Organic Chemistry 1 · Functional groups and connectivity
Find the hemiacetal carbon in glucopyranose
Conditions: β-D-Glucopyranose; ring carbons numbered C1–C5 from the anomeric carbon, C6 is the CH2OH.
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The question
Which carbon of β-D-glucopyranose is a hemiacetal carbon?
Reveal the answer and explanation
Answer and explanation
Correct answer C1, the anomeric carbon
C1 bears an OH and the ring oxygen: OH plus OR on one carbon.
Not this one C5, the other carbon next to the ring oxygen
C5 is bonded to the ring oxygen and to C6 but has no OH of its own.
Not this one C6, the CH2OH carbon
C6 is a primary alcohol carbon with only one oxygen.
Not this one C4
C4 carries one OH and no ring oxygen: a secondary alcohol carbon.
Worked solution
A hemiacetal carbon carries one OH and one OR on the same carbon. In glucopyranose only C1 is bonded to both an OH and the ring oxygen; it is the former aldehyde carbon, called the anomeric carbon. C5 is bonded to the ring oxygen but carries no OH, so it is an ether carbon.
What you'll practice
Locate the hemiacetal carbon of a cyclic sugar by finding the carbon bonded to both an OH and the ring oxygen.
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