Q.Write the open-chain structure of D-fructose and state the size of ring it forms when it cyclises.

Numbering D-fructose's six carbons: C1 is a primary $-\text{CH}_2\text{OH}$ group, C2 carries the ketone carbonyl ($\text{C}{=}\text{O}$), C3 has $-\text{OH}$ on the left, C4 has $-\text{OH}$ on the right, C5 has $-\text{OH}$ on the right, and C6 is the terminal $-\text{CH}_2\text{OH}$. In condensed form: $\text{CH}_2\text{OH}{-}\text{CO}{-}(\text{CHOH})_3{-}\text{CH}_2\text{OH}$.

Like glucose, fructose does not exist mainly as this open chain in solution -- it cyclises intramolecularly. Because fructose's carbonyl group is at C2 rather than C1, it is the C5 hydroxyl group that attacks C2 (not C1, as in glucose), and since this ring spans C2 through C5 plus the ring oxygen, it forms a smaller five-membered ring rather than glucose's six-membered ring. A five-membered ring containing one oxygen is called a furanose ring, so fructo …

Locate the ketone carbon (C2) as the site cyclisation will occur at, then count the ring members formed when the C5 hydroxyl attacks it, to see why the resulting …

Assuming fructose forms the same size ring as glucose (a pyranose) by default -- its usual cyclic form is the smaller furanose ring, precisely because its carbonyl carbon (C …