Which carbon in this molecule is the anomeric carbon? (Indicate by the appropriate letter.)

Question

BlackTom AI · Accepted Answer

The anomeric carbon is the former carbonyl carbon (C-1 in an aldose) that becomes a new stereocenter when the ring forms; it is the carbon bonded to two oxygens (one being the ring oxygen and the other being a hydroxyl or OR group).

1) Relevant concepts:
- Ring closure of an aldose converts the carbonyl carbon into a hemiacetal (anomeric) carbon.
- The anomeric carbon is directly bonded to the ring oxygen and to an OH (or OR) group; this distinguishes it from other ring carbons that are bonded only to one oxygen (an alcohol) and to carbons.

2) Step-by-step identification:
- Locate the ring oxygen in the drawing (the O in the top-right of the ring).
- Find the carbon directly adjacent to that oxygen that also bears a hydroxyl group. That carbon is the former carbonyl (C-1) and therefore the anomeric carbon.
- In the pictured structure, the carbon labeled A is directly next to the ring oxygen and carries an OH substituent.

3) No numerical calculations are needed; the “calculation” is the inspection of connectivity: A is bonded to two oxygens (the ring O and the OH). B, C, D are ring carbons bonded to only one oxygen (an OH or the ring O depending on position) and to neighboring carbons; E is the exocyclic CH2OH (C-5 primary alcohol), not the hemiacetal center.

4) Why answer A is correct:
- A matches the definition of the anomeric carbon: it is the carbon that was originally the carbonyl carbon and is now bonded both to the ring oxygen and to a hydroxyl group, creating the stereocenter (anomer).

5) Why the other choices are incorrect:
- B and C are internal ring carbons that bear single hydroxyls but are not bonded to the ring oxygen as their second oxygen substituent, so they are not the hemiacetal carbon.
- D is another ring carbon with an OH but not the former carbonyl carbon; it is not bonded to both the ring oxygen and an OH in the way an anomeric carbon is.
- E is the CH2OH exocyclic carbon (C-5), a primary alcohol, not a hemiacetal center.

6) Conclusion:
Because the anomeric carbon is the ring carbon bonded to both the ring oxygen and an OH, the correct choice is A — confirming the provided answer A.

Which carbon in this molecule is the anomeric carbon? (Indicate by the appropriate letter.)单项选择题

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