JEE Advanced 2024 · Paper 2 · Chemistry Q6

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ChemistryMultiple correct+4 / -2Question 40 of 51

The option(s) with correct sequence of reagents for the conversion of P to Q is(are)

Compound P: a cyclopentanone. C1 is the ketone C=O; C2 carries the chain CH₂-C≡C-CH₂CH₂CH₂-CO₂Et; C3 carries the chain CH₂-C≡C-CH₂CH₂CH₂-CN; C4 carries an acetate ester O-C(=O)CH₃; C5 is CH₂.

An arrow labelled 'reagents' leads to compound Q: a cyclopentane-1,3-diol (OH on C1 and C4 of the ring, corresponding to P's ketone and acetate carbons) with the chain CH₂-CH=CH-CH₂CH₂CH₂-CO₂H on C2 and the chain CH₂-CH=CH-CH₂CH₂CH₂-CHO on C3. Both alkynes have become alkenes.

i) Lindlar's catalyst,

\ce{H2}

; ii)

\ce{SnCl2}

/HCl; iii)

\ce{NaBH4}

; iv)

\ce{H3O+}

i) Lindlar's catalyst,

\ce{H2}

; ii)

\ce{H3O+}

; iii)

\ce{SnCl2}

/HCl; iv)

\ce{NaBH4}

\ce{NaBH4}

; ii)

\ce{SnCl2}

/HCl; iii)

\ce{H3O+}

; iv) Lindlar's catalyst,

\ce{H2}

i) Lindlar's catalyst,

\ce{H2}

; ii)

\ce{NaBH4}

; iii)

\ce{SnCl2}

/HCl; iv)

\ce{H3O+}

Answer and worked solution

Official answer: C, D

Changes needed (P → Q):

ketone → secondary alcohol ( $\ce{NaBH4}$ );
acetate ester → alcohol ( $\ce{H3O+}$ hydrolysis);
ethyl ester → $\ce{COOH}$ ( $\ce{H3O+}$ );
$\ce{C#N}$ → $\ce{CHO}$ (Stephen reduction: $\ce{SnCl2}$ /HCl to the iminium salt, then $\ce{H3O+}$ );
both $\ce{C#C}$ → cis $\ce{C=C}$ (Lindlar).

Constraint on order. $\ce{NaBH4}$ must act before the aldehyde exists, or it would reduce the $\ce{CHO}$ as well. $\ce{NaBH4}$ does not touch esters, nitriles or alkynes, and Lindlar's catalyst does not reduce $\ce{C=O}$ , so Lindlar may come first or last.

(A) $\ce{NaBH4}$ comes after $\ce{SnCl2}$ /HCl and would reduce the imine/aldehyde to an amine/alcohol. ✗
(B) $\ce{NaBH4}$ comes last, after the $\ce{CHO}$ has formed, so it reduces it. ✗
(C) $\ce{NaBH4}$ (ketone) → $\ce{SnCl2}$ /HCl → $\ce{H3O+}$ (gives $\ce{CHO}$ and hydrolyses both esters) → Lindlar. ✓
(D) Lindlar → $\ce{NaBH4}$ → $\ce{SnCl2}$ /HCl → $\ce{H3O+}$ . ✓

Topics:Oxidation and reduction Alkynes Cyanides and isocyanides Acid derivatives

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JEE Advanced 2024 · Paper 2, Chemistry question 6