JEE Advanced 2018 · Paper 2 · Chemistry Q4

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ChemistryMultiple correct+4 / -2Question 22 of 54

The Fischer presentation of D-glucose is given below.

The correct structure(s) of β-L-glucopyranose is (are)

Fischer projection of D-glucose: CHO at top; C2 H left, OH right; C3 HO left, H right; C4 H left, OH right; C5 H left, OH right; CH₂OH at bottom.

Haworth pyranose (ring O at back right): C5 has H up and CH₂OH down; C1 has OH up, H down; C2 has OH up, H down; C3 has H up, OH down; C4 has OH up, H down (α-L-glucopyranose).

Haworth pyranose: C5 has H up and CH₂OH down; C1 has OH up, H down; C2 has H up, OH down; C3 has H up, OH down; C4 has OH up, H down.

Haworth pyranose: C5 has CH₂OH up and H down; C1 has OH up, H down; C2 has OH up, H down; C3 has OH up, H down; C4 has H up, OH down. (β-D-mannopyranose)

Haworth pyranose: C5 has H up and CH₂OH down; C1 has H up, OH down; C2 has OH up, H down; C3 has H up, OH down; C4 has OH up, H down (β-L-glucopyranose).

Answer and worked solution

Official answer: D

L-glucose is the mirror image of D-glucose. Every stereocentre is inverted.

Haworth conventions. Draw the ring with O at the back right and C-1 on the right. In the L-series the C-5 $\ce{CH2OH}$ points down. For L-glucose the OH groups are: C-2 up, C-3 down, C-4 up. These are the reverse of D-glucose (down, up, down).

β anomer. The anomeric OH is trans to the C-5 $\ce{CH2OH}$ , so it points down at C-1.

Check the options.

(A) has OH up at C-1: α-L.
(B) has OH down at C-2: wrong configuration.
(C) has $\ce{CH2OH}$ up: a D-sugar (β-D-mannose).
(D) has $\ce{CH2OH}$ down; C-1 OH down; C-2 up; C-3 down; C-4 up. This is β-L-glucopyranose.

(D)

Topics:Carbohydrates Optical isomerism and chirality

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JEE Advanced 2018 · Paper 2, Chemistry question 4