Ques. 1AIPMT 2002
Which of the following is a reducing sugar?
- Galactose
- Gluconic acid
- -methyl galactoside
- Sucrose
Answer
(1) Galactose
Solution
A reducing sugar has a free (or potentially free) aldehyde or ketone group at its anomeric carbon, which reduces Fehling's / Benedict's solution.
Galactose is an aldohexose whose anomeric carbon is free, so it is a reducing sugar.
In -methyl galactoside the anomeric -OH is replaced by -OCH3 (glycoside), so the ring cannot open and it is non-reducing.
Sucrose is non-reducing because the anomeric carbons of glucose and fructose are joined in the glycosidic bond, and gluconic acid is an oxidised product (aldonic acid) with no free aldehyde group.
∴ Galactose is a reducing sugar.
Answer: (1)