JEE Advanced 2024 Paper 2 Q03 Chemistry Organic Chemistry Hydrocarbons Medium

JEE Advanced 2024 Paper 2 · Q03 · Hydrocarbons

In the following reaction sequence, the major product Q is

L-Glucose $\xrightarrow[\text{ii) }Cr_2O_3,\ 775\ K,\ 10\text{-}20\ \text{atm}]{\text{i) HI, }\Delta}$ P $\xrightarrow[\text{UV}]{Cl_2\ (\text{excess})}$ Q

(Options are four chlorinated benzene/cyclohexane structures.)

  1. A. 1,2,3,4-tetrachlorobenzene (1,2,3,4-Cl4 on benzene ring)
  2. B. 1,2,4,5-tetrachlorobenzene (1,2,4,5-Cl4 on benzene ring)
  3. C. 1,4-dichlorobenzene (para-dichlorobenzene)
  4. D. 1,2,3,4,5,6-hexachlorocyclohexane (BHC / lindane, all six Cl on cyclohexane)
Reveal answer + step-by-step solution

Correct answer:D

Solution

Step 1: L-Glucose $+$ HI/$\Delta$ reduces all hydroxyl groups and gives n-hexane (the standard reductive deoxygenation of an aldohexose with HI). Step 2: n-Hexane over $Cr_2O_3$ at 775 K, 10-20 atm undergoes catalytic dehydrocyclization (aromatization) to give benzene (P). Step 3: Benzene with excess $Cl_2$ under UV light undergoes radical addition (photochlorination) and the major product is 1,2,3,4,5,6-hexachlorocyclohexane (BHC / lindane). Hence Q is the fully chlorinated cyclohexane in option (D).

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