Ques. 1AIPMT 2011 (Prelims)
Which one of the following statements is not true regarding (+) lactose?
- On hydrolysis (+) lactose gives equal amount of D(+) glucose and D(+) galactose.
- (+) Lactose is a -glucoside formed by the union of a molecule of D(+) glucose and a molecule of D(+) galactose.
- (+) Lactose is a reducing sugar and does not exhibit mutarotation.
- (+) Lactose, C12H22O11 contains 8 –OH groups
Answer
(3) (+) Lactose is a reducing sugar and does not exhibit mutarotation.
Solution
As per NCERT, lactose (milk sugar) is composed of β-D-galactose and β-D-glucose; the linkage is between C1 of galactose and C4 of glucose.
(1) Hydrolysis gives one molecule each of D(+)-glucose and D(+)-galactose: true.
(2) It is formed by the union of glucose and galactose through a β-glycosidic linkage: true as stated.
(3) The glucose unit has a free hemiacetal –OH at C1, so lactose is a reducing sugar and it also shows mutarotation the statement 'does not exhibit mutarotation' is not true.
(4) Lactose has 8 free –OH groups (4 in the galactose unit + 4 in the glucose unit): true.
Answer: (3)