A solution of D-glucose in water has stood for a day…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A solution of D-glucose in water has stood for a day. It holds of glucose. In a tube of it gives an observed rotation of degrees. Treat the solution as a mixture of only the two pyranose anomers. Use degrees for the α form and degrees for the β form. Find the specific rotation of the solution, the percentage of the α form and the percentage of the β form.
Need a reminder? The key ideaShow
The anomeric carbon is C-1 of an aldose. A group on the right of the Fischer projection points down in the Haworth ring and a group on the left points up. In a D sugar, the anomeric of the α form points down and that of the β form points up. The specific rotation is , with in dm and in g/mL.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Plan. Find the specific rotation, then use it as a weighted mean of the two anomers.
Step 1. Specific rotation. degrees.
Step 2. Fraction of α. , so .
Step 3. Percentages. α is and β is .
Answer: degrees, α and β.
Check. The equilibrium value is degrees, with about α. Our solution is close to equilibrium, so the answer is sensible.
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