What is meant by reducing sugar & non-reducing sugar?

Question

For example, sucrose is a non-reducing sugar & maltose is a reducing sugar. Please explain.

Answer 1

Reducing Sugars :
Sugars that contain aldehyde groups that are oxidised to carboxylic acids are classified as reducing sugars.
Common test reagents are :
Benedicts reagent (CuSO4 / citrate)
Fehlings reagent (CuSO4 / tartrate)
They are classified as reducing sugars since they reduce the Cu2+ to Cu+ which forms as a red precipitate, copper (I) oxide.
Remember that aldehydes (and hence aldoses) are readily oxidised (review ?)
In order for oxidation to occur, the cyclic form must first ring-open to give the reactive aldehyde.
So any sugar that contains a hemi-acetal will be a reducing sugar.
But glycosides which are acetals are not reducing sugars.

A reducing sugar is any sugar that, in basic solution, forms some aldehyde or ketone. This allows the sugar to act as a reducing agent, for example in the Maillard reaction and Benedict's reaction. Reducing sugars include glucose, glyceraldehyde, lactose, arabinose and maltose. All monosaccharides which contain ketone groups are known as ketoses, and those which contain aldehyde groups are known as aldoses. Significantly, sucrose is not a reducing sugar. It is in fact known as a non-reducing sugar.

Benedict's reagent is used to determine if a reducing sugar is present. If it is a reducing sugar, the mixture will turn green/orange/red. Fehling's solution can also be used for the same purpose, as both contain copper (II) ions, which are reduced to a brick red precipitate of copper (I) oxide when the solution is heated.

A reducing sugar occurs when its anomeric carbon is free. 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 hydroxylgroup is free, it is not locked, not linked to another (sugar)molecule, the aldehyde (or keto-) form (i.e. the chain-form) is available for reducing copper (II) ions. When a sugar is oxidized its carbonyl group (i.e. aldehyde or ketone group) is converted to a carboxyl group.
Non REDUCING SUGARS :
Fructose, glucose and galactose are all hexoses. However, whereas glucose and galactose are aldoses (reducing sugars), fructose is a ketose (a non-reducing sugar). It also has a five-atom ring rather than a six-atom ring. Fructose reacts with glucose to make the dissacharide sucrose.

the chemical difference between reducing and non-reducing sugars

Reducing sugars (monosaccharides, e.g. glucose and some disaccharides, e.g. lactose and maltose) act as reductants when heated with a weakly alkaline solution of copper (II) sulfate (e.g. Benedict’s solution) to form an orange-brown precipitate of copper (I) oxide.

The Cu2+ is reduced to Cu+, which reacts with OH- to give Cu2O.

2Cu2+(aq) + 2e- + H2O(l) → Cu2O(s) + 2H+(aq)

Aldehyde (-CHO) group in a reducing sugar can be the source of electrons that reduces copper (II) to copper (I)


R-CHO (aq) + H2O(l) → R-COOH(aq) + 2H+(aq) + 2e-

Benedict test of glucose and sucrose, University of Manitoba, Winnipeg, Manitoba, Canada.

Fehling’s solution (strongly alkaline copper sulfate) and Tollen’s reagent (silver nitrate) can also be used.

Negative (1) and positive(2) Fehling test , Sciences de la vie et de la terre, Didier,(Sciences of life and the earth) Paris, France.

The reaction only occurs if the carbohydrate exists in equilibrium between a ring and open-chain form. The open-chain form contains –CHO and –CO-CH2OH functional groups, which are easily oxidised to carboxylic acids R-COOH and R-CHOH-COOH respectively. Since the carbohydrates are oxidised the reagent must have been reduced, therefore the carbohydrates are reducing agents. Thus the sugars that cause a reaction are called reducing sugars.

Answer 2

Reducing Sugar

Answer 3

1

Answer 4

2

Answer 5

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When you're looking at pure sugars, they are either reducing or non-reducing sugars. So, glucose is a reducing sugar, but sucrose is a non-reducing sugar. When you start looking at other things, its quite possible that it has a mixture of reducing and non-reducing sugars. So, for example, honey has glucose, fructose and sucrose in it. Glucose is a reducing sugar, fructose acts like one (but it is really not), and the sucrose is not a reducing sugar. Does that help?

Answer 6

ya reducing sugars are those which acually acts as a reducing agent while undergoing reaction with reagents like Tollen's or Fehling

Answer 7

Difference between Reducing Sugar and Non-reducing sugar is in its structure. Reducing sugars have a aldehyde group or carbonyl functional group, which can be reduced to -OH group by chemical reaction. Some sugars like Fructose get re-arranged in specific conditions like alkaline pH, and form a compound that is reducible. Most common reducing agents to decide this nature are Tollen's reagent, Fehling's reagent, Benedict's reagent, etc. Maltose has an aldehyde group in them, and that is why it is a reducing sugar.
Non-reducing sugars do not reduce the above because of absence of an aldehyde group. (E.g. sucrose)
Fructose does not have an aldehyde group, yet it is reducing, because it gets rearranged to the reducing glucose in basic solution.

Answer 8

reducing sugars posses carbonyl(-CHO) group while as non -reducing sugars do not posses this group due to which reducing sugars can reduce tollen's reagent ,Bendicty's reagent. while non-reducing sugars don't show this property.

Answer 9

Reducing sugars are those which act as reducing agents and reduce Tollen's reagent, Fehling's reagent, Benedict's reagent, etc. This is because of the presence of an aldehydic group in them. (E.g. Maltose, etc)
Non-reducing sugars do not reduce the above because of absence of an aldehydic group. (E.g. sucrose)

Fructose does not have an aldehydic group, yet it is reducing, because it gets rearranged to the reducing glucose in basic solution.

Answer 10

Is Water a Nonreducing sugar or not?

Answer 11

sugars which can reduce tollen and fehlings reagent are called reducing sugars and those which cannot reduce these reagents are called non reducing sugars