What is the total number of chiral carbons present in glucopyranose and fructofuranose respectively?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 5 and 4
A chiral carbon (asymmetric carbon) has four different groups attached to it. A \( \ce{CH2OH} \) carbon carries two H atoms, so it is never chiral. When the open chain of a sugar closes into a ring, the carbonyl carbon becomes the anomeric carbon: it now carries an –OH, the ring oxygen and two different carbon or hydrogen groups, so it becomes a new chiral centre.
Glucopyranose (six-membered ring, C1 joined to the oxygen of C5): C1 (anomeric carbon: H, OH, ring O and C2), C2, C3 and C4 (each with H, OH and two different ring carbons) and C5 (H, \( \ce{CH2OH} \), ring O and C4) are chiral. C6 is \( \ce{CH2OH} \).
\( \displaystyle \text{glucopyranose: C1, C2, C3, C4, C5} = 5 \)
Fructofuranose (five-membered ring, C2 joined to the oxygen of C5): C2 (anomeric carbon: OH, \( \ce{CH2OH} \) (C1), ring O and C3), C3, C4 and C5 are chiral. C1 and C6 are both \( \ce{CH2OH} \).
\( \displaystyle \text{fructofuranose: C2, C3, C4, C5} = 4 \)
So the numbers of chiral carbons are 5 and 4, respectively.