Question

In the ring structure of fructose, the anomeric carbon is

• A. C - 1
• B. C - 2
• C. C - 3
• D. C - 5

Hint:

The anomeric carbon is the old carbonyl carbon. Fructose is a ketose with C=O at C-2, so C-2 becomes anomeric on ring closure.

Answer 1

The Correct Option is B

Step 1: Compare an aldose with a ketose:
In glucose the carbonyl is an aldehyde at C-1, so glucose has its anomeric carbon at C-1.
In a ketose the carbonyl is a ketone group inside the chain, one place further along.

Step 2: Locate the carbonyl in fructose:
Fructose is $\mathrm{CH_2OH-CO-CHOH-CHOH-CHOH-CH_2OH}$.
Counting from the end that gives the lowest number, the ketone carbon is C-2.
C-1 is the CH2OH group next to it.

Step 3: What happens on cyclisation:
A hydroxyl group reacts with the C=O group and gives a cyclic hemiketal.
In fructose the C-5 hydroxyl attacks C-2, so a furanose ring forms.
The old carbonyl carbon C-2 now carries two different oxygen groups, OH and the ring O.
This gives two forms, $\alpha$ and $\beta$, which differ only at C-2. These forms are called anomers.

Step 4: Reject the distractors:
The two anomers differ only at C-2, so no other carbon can be called anomeric.
C-1, C-3 and C-5 keep the same configuration in both anomers.

Final Answer:
The anomeric carbon of fructose is C-2, option (B).
\[ \boxed{\text{C-2}} \]