QUESTION 151 (5 marks)
The diagrams show two cyclic forms of D-glucose. Both have formula C₆H₁₂O₆.
(a) Identify X and Y as α-glucose or β-glucose. Explain using the OH group on the carbon next to the ring oxygen and the CH₂OH group. [3 marks]
(b) Two glucose molecules form a disaccharide by one condensation step. Determine the disaccharide formula and name the linkage formed. [2 marks]
Practice marking scheme
Answer
X is α-glucose and Y is β-glucose. The disaccharide formula is and the linkage is glycosidic.
Working
In the Haworth representations shown, the reference CH₂OH group is above the ring. In X the anomeric OH is below it, on the opposite side: α. In Y that OH is above it, on the same side: β. Condensation joins two monosaccharides through a glycosidic linkage and removes H₂O from their combined formula.
Marking criteria
- Identifies X as α-glucose. [1 mark]
- Identifies Y as β-glucose. [1 mark]
- Uses the opposite/same side relationship of anomeric OH and CH₂OH correctly. [1 mark]
- Determines C₁₂H₂₂O₁₁ by subtracting one water molecule. [1 mark]
- Names the glycosidic linkage. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.