Answer : (B) C-2
Step 1 — Recall what the anomeric carbon is
The anomeric carbon is the former carbonyl carbon, which becomes a new chiral centre when the open-chain sugar cyclises into a hemiacetal or hemiketal.
Step 2 — Locate the carbonyl group in fructose
Fructose is a ketohexose — its carbonyl group is a ketone at C-2, not an aldehyde at C-1 :
$$CH_2OH-\underset{\|}{\overset{O}{C}}-(CHOH)_3-CH_2OH$$
$$\ \ \ C_1 \qquad C_2 \qquad \qquad \ \ C_6$$
Step 3 — Identify the anomeric carbon
On cyclisation, the $-OH$ at C-5 attacks the carbonyl carbon at C-2, closing a five-membered furanose ring. C-2 therefore becomes the new chiral centre.
$$\textbf{Anomeric carbon of fructose} = \textbf{C-2}$$
Step 4 — Contrast with glucose
Glucose is an aldohexose, with its $-CHO$ group at C-1. Its anomeric carbon is therefore C-1.
| Sugar | Type | Carbonyl at | Anomeric carbon |
|---|
| Glucose | Aldohexose | C-1 | C-1 |
| Fructose | Ketohexose | C-2 | C-2 |
Step 5 — Note the consequence
Because C-2 is the anomeric carbon, fructose exists as $\alpha$- and $\beta$-D-fructofuranose, and it shows mutarotation just as glucose does.
