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From sodium butanoate to 1-phenylphthalazine: Kolbe electrolysis, Friedel-Crafts acylation, Rosenmund reduction and the tests on Q, R, S and T

JEE Advanced 2026 Paper 1 · Question 8 Chemistry Reaction sequence - Kolbe electrolysis, Friedel-Crafts acylation, Rosenmund reduction, Tollens test and hydrazine ring closure Moderate

A four-carbon salt goes in and a nitrogen heterocycle comes out. Press start and the lab does the rest: Kolbe's electrolysis couples two propyl radicals to n-hexane, V2O5 at 500 C closes it into benzene, Friedel-Crafts with phthalic anhydride gives 2-benzoylbenzoic acid, PCl5 and a poisoned Rosenmund catalyst give 2-benzoylbenzaldehyde, and hydrazine closes 1-phenylphthalazine. Every structure is drawn from its bonds, and each option is tested - Cl2/UV on Q, cyclisation plus Clemmensen on R, Tollens on S, a ring-atom audit on T - before the answer is revealed. No prediction stage - input, run, watch, result.

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Sub-topics covered

Kolbe's electrolysis of carboxylate salts
Aromatisation of n-hexane over V2O5 (or Cr2O3/Mo2O3) at high temperature
Free-radical addition of Cl2 to benzene under UV - benzene hexachloride, gammaxane
Friedel-Crafts acylation with a cyclic anhydride
Acid-catalysed intramolecular acylation to anthraquinone and Clemmensen reduction
Acid chlorides from PCl5 and the Rosenmund reduction
Tollens' test for aldehydes
Condensation of 1,4-dicarbonyls with hydrazine - phthalazines, heterocycles

What it teaches

Kolbe's electrolysis: 2 CH3CH2CH2COO- -> CH3(CH2)4CH3 + 2CO2 + 2e- (two propyl radicals couple)
n-Hexane over V2O5 at 500 C and 10-20 atm cyclises and loses 4 H2: Q = benzene
Benzene + excess Cl2 under UV adds three Cl2 to give C6H6Cl6 (BHC); its gamma-isomer is gammaxane (lindane): (B) true
Friedel-Crafts acylation with phthalic anhydride/AlCl3 gives R = 2-benzoylbenzoic acid
Acid cyclisation of R gives anthraquinone; Clemmensen (Zn-Hg/HCl) reduces both C=O to CH2, giving 9,10-dihydroanthracene C14H12, not 9,10-dihydroxyanthracene C14H10O2: (D) false
PCl5 then H2 over poisoned Pd/BaSO4 (Rosenmund) stops at the aldehyde: S = 2-benzoylbenzaldehyde, which gives a silver mirror with Tollens' reagent: (A) true
The aldehyde and ketone of S are 1,4-related; hydrazine condenses with both (-2H2O) to give 1-phenylphthalazine, a heterocycle with N-N in the ring: (C) true

Simulation ID

ADV-2026-P1-CHE-Q08 · revision 1

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