Any carbohydrate which is capable of being oxidized and causes the reduction of other substances without having to be hydrolysed first is known as reducing sugar, but those which are unable to be oxidised and do not reduce other substances are known as non-reducing sugars. Generally, all the free monosaccharides having free aldehyde or hydroxyl ketonic group are capable of being oxidised. After being oxidised they cause the reduction of the other substances and so known as reducing sugars.
Fehling’s solution and Benedict’s solution are carry out the oxidation e.g.,
Reducing carbohydrate + Fehling’s solution (Deep blue) -> Oxidised Carbohydrates + 2 Cu+
2Cu+ + 2OH -> 2CuOH (Light yellow or green) -> Cu2O (Red) + H2O
C6H12O6 (Glucose) + 2Cu(OH)2-> C6H12O7 (Gluconic acid) + Cu2O (Red) + H2O
Reducing and Non-reducing sugars
Such sugar bear a free aldehyde
(-CHO) or ketonic (-CO) group
These sugars do not have such groups
Reducing sugars have the capacity to reduce cupric ions of Benedict’s or Fehling solution to cuprous ions.
Non reducing sugar fail to reduce the cupric ions of Benedict’s solution to cuprous ions.