Q.The number of sp² and sp³ hybridised carbon in fructose are respectively

Fructose, an open-chain 2-ketohexose, has 3 chiral carbons and therefore $2^3=8$ possible optical isomers.

Open-chain structure of fructose ($C_6H_{12}O_6$, a ketohexose):

$\begin{aligned} &C1:\ CH_2OH \\ &C2:\ C{=}O \quad (\text{keto group}) \\ &C3:\ H-C-OH \\ &C4:\ HO-C-H \\ &C5:\ H-C-OH \\ &C6:\ CH_2OH \end{aligned}$

written linearly as $CH_2OH-CO-CHOH-CHOH-CHOH-CH_2OH$.

Locating the chiral carbons: A carbon is chiral (asymmetric) if it is bonded to four different groups. C1 ($CH_2OH$, bonded to C2 which bears the keto group) and C2 (the carbonyl carbon, only 3 groups, $sp^2$) are not chiral. C6 ($CH_2OH$, terminal) is also not chiral. Carbons C3, C4 and C5, however, are each bonded to an $-H$, an $-OH$, and two different carbon-chain fragments (one side leading towards the keto/C1 end, the other towards the terminal $CH_2OH$/C6 end) - so each of these three is a genuine chiral centre.

Number of chiral carbons = 3.

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