JEE Main 2020 · 6 September · evening · Single correct · +4 / −1
Which one of the following statements is not true?
- (a) Lactose contains α-glycosidic linkage between C₁ of galactose and C₄ of glucose
- (b) Lactose is a reducing sugar and it gives Fehling’s test
- (c) On acid hydrolysis, lactose gives one molecule of D(+)-glucose and one molecule of D(+)-galactose
- (d) Lactose (C₁₁H₂₂O₁₁) is a disaccharide and it contains 8 hydroxyl groups
Answer and solution
Answer (a) Lactose contains α-glycosidic linkage between C₁ of galactose and C₄ of glucose
Solution
Chemistry ma’am · 3 steps
In lactose, galactose C1 joins glucose C4 through a β-link, not an α-link. That makes (a) the false one.
- Structure of lactose. Lactose is milk sugar, C₁₂H₂₂O₁₁. It is built from β-D-galactose and β-D-glucose. C1 of galactose bonds to C4 of glucose, so the linkage is a β-1,4 glycosidic linkage.
- Free anomeric carbon. Only galactose C1 is used in the bond. C1 of the glucose unit stays free, so that ring can open to an aldehyde. This is why lactose reduces Fehling's solution.
Statement What is actually true Verdict (a) α-linkage, galactose C1 to glucose C4 The atoms are right, but the link is β-1,4 Not true (b) Reducing sugar, gives Fehling's test Glucose C1 is free, so yes True (c) Hydrolysis gives D(+)-glucose and D(+)-galactose One of each, and both are dextrorotatory True (d) Disaccharide with 8 OH groups 4 OH on galactose (C2, C3, C4, C6) and 4 on glucose (C1, C2, C3, C6) True - So. The only wrong claim is the α-linkage. The answer is (a).
The idea to keep. Learn the three linkages together: maltose is α-1,4, lactose is β-1,4, sucrose is α-1,β-2. If a disaccharide keeps one anomeric carbon free, it is reducing. Sucrose keeps none.
Answer from the NTA final answer key. Solution by PrepWiser.