JEE Advanced 2020 · Paper 1 · Chemistry Q9

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ChemistryMultiple correct+4 / -2Question 27 of 54

In the reaction scheme shown below, Q, R, and S are the major products.

The correct structure of

P (6-tert-butyl-1,1-dimethylindane) reacts with succinic anhydride and AlCl₃ to give Q; Q with (i) Zn-Hg/HCl, (ii) H₃PO₄ gives R; R with (i) CH₃MgBr, (ii) H₃O⁺, (iii) H₂SO₄/Δ gives S.

S is

A tricyclic compound: the indane ring of P (6-tert-butyl-1,1-dimethylindane) with a further six-membered ring fused across C4-C5; that ring has a C=C double bond, and its carbon attached to C4 (next to the five-membered ring) carries a CH₃ group.

Q is

6-tert-butyl-1,1-dimethylindane carrying a -C(=O)CH₂CH₂CO₂H group at C4 (the aromatic carbon next to the CH₂ end of the five-membered ring).

R is

A tricyclic ketone: the indane ring of P with a six-membered ring fused across C4-C5; the ring C=O is on the carbon attached to C4 (next to the five-membered ring), and the CH₂ chain is attached to C5.

S is

A tricyclic compound: the indane ring of P with a six-membered ring fused across C4-C5; that ring has a C=C double bond, and its carbon attached to C5 (next to the tert-butyl group) carries a CH₃ group.

Answer and worked solution

Official answer: B, D

P is 6-tert-butyl-1,1-dimethylindane. Its free ring positions are C4, C5 and C7.

Q. Friedel-Crafts acylation with succinic anhydride (first step of the Haworth synthesis). C7 lies between two bulky groups and C5 is next to tert-butyl, so acylation goes to C4. That gives a 4-oxobutanoic acid side chain at C4, structure (B).

R. Clemmensen reduction converts $\ce{C=O}$ to $\ce{CH2}$ . $\ce{H3PO4}$ then closes the ring by intramolecular acylation onto the only free ortho position, C5. The ketone is therefore attached to C5, next to the tert-butyl group. Option (C) puts it on the C4 side, so (C) is wrong.

S. $\ce{CH3MgBr}$ turns the ketone into a tertiary alcohol, which dehydrates to the ring alkene conjugated with the arene. The $\ce{C(CH3)=CH}$ carbon is attached to C5, next to tert-butyl, structure (D). Option (A) places it next to the five-membered ring.

(B), (D)

Topics:Aromatic hydrocarbons and electrophilic substitution Oxidation and reduction Alcohols

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JEE Advanced 2020 · Paper 1, Chemistry question 9