O....
 to the end of the chain. Therefore, keto sugars are also sometimes reducing.

Benedict's reagent
Benedict's reagent

Benedict's reagent is a chemical reagent named after an American chemist, Stanley Rossiter Benedict.Benedict's reagent is used as a test for the presence of all monosaccharides, and generally also reducing sugars....
 and Fehling's solution
Fehling's solution

Fehling's solution is a solution used to differentiate between water soluble aldehyde and ketone functional groups, although ketose monosaccharides will also test positive, due to conversion to aldoses by the base in the reagent ....
 are used to test for the presence of a reducing sugar. The reducing sugar reduces copper(II)
Copper

Copper is a chemical element with the symbol Cu and atomic number 29.It is a ductile metal with very high thermal and electrical conductivity....
 ions in these test solutions to copper(I), which then forms a brick red copper(I) oxide
Copper(I) oxide

Copper oxide or cuprous oxide is an oxide of copper. It is insoluble in water and organic solvents. Copper oxide dissolves in concentrated ammonia solution to form the colorless complex [Cu2]+, which easily redox in air to the blue [Cu42]2+....
 precipitate. 3,5-Dinitrosalicylic acid
3,5-Dinitrosalicylic acid

3,5-Dinitrosalicylic acid is an aromatic compound that reacts with reducing sugars and other reducing molecules to form 3-amino-5-nitrosalicylic acid, which absorbs light strongly at 540 1 E-9 m....
 is another test reagent that allows quantitative spectrophotometric measurement of the amount of reducing sugar present.

Sugars having acetal or ketal linkages are not reducing sugars, as they do not form free aldehyde chains. They therefore do not react with any of the reducing-sugar test solutions. However, a non-reducing sugar can be hydrolysed using dilute hydrochloric acid
Hydrochloric acid

Hydrochloric acid is the solution of hydrogen chloride in water. It is a highly corrosive, strong acid mineral acid and has major industrial uses....
 to convert the acetal or ketal into a hemiacetal or hemiketal. After hydrolysis and neutralization of the acid, the product may be a reducing sugar that gives normal reactions with the test solutions.

All carbohydrates respond positively to Molisch's reagent
Molisch's test

Molisch's Test is a sensitive chemical test for the presence of carbohydrates, based on the dehydration of the carbohydrate by sulfuric acid to produce an aldehyde, which condenses with two molecules of phenol resulting in a red- or purple-colored compound....
.