JEE Advanced 2022 · Paper 2 · Chemistry Q18

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ChemistrySingle correct+3 / -1Question 54 of 54

Treatment of D-glucose with aqueous NaOH results in a mixture of monosaccharides, which are

Three Fischer projections (structures shown): D-mannose, D-galactose and D-talose

Fischer projection of D-mannose: CHO at top, OH on C2 left, C3 left, C4 right, C5 right, CH2OH at bottom.

Fischer projection of D-galactose: CHO at top, OH on C2 right, C3 left, C4 left, C5 right, CH2OH at bottom.

Fischer projection of D-talose: CHO at top, OH on C2 left, C3 left, C4 left, C5 right, CH2OH at bottom.

Three Fischer projections (structures shown): D-fructose, D-altrose and D-talose

Fischer projection of D-fructose: CH2OH at top, C2 as C=O, OH on C3 left, C4 right, C5 right, CH2OH at bottom.

Fischer projection of D-altrose: CHO at top, OH on C2 left, C3 right, C4 right, C5 right, CH2OH at bottom.

Three Fischer projections (structures shown): D-glucose, D-fructose and D-mannose

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

Three Fischer projections (structures shown): D-galactose, D-glucose and D-mannose

Answer and worked solution

Official answer: C

The reaction. In aqueous base, D-glucose forms an enediol (the Lobry de Bruyn-van Ekenstein rearrangement). The enediol can re-ketonise in three ways:

back to D-glucose;
to its C-2 epimer, D-mannose;
to the ketose D-fructose.

C-3 to C-5 keep the D-glucose configuration (C-3 OH left, C-4 and C-5 OH right).

Match the Fischer projections. Option (C) shows exactly D-glucose, D-fructose and D-mannose. The other options include sugars that differ from glucose at C-3 or C-4 (galactose, talose, altrose), which this reaction cannot produce. (C).

Topics:Carbohydrates Optical isomerism and chirality

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JEE Advanced 2022 · Paper 2, Chemistry question 18