JEE Advanced 2024 · Paper 2 · Chemistry Q3

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ChemistrySingle correct+3 / -1Question 37 of 51

In the following reaction sequence, the major product Q is

$\text{L-Glucose} \xrightarrow[\text{ii) } \ce{Cr2O3},\ 775\,\mathrm{K},\ 10\text{-}20\,\mathrm{atm}]{\text{i) HI},\ \Delta} \mathbf{P} \xrightarrow[\text{UV}]{\ce{Cl2}\ \text{(excess)}} \mathbf{Q}$

Hexachlorobenzene: benzene ring with Cl on all six carbons.

1,2,3,5-Tetrachlorobenzene: benzene ring with Cl on three adjacent carbons and on the carbon para to the middle one.

1,4-Dichlorobenzene: benzene ring with two Cl para to each other.

1,2,3,4,5,6-Hexachlorocyclohexane: saturated six-membered ring (no double bonds) with one Cl on each carbon.

Answer and worked solution

Official answer: D

HI, heat: glucose is fully reduced to n-hexane.
$\ce{Cr2O3}$ , 775 K, 10-20 atm: n-hexane is cyclised and dehydrogenated (aromatised) to benzene, P.
Excess $\ce{Cl2}$ , UV: benzene undergoes free-radical addition of three $\ce{Cl2}$ , not substitution:
$\ce{C6H6 + 3Cl2 ->[h\nu] C6H6Cl6}$

Q is 1,2,3,4,5,6-hexachlorocyclohexane (BHC, gammexane). The configuration of L-glucose is lost in the first step, so it does not matter.

Topics:Aromatic hydrocarbons and electrophilic substitution Carbohydrates Polyhalogen compounds

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JEE Advanced 2024 · Paper 2, Chemistry question 3