what is the difference between reducing sugar and non-reducing sugar?plz atleast give 3 points and answer as soon as possible cuz i have my exam the day after tomorrow.
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.
Differences between reducing and non-reducing sugars are:
1. Possess a free aldehyde(-CHO) or ketone (-C=0) Group.
2. Can reduce the Cu2+ cupric ions (blue)in Fehling’s or Benedict’s Solution to Cu+ cuprous ions (reddish) that precipitate out as Cu2O(cuprous oxide).
3. Maltose. lactose, melibiose, gentiobiose, cellobiose, mannotriose, rhamnotriose.
1. A free aldehyde or ketonic group is lacking.
2. No such reaction.
3. Sucrose, trehalose,raffinose, gentiarose, melezitose.