IIT-JEE 2012 · Paper 1 · Chemistry Q38

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ChemistryInteger+4Question 38 of 60

When the following aldohexose exists in its D-configuration, the total number of stereoisomers in its pyranose form is
$\ce{OHC-CH2-CH(OH)-CH(OH)-CH(OH)-CH2OH}$

Open-chain 2-deoxyaldohexose drawn vertically: CHO, CH₂, CHOH, CHOH, CHOH, CH₂OH.

Your answer

Answer and worked solution

Official answer: 8

The sugar is a 2-deoxyaldohexose: C2 is $\ce{CH2}$ , and C3, C4, C5 are stereocentres.

D-configuration fixes C5. C3 and C4 can each be either way: $2^2=4$ open-chain D-isomers.

Pyranose form. Ring closure creates a new stereocentre at C1 (α or β anomer), doubling the count: $4\times2=8$ .

Answer: 8

Topics:Carbohydrates Optical isomerism and chirality

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IIT-JEE 2012 · Paper 1, Chemistry question 38